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Manufacturability of Different Engineering Materials Ferrous Materials

Steel a. Hot Formability

A cold steel ingot cannot be converted into other shapes. Hot working does the job of shaping the ingot into structural shapes. With the help of hot forming the mechanical properties of steel can be improved and the grain structure can be refined. Forging, rolling and extrusion are the most popular techniques used in hot forming of steel.

b. Cold Formability

The chemical composition, mechanical properties, metallurgical microstructure, surface condition, thickness, edge condition, and forming direction in relation to the rolling direction of the steel, will have an influence on the forming properties. Cold forming due to the grain structure of the material being elongated and forced to follow the contours of the part thus increasing its strength. Additionally, during cold forming the part undergoes work-hardening.

AHMET ARAN - MFG PROP V1

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Steel Types For Cold Forming

Low strength steels are generally more ductile than higher strength steels and are therefore capable of being shaped to more restrictive forming radii. Generally low carbon content is a prerequisite to good formability and higher carbon steels have limitations in this respect.

Commercial Steel is intended for applications in which simple bending or moderate forming is used to make the finished part.

c. Drawing Steel is more ductile and formable. These properties result from more restrictive chemical analysis, and either different production practices or higher standards for process control when practices are identical to those used for CommercialQuality. Deep Drawing Steel (DDS) provides a higher level of formability in both drawing and stretching operations. These are a result of close control of chemical analysis and carefully tailored processing subject to the most stringent process standards.


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Machinability

The carbon content of steel greatly affects steel machinability. High-carbon steels

are difficult to machine because they are strong and because they may contain carbides that abrade the cutting tool. On the other end of the spectrum, low-carbon steels are troublesome because they are too soft and stick to the cutting tool, resulting in a built up edge that shortens tool life. Therefore, steel has the best machinability with medium amounts of carbon, about 0.20%.

Chromium, molybdenum and other alloying metals are often added to steel to improve its strength. However, most of these metals also decrease machinability. Inclusions in steel, especially oxides, may abrade the cutting tool. Machinable steel should be free of these oxides.

There are a variety of chemicals, both metal and non-metal, that can be added to steel to make it easier to cut. These additives may work by lubricating the tool-chip interface, decreasing the shear strength of the material, or increasing the brittleness of the chip. Historically, sulphur and lead have been the most common additives, but bismuth and tin are increasingly popular for environmental reasons.


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Lead can improve the machinability of steel because it acts as an internal lubricant in the cutting zone.[9] Since lead has poor shear strength, it allows the chip to slide more freely past the cutting edge. When it is added in small quantities to steel, it can greatly improve its machinability while not significantly affecting the steel's strength.

Sulphur improves the machinability of steel by forming low shear strength inclusions in the cutting zone. These inclusions are stress risers that weaken the steel, allowing it to deform more easily. Stainless steel Stainless steels have poor machinability compared to regular carbon steel because they are tougher and tend to work harden very rapidly.Slightly hardening the steel may decrease the toughness and make it easier to cut. AISI grades 303 and 416 are easier to machine because of the addition of sulphur and phosphorus.

http://en.wikipedia.org/wiki/Machinability#cite_note-kalpakjian-8


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d. Weldability

Low Carbon Steels (0,04-0,3% carbon content) Low carbon steels (with the carbon content between 0,04-0,15%) have perfect weldability. No hardening occurs. No hot or cold cracking. If the carbon content is 0,15-0,3%, weldability is generally good but hardening of steel becomes a problem. Hardening makes the base metal (heat affected zone) martensite. Shinkage of the melted metal can create cracks. Preheating process can be solution for the hardening and crack problems.

Medium Carbon Steels (0,3-0,6 % carbon content) 0,3% carbon content is good for weldability. But the 0,6% carbon content can transform the heat affected zone into martensite completely. This will decrease the toughness and the ductility of heat affected zone. Developments of cracks while cooling to room temperature and fractures during service are possible results.


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High Carbon Steels (above 0,6%) Poor weldability. Hard, brittle martensite form occur. In order to solve the hardness problem, preheating to high temperatures tempering with postheating is required. But each welding need special process.

High Strength Low Alloy Steels These steels are designed to meet specific mechanical properties. As the yield strength increases, weldability decreases. Welding properties are the same with the carbon steels that have similar carbon equivalents.

Quenched and Tempered Steels Welding can change the microstructure and so the mechanical properties. In order not to change microstructure or to control the changes preheating and slow cooling is required.


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Chromium-Molybdenum Steels Chromium and molybdenum make the steel more resistant to corrosion and increases the materials elevated-temperature strength. But the alloying element and approximate 0,15% C make the steel air hardenable. For this reason chromium-molybdenum steels are supplied in quenched and tempered condition and have the same weldability with Quenched and Tempered Low Alloy steels. e. Castability

The mechanical properties of steel make it an attractive engineering material. However, great difficulties are faced by the foundry specializing in steel. We use steel castings only if,

Strength and ductility of cast iron is not enough.

Manufacturing by casting is the most economic way (especially for very big parts and complicated parts).

Welding is necessary.

The selected steel is not suitable for other methods.


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f. Finishing

Especially, after hardening steels become really difficult to machine. Therefore, finishing processes are done on steels to make them suitable for usage. Some of the common types of finishing processes are briefly explained below:

Grinding to abrade, to wear away by friction, to sharpen. Done by removing metal by a rotating abrasive wheel. Grinding is used to achieve high quality surface finish and accuracy.

Honing an abrading process in which the cutting action is slowly done by abrasive sticks placed on a metal mandrel. cutting speed is lower than grinding, heat and pressure are minimized, resulting in desired size and a smooth finish for the material.

Polishing Polishing is an abrading operation done for the removal or smoothing out of

grinding lines, scratches, pits and certain other surface defects. It is performed with a belt or a wheel that has an abrasive bonded to it.


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Buffing Buffing is a process for producing smooth, reflective and scratch-free surfaces by putting the workpieces into contact with revolving cloths or buffing wheels charged with a suitable compound.

Lapping Lapping is used on flat, cylindrical, spherical or specially formed surfaces. It is done by having two work surfaces in contact with a lap, having motion with one another with an abrasive being introduced between them

Barrel Finishing Barrel finishing is done by the tumbling or rolling of parts in rotating barrels for cleaning, and burnishing. Depending on the purpose it can be done either dry or wet

Superfinishing All machining processes leave a surface coated with fragmented or smear metal that results in excessive wear and lubrication difficulties. Superfinishing is a surface improving process that removes this undesirable fragmented metal..


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Cast Irons a. Castability

Effects on Castability

Fluidity (in cast iron it depends on carbon and silicon content, and temperature)

Thermal conductivity

Melting point

Chilling tendency

Formation and content of carbon (Low carbon content reduces liquidity of cast iron and graphite flakes cause more stress than spheroid graphite)

Cooling rate (defined by section thickness variations and minimum section thickness. Casting design is often described in terms of section sensitivity. This is an attempt to correlate properties in critical sections of casting with combined effects of composition and cooling rate.)


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Gray cast iron: Lubricant, low melting point, low shrinkage, best castability (especially eutectic gray irons have excellent castability) White cast iron: High hardness, brittle, high wear resistance, relatively low castability. Ductile iron: High ductility, strength and toughness. Its spherical graphite reduces stress concentrations which provide it greater stiffness than gray iron. Malleable iron: Excellent castability. With high temperature and silicon or carbon on the high side, small castings down to 1/16 inch section thickness may be cast. Comparison between ferritic graphite (FG), cementitic graphite (CG) and spheroid graphite (SG): Fluidity: FG > CG > SG Shrinkage: SG > CG > FG Chilling tendency: CG >FG


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b. Weldability Welding is used for, repairing defects to salvage or upgrading a casting before service, repair damage or worn castings, and to fabricate castings into welded assemblies. Mainly, arc welding, oxyfuel welding, and brazing welding are applied.

Arc Welding More than 90% of all cast iron welding is done by arc welding. Shielded metal arc, gas metal arc, flux cored arc, gas tungsten arc (not consumable), submerged arc welding (not consumable) Electrods used are also so important due to the welded part would be machined and ductility. Nickel based, steel based, stainless steel, cast iron electrodes are mostly used. Filler metal is mostly nickel based.


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Gray Cast Iron Because of including graphite in flake form, carbon can be easily introduced into the weld pool, thus causing weld metal embrittlement. Consequently, techniques that minimize base metal dilution are recommended. Shrinkage must be taken care and low strength filler metals help reduce cracking without sacrificing all joint strength. Gray iron weldments are susceptible to formation of porosity. This can be controlled by lowering the amount of dilution of the weld metal, or by slowing the cooling rate so the gas has time to escape. Preheat helps reduce porosity and reduces cracking tendency. Ductile Cast Iron Ductile irons have greater weldability than gray cast irons but require specialized welding processes and filler metals. Pearlitic ductile iron produces a large amount of martensite in the HAZ than Ferritic ductile iron and generally more susceptible to cracking. If machinability is desired, castings should be annealed immediately after welding. Preheat reduces heat input and reduces the amount of HAZ carbides and martensite. Ferritic ductile iron, however, can usually be welded without preheat.


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Malleable Cast Iron During welding, the ductility of HAZ of malleable iron is severely reduced because graphite dissolves and precipitates as carbide. Malleable iron castings should not be repaired by welding to correct a failure caused by overstressing of the part. Ferritic malleable grades have the highest weldability of the malleable irons although impact strength is reduced. Pearlitic malleable irons, due to their higher combined carbon content, have lower impact strength and higher cracking susceptibility when welded. Chilled and White Cast Iron These irons are abrasion- resistant cast irons having structures free of graphite carbon. Due to their extreme hardness and brittleness, they are generally considered unweldable. Because abrasion resistant cast iron have limited resistance to thermal shock, welding is generally not recommended. Welding is sometimes applied in repair operations or to attach parts to other machine components.


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Corrosion Resistant Cast Iron These cast irons include high silicon, high chromium, or high nickel irons. Specifications for many of these irons permit welding for repair of minor casting defects. Heat Resistant Cast Iron Heat resistant cast irons provide high strength at elevated temperatures, as well as resistance to scaling. They are produced as flake graphite irons or ductile (spheroid graphite) irons.


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Oxyfuel Welding The OFW process is ideal for welding thin sections, tubes, and small diameter pipe. OFW is widely used to repair minor defects in gray cast iron but is used less frequently for ductile irons. The slow heating rate of OFW causes a large HAZ to develop but prevents the formation of the brittle martensitic structure.

Gray Cast Irons OFW is generally is done for repairing and also it can be done for producing a simple assemblies. Immediately after completing welding, the part should be placed in furnace heated to the same temperature as the casting. And it should be cooled slowly, when the welded casting is not stress relieved. Ductile Cast Irons OFW is often used for repairing of defects. It has also been used for hard facing of specific areas to increase abrasion resistance. Processes have been developed to cause nodularization by adding magnesium or cerium. If these elements are added, a postweld ferritizing anneal is necessary for ductility. A major problem in the welding of ductile iron is the complete loss of ductility in the HAZ.


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Malleable and White Cast Irons OFW is used in the foundry repair of small defects on rough casting while the casting in the white iron condition, before malleabilizing.

Braze Welding Unlike brazing, the filler metal in braze welding is not distributed in the joint capillary. Brazing welding is developed for repairing cracks and or broken cast iron parts. Advantages of the braze welding are less heat required, less distortion, excellent machinability and low residual stresses due to the soft and ductile deposited weld metal, possibility of adequate strength in joint application. The limits of braze welding are joint strength controlled by weld metal, low service temperature, no good color match. Braze welding can be used to join castings to castings or other metals. The malleable cast iron grades are the easiest to braze welding. Ductile iron is more difficult to braze due to its higher total carbon content. The graphite flakes in gray cast iron make it the most difficult to braze weld; special groove preparation is necessary.


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c. Machinability The presence of graphite provides the free machining characteristic of iron, and shape and amount of graphite establish the potential surface finish obtainable with a cutting process and the necessary cutting force. Different phase’s different characteristics: Ferrite; no carbon, low hardness, silicon contents reduce machinability Pearlite; medium strength and hardness, fine and coarse perlite, fine is harder and machine low speed Martensite; unmachinable, but tempered and good machinability at the same hardness of perlite Acicular structures (involve bainite); hard to machine Austenite; soft good machinability, but chromium carbides is decrease machinability Carbides; hard, reduce tool life, as little as 5% free carbides in the matrix structure has a significantly detrimental effect. Steadite (phosphorus); 0.20% P no effect tool life, but 0.40%P effect machinability and reduce tool life Mixed-matrix structures; amount of components % perlite, % bainite are effective


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Gray Iron; Machinability of gray cast iron is good. Graphite flakes; discontinuity of matrix and chip breaker, and also lubricant. But gray irons have some difficulties:

The presence of chill at the corners and in light section; light section the cooling rate is different and different micro structure is occurred.

The presence of adhering sand on the surface of the casting; sand casting sang particle adhere on the surface. Slow speed machining is good. Carbide tools for resisting the extreme abrasion

Swells, usually the result of soft molds; metals stick on the wall of part

Shifter castings; similar swells

Shrinks; in drilling shrinkage section is less resistant to drilling and the drill follow the shrinkage section

Ductile iron; Balls or spheres, the same as flakes lubricant and chip breaker, also tensile strength is higher the same hardness on cast irons Compacted graphite iron; machinability is between ductile and gray


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Malleable iron; the machinability of malleable iron is 25% better than free cutting steel, but in some application White and High-Silicon Irons; is the least machinability in cast iron High silicon cast irons have hardnesses of approximately 500 HB and are normally considered machinable only by grinding, but it is alloyed with carbon or other component and increasing machinability. On the other hand strength and resistance to corrosion are reduced. Cutting Fluids: Free graphite performs an important lubrication role, reduces strain and friction, lower the tool and part temperature, but other lubricant can be used to increase of machining especially drilling. Lubricant increases the machining speed and tool life. Grinding: Some cast irons are extremely difficult to machine. Grinding is commonly used finishing operation for iron castings that is characterized by minimal stock removal, fine surface finish, and close dimensional tolerances. Grinding wheels control their grinding action: the abrasive. The abrasives most frequently recommended for use on iron castings are SiC and Al2O3..


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Stainless Steels

Concerning machinability there are two groups; free machining alloys and non-free machining alloys. Free-machining alloys: They contain a free-machining additive such as sulphur, selenium, lead, oxides to form inclusions which improve all machining characteristics. These elements can reorganize the matrix of the materials and improve the machinability. Some of the properties may be influenced negatively such as corrosion resistance, ductility, hot/cold formability by these additives.

a. Machinability

Free machining martensitic and ferritic alloys are the easiest of stainless steels

to machine and austenitic stainless steels are the most difficult grade to machine because of high work hardening rate and ductility. The machinability of duplex stainless steels is restricted by their strength level. The machinability of precipitation-hardenable stainless steels depends on type of alloy and its hardness level.


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In each kind of machining process of stainless steels need a cooling fluid (due to low thermal conductivity) to avoid overheating and to reduce friction between cutting tool and workpiece.

Generally hard and high strength cutters are used for machining of stainless steels because of high hardness level and strength. Lastly, cutters should be as rigid as possible.

It is recommended to use free machining alloys for machining stainless steels to minimize the difficulty of removing chips (ex: tapping) and non-free machining stainless steels produce long and continuous chips but these chips are not desirable since they may cause some problems.

Some difficulties may occur during machining stainless steels due to high strain hardening rate (especially austenitic type). Sometimes the reason of the problem can arise from previous operations; especially with non-free machining alloys since they have high strain hardening rate.


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Grinding: The thermal conductivity of all stainless steels is low and their frictional qualities are high. If stainless steel is subject to very heavy grinding operations, a very thick oxide layer will form. Most notably, the low thermal conductivity of stainless steels all lead to build-up surface oxidation and in extreme cases even sensitisation of austenitic stainless steels or "burning" (re-hardening) of heat treated martensitic grades. Techniques that help prevent build-up of surface heat include, use of lower speeds and feeds and careful selection of lubricants to minimize loading of the abrasive. The austenitic stainless steels generally have higher tensile strengths and elongation than the ferritic stainless steels but lower yield strengths and the martensitic stainles steels are very brittle. So the grinding is difficult for martensitic steels. Grinding is best for austenitic stainless steels.


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b. Formability The main factors which affect formability of stainless steels are yield strength, tensile strength, ductility and strain hardening rate.

The austenitic and ferritic stainless steels are well suited for forming processes but austenitic grade is the best one due to high strain hardening rate, large elongation and martensitic grade is the worst for forming because they are brittle.

Usually a lubricant is used in forming of stainless steels to reduce friction and to have a high quality surface finish. During hot extrusion, glass is used as a lubricant for stainless steels.

Because of high strength and toughness of stainless steels, dies must withstand high stresses, they are made from tool steel or carbide.

Work hardened stainless steels can be formed in very limited degrees but when the material is heated to high temperatures the material becomes more ductile so forming becomes easier.

Generally low speeds and small reductions are essential for forming of stainless steels to slow strain hardening.


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c. Weldability

Martensitic Steels Higher carbon contents will cause greater hardness and more tendency to cracking. Because of their sensitivity to welding thermal cycles, they are the most difficult stainless steels group to be welded. The most important factor for welding of martencitic steels is hardenability.

Ferritic Steels Purity is important because of its effect on ductility, toughness and so weldability. If coefficients of thermal expansion are different so there is the risk of excessive stresses.

Austenitic Steels Austenitic Steels are the best weldable type of stainless steels. Purity is important. The HAZ round the weld may become sensitive to intergranular corrosion. Welded joints have tendency to hot cracking under the stresses during cooling because of large thermal contraction.


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Duplex Steels The weldability and welding characteristics of duplex stainless steels are better than ferritic stainless steel but generally not as good as austenitic materials.

Precipitation-Hardening Steels PH stainless steels which solidify as primary ferrite have good resistance to

cracking. Intergranular Corrosion: In regions where all intermediate temperatures exist and where the temperature causes carbide precipitation sensitized zones in the welded part are created.


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d. Castability Stainless steels alloys are somewhat easier to shape by die-casting than the low

alloys structural steels. Sounder castings with better surfaces are more easily obtained and longer die lives are realized.

Their shrinkage ratio is high and they have low fluidity; therefore stainless steels

are not proper for casting as other steels. The most notable effect with stainless steels is that the resistance to corrosion can be negatively influenced, if the cooling is not rate is not high enough to subpress the precipitation of chromiumcarbides.


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e. Heat Treatment

Ferritic Stainless Steels Ferritic stainless steels contain from 10.5 to 27% chromium. Low in carbon content, they cannot be hardened by heat treating; always used in the annealed condition.

Austenitic Stainless Steels But the austenitic stainless steels cannot be hardened by thermal treatments.

Martensitic Stainless Steels They can be hardened by heat treatment and have high strength and moderate

toughness in the hardened-and-tempered condition.


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3.2 Non-ferrous Materials 3.2.1 Aluminum and Alloys

a. Castability

The properties of aluminum which affect the castability:

It has low melting point, 660 °C. 660 °C is easy to reach.

Aluminum has a very high specific heat, 0.2259 cal/g-K, so it affects bad on the cost.

It has a low latent heat of fusion 10.71 kj/mol. Melting doesn’t spend so much energy

Thermal conductivity of aluminum is 3.00 cm °C. It is higher than most manufacturing metals. Thermal conductivity determines cooling rate.

Thermal diffusivity, measure of the rate at which a temperature disturbance at one point travels to another point. Aluminum has a high thermal diffusivity value like thermal conductivity.


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The coefficient of expansion of aluminum is very high according to its’ competitors. Changes in expansion aren’t wanted in casting

b. Machinability

Aluminum is much softer than steel, and techniques to improve its machinability usually rely on making it more brittle. Alloys 2007, 2011 and 6020 have especially good machinability

Traditional machining operations such as turning, milling, boring, tapping, sawing etc. are easily performed on aluminum and its alloys. But for pure aluminum we can not say that it has good machinability. Since it has very high ductility that causes continuous chips. Machinability varies significantly from alloy to alloy and even for various tempers for a given alloy, based on factors such as alloy composition and temper along with second phase particle type and volume fraction.

The specific properties of aluminum alloys must be considered:

Their density allows high speeds of rotation.

Their modulus of elasticity, one third that of steel must be considered.


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The alloy’s thermal conductivity assists with heat dissipation.

A coefficient of linear expansion that is twice that of steel makes heating undesirable if criteria of dimensional stability are to be satisfied.

Some alloying elements like Pb and Bi these elements form low-melting point eutectic phases that partially melt as a result of the heat generated during machining, expand, and facilitate the formation of small chips at the tool/work piece interface. Si, Mn, Fe, and Ni additions were considered, with Si showing the most promise in producing an alloy with machinability Cutting Force:The specific cutting force needed to machine aluminum alloys is far less than is required for steel (one third) Tooling: For aluminum alloys, tool life is much longer than for machining steels, when all other factors being equal. Cutting Speeds: All wrought alloys can be machined very rapidly. c. Formability


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Aluminum alloys are used frequently because of their high formability (fcc lattice structure) and strength/density ratio. They can be used in both bulk deformation processes and sheetmetal working. They are suitable for cold forming with their low yield strengths and ductility. They can also be hot formed at low temperatures compared to ferrous metals (at ~400oC). But their high thermal expansion coeffient is problematic.

d. Weldability

Aluminum is the most difficult alloy to weld because of its high thermal expansion coefficient (twice that of steel).

Heat treatable aluminum alloys get their strength from ageing. Significant decrease in tensile strength can occur when welding aluminum due to over aging.

Non-heat-treatable alloys: Material strength depends on the effect of work hardening and solid solution hardening of alloy elements such as magnesium, and manganese. When welded, these alloys may lose the effects of work hardening which results in softening of the HAZ adjacent to the weld.


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Imperfections in welds: Aluminum and its alloys can be readily welded providing appropriate precautions are taken. The most likely imperfections in fusion welds are: Porosity, cracking, poor weld profile

e. Finishing There is a clear, colorless and protective oxide film on aluminum, moreover that film provides a permanent and highly attractive surface finish. An aluminum product doesn’t need paint and other finish types that are elaborate and expensive. Most other metals needs costly finishes. 3.2.2. Copper and Alloys a. Machinability

Cast copper-base alloys are easy to machine(especially when compared to stainless steels and titanium, their main competitors for corrosion resistance). Easiest to


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machine are the leaded copper-base alloys. These alloys are free cutting and form small, fragmented chips while generating little heat.

Next in order of machinability are moderate to high-strength alloys with second phases in their microstructures such as unleaded yellow brasses, manganese bronzes, and silicon brasses and bronzes. These alloys form short, brittle, tightly curled chips that tend to break into manageable segments. While the surface finish on these alloys will be good, the cutting speed will be lower and tool wear higher. Copper alloys like brass and bronze have lead added to make them easy to machine. They can be easily machined using very hard tool materials. Bronze is stronger and less machinable than brass. Lead, selenium, tellurium and sulfur are added to copper alloys to improve machinability.

The most difficult copper-base alloys for machining are the single-phase alloys such as high conductivity copper, chromium copper, beryllium copper, aluminum bronze and copper nickel. A general tendency during machining is to form long, stringy chips that interfere during high-speed machining operations

The principal copper alloys contain from 2% to 45% alloying element by weight. The amount of copper increases the thermal conductivity of the alloy while reducing the


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hardness and modulus of rupture. Being non-porous, standard water soluble coolants may be used if desired, but are not required. b. Formability

The formability of copper alloys is dependent upon a number of variables including alloy, temper, bending direction, strip thickness, width, and method of forming. Copper and the majority of its alloys are highly workable hot or cold, making them readily commercially available in various wrought forms: forgings, bar, wire, tube, sheet, and foil.

As with other metal systems, copper is intentionally alloyed to improve its strength without unduly degrading ductility or workability. However, it should be recognized that additions of alloying elements also degrade electrical and thermal conductivity by various amounts pending on the alloying element, its concentration and location in the microstructure.

Copper is hot worked over the temperature 750 to 875°C (1400 to 1600°F), annealed between cold working steps over the temperature range 375 to 650°C and is thermally stress relieved usually between 200 and 350°C.


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Many of the applications of copper alloys take advantage of the work-hardening capability of the material, with the cold processing deformation of the final forming steps providing the required strength and ductility for direct use. Copper is easily deformed to more than 95% reduction in area. The amount of cold deformation between softening anneals is usually restricted to 90% avoid excessive crystallographic texturing, especially in rolling of sheet and strip. As a result, the copper alloys provide the optimum combination of excellent formability and the highest strength attainable. c. Castability

All copper alloys can be successfully cast in sand. Sand casting is the most economical casting method and allows the greatest flexibility in casting size and shape. High fluidity often ensures good castability, but it is not solely responsible for that quality in a casting alloy.

Foundry alloys generally are classed as "high shrinkage" or "low shrinkage". To the former class belong the manganese bronzes, aluminum bronzes, silicon bronzes, silicon brasses and some nickel silvers. They are more fluid than the low-shrinkage red


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brasses, more easily poured, and give high-grade castings in sand, permanent mold, plaster, die and centrifugal casting processes.

d. Weldability

Many of the physical properties of copper alloys are important to the welding processes, including melting temperature, coefficient of thermal expansion, and electrical and thermal conductivity. Certain alloying elements greatly decrease the electrical and thermal conductivities of copper and copper alloys.

Several alloying elements have pronounced effects on the weldability of copper and copper alloys. Small amounts of volatile, toxic alloying elements are often present in copper and its alloys. As a result, the requirement of an effective ventilation system to protect the welder and/or the welding machine operator is more critical then when welding ferrous metals. Effect of Alloying Elements

Zinc reduces the weldability of all brasses in relative proportion to the percent of zinc in the alloy. Zinc has a low boiling temperature, which results in the production of toxic vapors when welding copper-zinc alloys.


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Tin increases the hot-crack susceptibility during welding when present in amounts from 1 to 10%. Tin, when compared with zinc, is far less volatile and toxic. During the welding tin may preferentially oxidize relative to copper. The results will be an oxide entrapment, which may reduce the strength of the weldment.

Beryllium, aluminum, and nickel form tenacious oxides that must be removed prior to welding. The formation of these oxides during the welding process must be prevented by shielding gas or by fluxing, in conjunction with the use of the appropriate welding current. The oxides of nickel interfere with arc welding less than those beryllium or aluminum. Consequently, the nickel silvers and copper-nickel alloys are less sensitive to the type of welding current used during the process. Beryllium containing alloys also produce toxic fumes during the welding.

Silicon has a beneficial effect on the weldability of copper-silicon alloys because of its deoxidizing and fluxing actions.

Oxygen can cause porosity and reduce the strength of welds made in certain copper alloys that do not contain sufficient quantities of phosphorus or other deoxidizers. Oxygen may be found as a free gas or as cuprous oxide. Most commonly welded copper alloys contain deoxidizing element, usually phosphorus, silicon, aluminum, iron, or manganese.


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Free-Machining Additives. Lead, selenium, tellurium and sulfur are added to copper alloys to improve machinability. Bismuth is beginning to be used for this purpose as well when lead-free alloys are desired. These minor alloying agents, while improving machinability, significantly affect the weldability of copper alloys by rendering the alloys hot-crack susceptible. The adverse effect on weldability begins to be evident at about 0.05% of the additive and is more severe with larger concentrations. Lead is the most harmful of the alloying agents with respect to hot-crack susceptibility. Other Factors

Effect of Thermal Conductivity. The behavior of copper and copper alloys during welding is strongly influenced by the thermal conductivity of the alloy. When welding commercial coppers and lightly alloyed copper materials with high thermal conductivities, the type of current and shielding gas must be selected to provide maximum heat input to the joint. This high heat input counteracts the rapid head dissipation away from the localized weld zone. Depending on section thickness, preheating may be required for copper alloys with lower thermal conductivities. The interpass temperature should be the same as for preheating. Copper alloys are not


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post-weld head treated as frequently as steels, but some alloys may require controlled cooling rates to minimize residual stresses and hot shortness.

Welding Position. Due to the highly fluid nature of copper and its alloys, the flat position is used whenever possible for welding. The horizontal position is used in some fillet welding of comer joints and T-joints.

Precipitation-Hardenable Alloys. The most important precipitation-hardening reactions are obtained with beryllium, chromium, boron, nickel, silicon, and zirconium. Care must be taken when welding precipitation-hardenable copper alloys to avoid oxidation and incomplete fusion.Whenever possible, the components should be welded in the annealed condition, and then the weldment should be given a precipitation-hardening heat treatment.

Hot Cracking. Copper alloys, such as copper-tin and copper-nickel, are susceptible to hot cracking at solidification temperatures. This characteristic is exhibited in all copper alloys with a wide liquidus-to-solidus temperature range. Severe shrinkage stresses produce interdendritic separation during metal solidification. Hot cracking can be minimized by reducing restraint during welding, preheating to slow the cooling rate and reduce the magnitude of welding stresses, and reducing the size of the root opening and increasing the size of the root pass.


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Porosity. Certain elements (for example, zinc, cadmium, and phosphorus) have low boiling points. Vaporization of these elements during welding may result in porosity. When welding copper alloys containing these elements, porosity can be minimized by higher weld speeds and a filler metal low in these elements.

Surface Condition. Grease and oxide on work surfaces should be removed before welding. Wire brushing or bright dipping can be used. Miliscale on the surfaces of aluminum bronzes and silicon bronzes is removed for a distance from the weld region of at least 13 mm, usually by mechanical means. Grease, paint, crayon marks, shop dirt, and similar contaminants on copper-nickel alloys may cause embrittlement and should be removed before welding. Miliscale on copper-nickel alloys must be removed by grinding or pickling; wire brushing is not effective.

3.2.3. Beryllium

Beryllium is expensive, brittle, reactive and toxic. Its production is quite complicated

and hence the applications of Be alloys are very limited. Beryllium oxide (BeO) which is also toxic in a powder form, is used to make high-thermal conductivity ceramics.


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Its main characteristics of importance to machining are its toxicity and brittleness due to its hexagonal close packed structure. Machining dust of beryllium is extremely harmful if ingested into the lungs and special ventilation and clothing procedures must be followed. The brittleness of beryllium gives rise to discontinuous cutting and hence a tendency for chatter. There is also a tendency for sub-surface cracks to develop in machined beryllium surfaces which may be highly undesirable for certain space applications.

Extruding the beryllium that is a brittle material can not be done by conventional extrusion, but can be done by hydrostatic extrusion.

Beryllium – Bronze (copper and beryllium), containing up to about 2% beryllium, is easily formed in the annealed condition. It has a high tensile strength and fatigue strength in the hardened condition.

3.2.4. Magnesium and Alloys:


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a. Castability: Most common usage of magnesium is for die casting. In the die casting process, magnesium is able to be cast up to four times faster than aluminium and die lives are considerably longer than the aluminium alloys. Combined with the excellent castability of the common magnesium-aluminium alloys, large structural components can be successfully cast. This has resulted in significant production cost and weight savings in some instances. Generally, magnesium alloys that are normally sand cast are also suitable for permanent mold casting. Magnesium has the best strength-to-weight ratio for die cast products. Also magnesium alloys have high electrical and thermal conductivities , high impact resistance and good damping capacity. The best operating temperature range is from 300 F to 600 F. When magnesium becomes molten, it tends to react with oxygen and explode, unless care is taken to protect the molten metal surface against oxidation. This can be done with sulfur dioxide ( SO2 ). Magnesium is not very fluid just above its melting point , and so casting should be done above the melting point and also this causes danger of burning. Small amount of beryllium can prevent this effect.


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b. Formability Extrusion : Magnesium rods , bars , tubing can be produced in many sizes and shapes by the extrusion process. Suitable pressures for extruding magnesium are ; from 50 000 psi to 100 000 psi. In extrusion process temperature is very important. At low temperatures high strength , ductility and fine grain size can be obtained. When the temperature increases grain sizes increase. Rolling : Because of its close-packed hexagonal structure , it is hard to roll magnesium. Two different types of practices are used for rolling magnesium ; Cold Rolling ( 250 F or less ) : Because of the cold rolls , lubrications are not needed. Cold rolling of a material is generally done with four-high rolling mills because of the importance of uniformity of gauge. Hot Rolling (600 F) : In hot rolling much greater amount of deformation can be taken than cold rolling. The maximum hot rolling temperature is relatively lower than the other metals.

c. Machinability It is the fastest and easiest machining metal. Normal speeds for machining magnesium may reach up to ten times those for steel and twice those for aluminium


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alloys. Because of these free cutting qualities and the high thermal conductivity of the metal, the low cutting pressure, machining speeds increase and this reduces machining power need. Other advantages of magnesium alloys in machining are excellent surface finish and good tool life. Magnesium alloys are very flammable so great care should be taken. To avoid this potential hazard, cutting should be done carefully. Add to this large chips are less likely to burn. Magnesium should be machined dry. If a coolant is needed mineral oil or compressed air can be used.

d. Welding

There are a lot of arc and resistance welding ways for magnesium alloys. The strength of a weld joint is lower compared to the base metal as a result of recrystalization and grain growth in the heat-affected zone (HAZ). This effect can be prevented with gas metal arc welding because of the higher welding speeds. The gas tungsten arc welding process and the gas metal arc welding process are the recommended processes for joining magnesium. Gas tungsten arc is the best way for thin materials and gas metal arc is the best for thick materials. The choice of electrode wire or filler metal should be done according to the composition of the base metal.


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Other resistance welding processes which can be used for welding magnesium are ;

spot welding, seam welding, and flash welding.

Magnesium is a very active metal and the rate of oxidation increases as the temperature is increased. The melting point of magnesium is very close to that of aluminium, but the melting point of the oxide is very high. Because of this, the oxygen in the welding area should be removed.


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3.2.5. Nickel and Its Alloys

Commercially pure nickels and extra high nickel alloys: Nickel 200 and 201 are

primary wrought. Both of them contain 99.5% Ni. The cast grade, designated CZ-100, is recommended for use at temperatures above 600°F because its lower carbon content prevents graphitization and attendant ductility loss.

Nickel alloys generally have better high temperature strength than alloy steels. The principal alloying elements are chromium and cobalt; lesser elements include aluminum, titanium, molybdenum, niobium (Nb), and iron. Commercial nickel and nickel alloys are available in a wide range of wrought and cast grades; Monel, Hastelloy, Inconel, Incoloy are the most common wrought nickel alloys. However, considerably fewer casting grades are available. Nickel alloys have a high melting point and are not easy to cast. The casting alloys contain additional elements, such as silicon and manganese, to improve castability and pressure tightness.

Nickel and nickel-based alloys can be machined by the same techniques used for iron-based alloys.

Manufacturability:


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Most wrought-nickel alloys can be hot and cold worked, machined, and welded successfully. The casting nickel alloys can be machined or ground, welded and brazed.

Nearly any shape can be forged in nickel and nickel alloys. However, because nickel work hardens easily, severe cold-forming operations require intermediate soft annealing. Annealed cold-rolled sheet is best for spinning and other manual work. In general, cold-drawn rods have a better machinability than hot-rolled or annealed material.

With using shielded metal-arc, gas tungsten-arc, gas metal-arc, plasma-arc, electron-beam, oxyacetylene, and resistance welding, nickel alloys can be welded. Resistance welding methods include spot, seam, projection, and flash welding. Clearliness is the single most important requirement for successful welded joints in nickel alloys.

Nickel superalloys, are best worked at about 1,800 to 2,200°F. In the annealed condition, these alloys can be cold worked by all standard methods. These alloys work harden to a greater extent than the austenitic stainless steels, so they require more intermediate annealing steps.


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For developing optimum ductility and to minimizing distortion during subsequent machining both cold-worked and hot-worked Ni-Cu are heat treated. Stress relieving before machining is recommended to minimize distortion after metal removal. Stress equalizing of cold-worked Cu-Ni increases yield strength without marked effects on other properties.


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3.2.6. TITANIUM AND ALLOYS

Titanium is the most valuable and useful material for aeronautic industry, space industry and recently automobile industry. Structural titanium alloys are coming in for increased use because they are light, ductile and have good fatigue and corrosion-resistance properties. Titanium alloys resist corrosion better than the best stainless steels. Titanium alloys have high melting points it’s melting point is 1668 C degrees. Although, titanium is hard to manufacture, it is manufactured in any processes such as casting, forming, machining, joining, powder methods, heat treatment, finishing and special methods etc…

a. Castability

Casting is the first step in manufacturing. The difficulties of casting titanium stem from inherent characteristics, such as its high chemical reactivity, high melting point and the flow properties of the molten metal. As in ingot production, the molten metal and, in turn, the hot casting are sensible to atmospheric implication


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A second difficulty, one which is special to titanium, is in the maintenance of good flow over severe changes of dimension or direction within the mold.

b. Formability

Titanium and its alloys can be cold and hot formed on standard equipment using techniques similar to those of stainless steels. However, titanium possesses certain unique characteristics that affect formability, and these must be considered when undertaking titanium forming operations. The room temperature ductility of titanium and its alloys are generally less than that of the common structural metals including stainless steels.

Titanium has a relatively low modulus of elasticity, about half that of stainless steel. This results in greater springback during forming and requires compensation either during bending or in post-bend treatment. Heating titanium increases its formability, reduces springback, and permits maximum deformation with minimum annealing between forming operations.

The lower strength, more ductile titanium alloys can be roll-formed cold as sheet strip to produce long lengths of shaped products, including welded tubing and pipe. The


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sheet blank is often preheated as is the tooling. The softer, highly ductile grades of unalloyed titanium are often cold pressed or stamped in sheet strip.

c. Machinability Titanium is difficult to machine. Titanium presents a unique set of machining

problems. Grinding

In grinding, the difference between titanium and other metals is the activity of titanium at high temperatures. At the localized points of wheel contact titanium can react chemically with the wheel material. The most important facts to consider in order to prevent this and ensure successful grinding are:

1. Effective use of coolants

2. Correct wheel speeds.

3. Selection of proper wheels


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The low rigidity of titanium alloys makes them especially difficult to machine.Under cutting pressures, the "springy" materials move away from the tool.

d. Weldability

Titanium and most titanium alloys are readily weldable, using several welding processes. Properly made welds in the welded condition are ductile and, in most environments, are as corrosion resistant as base metal. Improper welds, on the other hand, might be embrittled and less corrosion-resistant compared to base metal.

The techniques and equipment used in welding titanium are similar to those required for other high-performance materials, such as stainless steels. Titanium, however, demands greater attention to cleanliness and to the use of auxiliary inert gas shielding than these materials.

e. Heat Treatment:


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Heat treatment of titanium fabrications is not normally necessary. Annealing may be necessary following severe cold work, if restoration of ductility or improved machinability are desired. A stress relief treatment is sometimes employed following severe forming or welding to avoid cracking or distortion due to high residual stresses, or to improve fatigue resistance. Cleanliness of titanium parts to be heat treated is important because of the sensitivity of titanium to contamination at increasing temperatures.

Although no special furnace equipment or protective atmosphere is required for titanium, a slightly oxidizing atmosphere is recommended to prevent pickup of hydrogen.

3.2.7 Cobalt Alloys

The main elements in these alloys are cobalt (around 40%) and chromium (perhaps 20%); other alloying elements include nickel, molybdenum, and tungsten. As a group, the cobalt-based alloys may be generally described as wear resistant, corrosion resistant, and heat resistant. Many of the properties of the alloys arise from the crystallographic nature of cobalt, of chromium, tungsten, and molybdenum, the


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formation of metal carbides, and the corrosion resistance imparted by chromium. Generally the softer and tougher compositions are used for high-temperature applications such as gas-turbine vanes and buckets. The harder grades are used for resistance to wear.

Rate of cooling and the temperature pouring are the properties that inform us about the solidification of carbide distribution. Forming of carbides usually occur when they are exposed to operating temperatures for a long time because cast alloy are not treated often. But hot work can be applied for wrought materials. When a heat treatment is applied between 1175-1230°C, material can be strengthened more.

Cobalt alloys are generally used at high temperatures. But tensile strength and ductility can affect how much hot or cold working the material can withstand and hardness influences machinability.

Also the effect of thermal history over the properties in room temperature, like elongation, should not be underestimated. For example when more precipitation exists, the ductility of the material decreases because of being inversely proportional. And the hardness of higher carbon alloys goes up in a faster way, while being exposed to high temperatures, when they are compared with lower carbon containing alloys.


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The castability of all the airmelt cobalt alloys is considered good to excellent, especially compared with nickel alloys and stainless steels. The compositions are characterized by high carbon contents with appreciable silicon, both providing fluidity. The alloys possess good hot strength (which is often the basis for their use in the first place) and are quite resistant to hot tearing. Some cracking may be experienced in the high carbon wear resistant alloys when rough treatment or severely restrictive gating arrangements are encountered.

3.2.8 Refractory Metals

As a group they provide a number of unique characteristics – such as resistance to

high heat, corrosion and wear – making them useful in a multitude of applications.

Excellent strength at high temperatures

Very high melting point (tungsten is more than double that iron and ten times that of lead) (2900C)

Exceptional resistance to corrosion

Excellent wear and abrasion resistance


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High resistance to thermal shock

Good electrical and heat conducting properties

Hardness

High specific density (13,6 g/cc) Main Refractory Metals 1 – Molybdenum 2 – Niobium 3 – Tantalum 4 – Tungsten Molybdenum is similar to Tantalum ; Niobium is similar to Tungsten

a. Castability Because of the high melting temperature and ease of oxidation, powder metallury is

preferred. (Tungsten: Melting Point: 3410C, Sintering: 1800C) In addition to this, other less common methods are: vacuum arc melting; electron

beam melting; zone melting and refining; slip casting and sintering; gas pressure bonding; and plasma spraying as fabrication progresses.


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b. Formability The cold working properties of niobium and tantalum are excellent. Because of their

bcc crystal structure, they are very ductile metals that can undergo cold reductions of more than 95% without failure. These metals can be easily forged, rolled or swaged directly from ingot at room temperature.

Tungsten should be fabricated well above its transition temperature. Failure to follow this rule will generally result in cracked or laminated parts.

Forming may be accomplished with the alloy in the annealed condition. The alloy has good ductility in the annealed condition and may be cold formed by conventional means

Bars or melted ingots which are fabricated by the powder method are extremely ductile. They can be cold rolled into sheet and foil and can be formed and drawn into many shapes.

It is important to care the ductile – brittle transition temperature of molybdenum and tungsten before the forming.

c. Machining:


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In general, refractory metals can be machined as well as the medium hard cast iron or stainless steel.

Due to they are abrasives and hard; cutting tools should be more abrasive and sharp and cutting speed should be higher than machining of steels. If the cutting speed is not enough, the tool life reduces.

Also coolants and lubricants are important and recommended for this process.

d. Weldability

Tantalum and niobium are easily weldable refractory metals without the brittleness problem encountered when welding tungsten and molybdenum. (However acetylene torch welding is destructive to tantalum.) Surfaces that are heated above 315C during welding must be protected with an inert gas or vacuum to prevent embrittlement.

Molybdenum parts can be welded by inertia, resistance, and spot methods in air; by TIG and MIG welding under inert atmospheres; and by electron beam welding in vacuum. The best welds are produced by inertia (friction) welding and electron-beam welding; welds produced by the other techniques are less ductile


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Tungsten can be welded to itself. However, the resulting weldment is always recrystallized and hence brittle. If tungsten parts which are not to be exposed to very high temperatures are to be joined, brazing is preferred to welding. As long as the braze material has a melting point below the recrystallization temperature of tungsten (under 1.200C), embrittlement can be avoided.

To reduce the tendency of grain growth in the HAZ and fusion zones, a high energy density welding process, such as EBW, is often more advantageous than GTAW.

Tungsten's high melting point and low vapor pressure make it very useful for welding electrodes. Helium or argon prevent oxidation of the electrode in gas tungsten arc, welding, as well as protecting the metal being welded.

e. Heat Treatment: Not heat treatable for hardness. However it can be annealed at 1.093C to 1.315C

and a stress relief anneal may be done at 870C for one hour. Welds in molybdenum and tungsten are brittle (<50% joint efficiency), and thus

these metals are difficult to join. Before welding, molybdenum and tungsten must be preheated above their ductile to brittle transition temperatures to prevent fracture.


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Heavy sections of molybdenum should be preheated and postheated when welded to reduce thermal stresses.

Tantalum and niobium alloys generally retain greater than 75% joint efficiency after gas tungsten arc welding. Preheating is not required, but postweld annealing can restore large amounts of ductility and toughness to commercial alloys.

Carbide Tool Machinability

Ratings


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Workpiece Material

Hardness Surface Speed (ft/min)

Cut Depth (in.)

Feed (in/rev)

Type Life (in3)

Removal Rate

(in3/min)

Relative Removal

Rate

Relative Removal

Cost

Steel

4130 200 BHN 445 0.12 0.019 C6 582 11.50 100.0 1

4130 54RC 90 0.12 0.004 C6 19 0.62 5.4 19

Superalloys

Rene 41 320 BHN 70 0.06 0.009 C2 23 0.47 4.1 25

Rene 41 365 BHN 70 0.06 0.009 C2 16 0.47 4.1 25

Refractory Metals


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TZM 217 BHN 350 0.06 0.009 C2 99 2.30 20.0 5

Niobium 112 BHN 300 0.12 0.005 C2 151 2.20 19.0 6

Unalloyed, Wrought Molybdenum

223 BHN 275 0.10 0.010 C1 132 3.30 29.0 4


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