NOVEL PROCESS DEVELOPMENT WITH CONTINUOUSS CASTING …

Green Technologies for Materials Manufacturing and Processing

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NOVEL PROCESS DEVELOPMENT WITH CONTINUOUSS CASTING AND PRECISE FORGING FOR AL-SI ALLOYS TO PRODUCE AN ENGINE PISTON O. Umezawa1, H. Takagi2, T. Sekiguchi3, T. Yamashita2, and N. Miyamoto3 1 Dept. of Mater. Sci. & Eng., Yokohama National University, Yokohama, 240-8501, Japan 2 Toyama Alloy Co., Imizu, Toyama, 934-8515, Japan 3 Miyamoto Industry Co., Shiotani, Tochigi, 329-2442, Japan ABSTRACT

A new vertical semi-continuous casting process using an adiabatic and rapid cooling mold has been developed to produce a billet with smooth surface for eutectic or hyper-eutectic Al-Si-Cu-Mg-(Ni, Fe, Mn) alloys. Without peeling and hot-extrusion processes the cast billet with 83 mm in diameter has been successfully applied to warm forging. Although primary Si crystals, intermetallic compounds and α dendrite structure were mostly refined under high cooling condition in the casting, coalesced Si crystals or compounds were still remained in the center part. A working and annealing step has been installed due to increase formability of the cast with a modification of its microstructure. An isothermal forging process with greener lubricant and precise mold technologies has been developed and a prototype of motorbike engine piston was produced. The forged alloys showed higher fatigue strength and creep resistance than conventional materials at high temperatures. This process is promised to provide an up-grade solution to the recycle of secondary aluminum alloy. INTRODUCTION

The products generally consist of many kinds of materials. On the other hand, it has been required to save resources and energy, and to recycle materials. Engineering subjects for the ideal recycling system in metals are to reduce impurity content from scrap melt, to immunize metals against impurities or make metals innocuous from impurities, and finally to replace metals with inherently recyclable metals. The concept of ecomaterials (environmental conscious materials) has been proposed in Japan since 1992.1 The consideration of environmental issues for all materials was deemed new and significant. The ecomaterials are associated with; minimal health hazards, minimal harmful emissions and wastes, minimal energy requirement, maximal recyclability and minimal material resource depletion, optimal physical properties and best technical performance. Traditional metals are pressed to meet new performance targets, with higher quality, at lower cost, with environmentally benign process and with globally source. Such paradigm shift in the metals industry has proceeded.2 Through discussion in the disassembly, recovery and waste treatment stages, furthermore, it is concluded that the developments of product design for easy disassembly and of easier recyclable material design are needed. That means that novel process technologies building high performance and/or service with low environmental burden are necessary. Those challenges facing ecomaterials may make an important role to develop the global eco-society. In particular, the secondary materials accepted for recycling should be considered to immunize their impurities by a microstructural control, since no disassemble operation fully renews them.

In the case of aluminum, the cascade of material flow is suitable for recycling, because producing aluminum from sources such as bauxite consumes large amount of energy. The application of secondary (scrap) metals, however, has been almost limited to cast materials, and

Environmental Issues and Waste Management Technologies in the Materials and Nuclear Industries XII: Ceramic Transactions Vol. 207, eds. A. Cozzi, T. Ohji, Wiley, NY, 2009, pp. 189-200


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the dilution by raw material has been inevitable. The reason why Si and Fe elements are problem in aluminum alloy recycle stage is to lead to very poor workability of aluminum production, and difficult to remove them, although Si, Fe and Cu are major detrimental elements in recycling aluminum products. Al-Si-Cu-Mg alloys are one of the major cast materials, and include almost no dislike elements for the ecomaterial. In addition, removal of Si from scrapped aluminum products is difficult and higher costly as well as that of Fe. Al-Si system having plural phases with low mutual solid solubility are effectively the in-situ metal-metal composites which mean one of simplified recycle models and give an adequate balance with properties such as strength, elongation, lightweight, good wear resistance and low thermal expansion. To control balanced properties without detrimental elements for the ecomaterials, fine microstructure with plural phases, mesocomplex structure, is one of candidates for the alloy design.3-4

Therefore, novel processes to refine microstructure of Al-Si-Cu-Mg-(Ni, Fe, Mn) alloys for high performance forging products have been developed and mechanical properties of the treated materials were characterized. PROCESSES TO DEVELOP FINE MICROSTRUCTURE

To improve mechanical properties of Al-Si-X cast materials, microstructural modifications have been commonly achieved by the addition of elements such as Sr and P into the melt, or by the hot-forging and long-time solution heat-treatment. Nevertheless, the ductility of the treated materials is not enough, and they can hardly be applied to wrought materials. Coarse Si crystals and/or coarse intermetallic compounds cause poor ductility and often give an origin of fatigue crack initiation site as well as inclusion. To improve both workability and mechanical properties, thus, the Si crystals and compounds must be refined to avoid the sample fracture due to their cracking. The most promising method of refining microstructures is rapid solidification. Spray-forming and thixoforming have been used for refining the Si in Al-Si alloys. However, those methods are not amenable to mass production, so the present work deals with a novel semi-continuous casting and thermomechanical treatment for Al-Si-X cast alloys.

Direct chill (DC) casting process has been widely adopted to produce aluminum cast billets. However, the billets need to peel their surface followed by hot-extrusion. Yamashita et al.5 has developed an adiabatic graphite mold casting (HIM) process to produce A6000 series alloys billets with a smooth surface. The cast billets directly provide forging materials and eliminate peeling and hot-extrusion process. Furthermore direct cooling of billets results in avoidance of segregation and improvement of mechanical properties. Therefore, it reduces the cost of the forging aluminum materials approximately 20% in the case of automotive suspension arms, and lightens them. Thus the direct cooling process with adiabatic mold was chosen to apply the Al-Si billet casting.

However, the heavy cold-work cannot be applied to the Al-Si alloy casts, because it causes severe cracking in the primary Si crystals and coarse intermetallic compounds. Generally, hot-working refers to deformation carried out under conditions of temperature and strain rate. Since recovery processes occur substantially during the deformation process, large strains can be achieved with essentially no strain hardening. It results in a decrease in the energy required to deform the metal and an increased ability to flow without cracking. Thus the higher working temperature is, the less cracked Si crystals are detected. Severe cracking in Si crystals occurs under cold-working operations. The cracks may lead to sample fracture, but be useful in refining the Si crystals. In order to avoid fracture, the cold-working operations must be carried out steps with intermediate annealing. Umezawa et al.3-4 have proposed a repeated thermomechanical


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treatment (RTMT) to produce a heavily deformable hyper-eutectic Al-Si-X alloys. Fragmentation of Si crystals and compounds and their dispersion in the matrix were achieved through repeated cold-working and annealing steps. This concept of microstructual modification was placed in the prior treatment step of isothermal forging instead of hot-extrusion. SEMI-CONTINUOUS CAST BILLET WITH SMOOTH SURFACE Development of Casting Technology

A new vertical semi-continuous casting process equipped with an adiabatic and rapid cooling mold (HI-RCM) has been developed to produce billets of 83 mm in diameter.6 Figure 1 shows the schematic illustration of new casting equipment. Direct water-cooling at lower edge of graphite mold is adopted to increase cooling rate in solidification, although no cooling of the mold was done in the HIM process.

(a) (b)

Figure 1. Schematic illustration of vertical semi-continuous casting equipment (a) and magnified image of adiabatic and rapid cooling mold (b).

Influence of casting conditions on surface roughness of the billet was examined. The

higher casting rate to pull down the billet was, the lower roughness was. But there was an upper-limit of casting rate to keep molten metal in the mold as shown in Figure 2. At higher flow rate of cooling water, smooth surface of the billet was available in lower casting rate. Figure 3 represents the outlook of A4032 (Al-12.7Si-0.9Cu-1.0Mg-0.5Fe-0.9Ni in mass%) alloy billets produced by semi-continuous casting methods. Surface roughness of DC billet (204 mm in diameter) was about 1 mm. In the HI-RCM process it was decreased less than 1.6 µm in average at the casting rate of 180 mm/min.6

Figure 4 shows the longitudinal section of A4032 HI-RCM billets with different casting rate. Chill layer with periodical triangle structure appears in the slow cooling condition and results in lapping surface shown in Figure 3(c). The triangle structure was neither a cold-shut in the DC casting nor a ripple in the hot-top casting.7-8 It may be related with a segregation in partial slow cooling zone or re-melted zone in the mold. The increase of casting rate also makes liquid-solid interface lowered in the center part as shown in Figure 5.


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Figure 2. Influence of casting conditions on billet surface for A4032

under the casting temperature of 973 K.

Figure 3. Outlook of A4032 billets produced by the conditions of (a) DC: 119 L/min, 95 mm/min,

(b) HIM: 25 L/min, 100 mm/min, (c) HI-RCM: 25 L/min, 100 mm/min and (d) HI-RCM: 25 L/min, 180 mm/min.

Figure 4. Macrostructure of the A4032 HI-RCM billets with casting rate of

(a) 100 mm/min and (b) 180 mm/min.


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Figure 5. Liquid-solid interface in A4032 HI-RCM billets with casting rate of

(a) 100 mm/min and (b) 180 mm/min. Test Alloys and Their Microstructure

Seven kinds of eutectic or hyper-eutectic Al-Si-Cu-Mg-(Ni, Fe, Mn) alloys listed in Table I were chosen to provide test billets to produce a prototype of motorbike engine piston by forging. In order to increase high temperature strength the alloys contain several percent of transition metals such as Cu, Ni, Fe and Mn.

Figure 6 shows microstructure near surface of the billets. Fine primary Si (about 10 µm in diameter) and eutectic phases are dispersed. Secondary dendrite arm spacing (DAS) was several µm, which was much lower than that of DC cast. On the other hand, coalesced Si crystals and/or compounds were observed in the center part of the billets for hyper-eutectic alloys as shown in Figure 7. In particular coarse AlSiFeMn compounds appear in UTM404 and 405 alloys (Figs. 7(c) and (d)). In order to avoid them total amount of Fe and Mn should be less than 1 mass%. DAS was about 15 µm in the center part. Fine rod-like AlSiCuNiFe compounds as well as eutectic Si were dispersed in the matrix. No remarkable segregation was detected in all billets. For higher Si alloy e.g. Al-17Si-3Cu-1Mg-1Ni, furthermore, fine structure was obtained with casting temperature above 1023 K.9 Therefore, new HI-RCM vertical semi-continuous casting process is suitable to provide the billets for forging. Table I. The chemical compositions of test alloys (mass%) Alloys Si Fe Cu Mn Mg Cr Ni Zr Ti P Ca Al UTM401 14.0 0.15 3.0 - 1.0 - 1.0 0.1 0.01 0.012 - Bal. UTM402 12.0 0.15 3.0 - 1.0 - 1.0 0.1 0.01 0.012 - Bal. UTM403 12.0 0.15 3.0 - 1.0 0.1 1.0 0.1 0.01 0.012 - Bal. UTM404 14.0 1.0 3.0 1.0 1.0 - - 0.1 0.01 0.012 - Bal. UTM405 12.0 1.0 3.0 1.0 1.0 - - 0.1 0.01 0.012 - Bal. UTM406 12.0 0.6 3.0 0.4 1.0 0.1 1.0 0.1 0.01 0.012 - Bal. UTM407 12.0 0.6 3.0 0.4 1.0 0.1 1.0 0.1 0.01 - 0.012 Bal.


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Figure 5. Solidification structure near surface in the longitudinal section for the billets of (a)UTM401, (b)UTM402, (c)UTM404 and (d)UTM405.

Figure 6. Solidification structure at the center part in the longitudinal section

for the billets of (a)UTM401, (b)UTM402, (c)UTM404 and (d)UTM405.


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Fatigue Strength and Crack Generation Load-controlling fatigue tests for the billets of UTM401-405 alloys were carried out

with stress ratio, R (σmin/σmax) = 0.01, in a sine wave at 296 K and 523 K. Influence of primary Si crystals and/or compounds on fatigue strength were examined. There was no big difference of tensile strength by the position in the billets and the alloys. For the hyper-eutectic alloys, however, distribution of primary Si crystals and/or compounds strongly affected to the fatigue strength as shown in Figure 7.

Fatigue crack initiation sites and fracture surfaces were analyzed by scanning electron microscopy. The crack length, crack width and distance from specimen surface to center of ellipse were determined, where the direction of crack length was parallel to the initial crack propagating direction. Maximum fatigue crack size in Stage II was taken from the ripple mark on fracture surface. Thus both initial and maximum crack sizes were experimentally given. A linear fracture mechanics program, SCAN,10 was adopted to evaluate the ΔK of a subsurface crack. The calculated stress intensity range resulted in the modeling of fatigue crack growth by integral of the Paris equation as follows:

€

dadN

= C(ΔK)m (1)

where C and m are constants. The constants in the Eq.(1) were chosen as C=8.08x10-13 and m=5.03. Those were resulted from crack propagating tests of AC8A cast alloy in the reference.11 Then the crack propagating life, Np, was estimated by the crack growth modeling. The number of cycles to failure, Nf, was obtained in the experimental. Based on the estimation coalesced compounds gave the fatigue crack initiation site as indicated by arrow in Figure 8. In order to improve fatigue strength of the alloys, further dispersion of Si crystals and compounds by forging process is required.

Figure 7. S-N data of (a) UTM401 billet and (b) UTM404 one at 296 K (R=0.01).


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Figure 8. Fatigue crack initiation sites in UTM404: at 296 K, σmax=139 MPa, Nf=981,000 cycles.

Arrow shows coalesced second phases giving the crack initiation site. A half ellipse indicates initial crack for the calculation.

PRECISE FORGING AND PROTO-TYPE PRODUCT Formability of Cast Alloys Deformation resistance at the temperature between 293 K and 693 K of the alloys casts was examined by direct extrusion, indirect extrusion and upset forging. Working rate was from 30% to 70% in reduction of area, respectively. Their formability was good to apply isothermal forging at above 600 K, while the deformation resistance was higher than that of A4032, especially at higher temperature.

Microstructural modification by RTMT was effective to improve tensile properties as mentioned in above.3-4 To increase formability, drawing-annealing step was applied to the cast alloys. The working from 10 to 70% in reduction was employed and T7 heat-treated at 773 K for 3600 sec followed by water quenched and aged at 483 K for 18000 sec. As shown in Figure 9 the treated material over 20% reduction in Al-11.2Si-2.2Cu-0.27Mg-0.25Fe alloy exhibits a significantly enhanced ductility and strength as well as RTMT material.

Figure 9. Effect of drawing reduction on the tensile properties of

Al-11.2Si-2.2Cu-0.27Mg-0.25Fe alloy.


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Precise Forging

The isothermal forging process with greener lubricant and precise mold technologies was developed. Test pieces of the alloys billets were solution treated at 783 K and peened their surface. Then greener lubricant which was water-soluble graphite coating on chemical oxidation of the test pieces was adopted. The isothermal forging with 400 ton press was done at sample temperatures of 633 K, 673 K and 693 K, respectively. The temperature of forging die was 543 K. All alloys were successfully forged into a motorbike engine piston to precise measure as shown in Figure 10. No defects were detected in the cross section of piston, and plastic flow indicated good formability. The product has merits of much less machining and lighter weight, since it gives a shape with fair accuracy. Further the combination of drawing-annealing step and isothermal forging on the alloy billets has an advantage to the manufacturing of high performance motorbike engine piston with lower cost and environmental load.

Figure 10. Prototype of motorbike engine piston (a) and macrostructure

in the section B-B of UTM401 forged at 693 K (b). Mechanical Properties of Forged Parts

Specimens were machined from the top part of forged pistons for all alloys as shown in Figure 11. The top part was controlled to keep its thickness for the specimen. The specimens were heat treated of T7 at 778 K for 4800 sec followed by water quenched and aged at 483 K for 21600 sec.

The test alloys showed higher strength such as tensile strength, high-cycle fatigue, creep and thermal fatigue than A4032 forged material and cast one at high temperature. Figure 12 shows S-N data at 523 K and 573 K of UTM407 compared with A4032 forged material and Al-12.7Si5Cu-1Mg-1Ni-0.15Ti (mass%) alloy cast. UTM407 showed the highest 107 cycles fatigue strength among test materials.

Through experiments mentioned in above the alloy, Al-(11~13)Si-2.5Cu-1Mg-1Ni-0.5Fe-0.1Mn-0.1Ti-0.1Zr (in mass%), was chosen as the candidate material of a motorbike engine piston. The combination of developed new semi-continuous casting process and precise forging process is promised to provide an up-grade solution to the recycle of secondary aluminum alloy.


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Figure 11. Schematic illustration of test specimen taken from forging parts.

Figure 12. S-N data of the forged UTM407 at 523 K (a) and 573 K (b).


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CONCLUSIONS Novel processes have been successful in achieving microstructural refinement of Al-Si-Cu-Mg-(Ni, Fe, Mn) cast materials and producing a prototype of forged engine piston. Major results were as follows: (1) A new vertical semi-continuous casting process was adopted an adiabatic and rapid cooling

mold to produce a billet with smooth surface. Without peeling and hot-extrusion processes the cast billet was applied to warm forging.

(2) Coalesced Si crystals or compounds were still remained in the center part of the billets for hyper-eutectic alloys. Those gave a fatigue crack initiation site and decreased high-cycle fatigue strength.

(3) The isothermal forging process with greener lubricant and precise mold technologies was developed. All test alloys were applicable to the forging. The test alloys showed higher strength such as high-cycle fatigue and creep than A4032 forged material and cast one at high temperature.

(4) A combination of drawing-annealing step and isothermal warm-forging on the Al-Si alloys billets has an advantage to the manufacturing of high performance motorbike engine piston with lower cost and environmental load. The product has merits of much less machining and lighter weight, since it gives a shape with fair accuracy.

(5) The alloy, Al-(11~13)Si-2.5Cu-1Mg-1Ni-0.5Fe-0.1Mn-0.1Ti-0.1Zr (in mass%), was chosen as the candidate material of a motorbike engine piston. This process is promised to provide an up-grade solution to the recycle of secondary aluminum alloy.

ACKNOWLEDGMENTS This study has carried out by the project “Novel process and product development with precise forging of Al-Si hyper-eutectic alloys towards high performance and low environmental load” in the regional research and development resources utilization program. Financial support from Japan Science and Technology Agency is gratefully acknowledged. REFERENCES 1K. Halada, Ecomaterial –New Step of Material Towards the 21st Century-, Bulletin Jpn. Inst. Metals, 31, 505-511 (1992). 2O. Umezawa, K. Halada and K. Shinohara, Ecomaterials in the Global Eco-society: Present Situation and Future Prospects, Materials Science Forum, 555, 1-7 (2007). 3O. Umezawa and K. Nagai, Microstructural Refinement of As Cast Al-12.6wt%Si Alloy by Repeated Thermomechanical Treatment to Produce a Heavily Deformable Material, Metall. Mater. Trans. A, 30A, 2221-2228 (1999). 4O. Umezawa, Mechanical Properties of Thermomechanical Treated Hyper-eutectic Al-Si-(Fe,Mn,Cu) Materials, Materials Transactions, 46, 2616-2623 (2005). 5T. Yamashita, T. Watanabe, Y. Kondou, H. Anada, S. Ikeno, S. Tada and K. Nakahira, Solidification Position and Surface Condition in Aluminum Billets Produced by a Vertical Semi-continuous Casting Process Using a Heat Insulating Mold, J. Jpn. Inst. Light Metals, 46, p 494-499 (1996). 6H. Takagi, M. Dohi, Y. Uetani, T. Watanabe, T. Yamashita and S. Ikeno, Microstructure of Eutectic Al-Si Alloy Billet Produced by Continuous Casting Process Used Heat Insulating and


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Rapid Cooling Mold, Proc. 11th Inter. Conf. on Aluminum Alloys, WILEY-VCH Verlag, in the press. 7R. Mitamura, Continuous Casting of Aluminum, J. Jpn. Inst. Light Metals, 30, 227-236 (1980). 8M. Matsuo, Continuous Casting of Aluminum, J. Jpn. Inst. Light Metals, 44, 510-525 (1994). 9H. Takagi, T. Watanabe, M. Dohi, T. Yamashita, T. Kawabata, K. Matsuda, S. Ikeno, Characteristics of Hypereutectic Al-Si Alloy Billet Fabricated by Continuous Casting Process Used a Heat Insulating and Rapid Cooling Mold, Proc.114th Conference of Jpn. Inst. of Light Metals, 233-234 (2008). 10M. Shiratori, Assessment of Residual Fatigue Lives for Surface-Cracked Structures by an Influence Function Method, Current Topics in Computational Mechanics, eds. J.F. Cory, Jr. and J.L. Gordon, ASME, 357-364 (1995). 11T. Kobayashi, Strength and Fracture of Aluminum Alloys, J. Jpn. Inst. Light Metals, 54, 333-347 (2004).

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NOVEL PROCESS DEVELOPMENT WITH CONTINUOUSS CASTING …