HIPing A Potent Post Casting Treatment For High Integrity Aluminium Castings

Transcription

1 HIPing A Potent Post Casting Treatment For High Integrity Aluminium Castings R.M.PILLAI Regional Research Laboratory (CSIR) Thiruvananthapuram , India rmpillai@rediffmail.com ABSTRACT The increasing use of high integrity Al castings for repeated loading applications warrants improved fatigue properties as well. The principal cause for the lower fatigue properties and reliability of aluminium castings is the presence of defects and inhomogenities, which are the preferred fatigue initiation sites. Hot Isostatic Pressing (HIPing) is a powerful post casting treatment for eliminating defects like shrinkage porosity and hydrogen pinholes leading to densification and enhanced mechanical properties of the castings meant for high performance applications. This paper brings out the current status of this potent post casting treatment for high integrity castings including basics of HIPing, its variants and their pros and cons and its effects on both microstructure and mechanical properties of aluminium alloy castings. Keywords : HIPing, LHIPing, Aluminium castings, Fatigue strength, 219/1

2 Introduction Applications in aerospace, automotive and general engineering, requiring high mechanical properties including resistance to fatigue failure and weight saving, look for aluminium castings. This is because of their excellent casting characteristics, corrosion resistance, near net shape capability and especially high strength to weight ratio. Performance of casting is dictated by many metallurgical features, such as, secondary dendrite arm spacing (SDAS), iron intermetallics, oxides/inclusions and porosity. Castings exhibit defects such as shrinkage and gas porosity, hot tears, inclusion and alloy segregation that generally result in lower and more variable mechanical properties than their wrought counterparts. The fatigue endurance of castings is very sensitive to the size of casting defects. On the other hand, solidification time as reflected by secondary dendrite arm spacing(sdas) and size of silicon particles also affects it to a lesser extent.these defects can be controlled reasonably by proper mold design and good foundry practice. In many engineering applications, the response to fatigue environment is very critical. Nonetheless, the complete elimination of the shrinkage defects is not always possible without the application of external forces to close voids and porosity. Hot Isostatic Pressing (HIPing) enables this. HIPing helps to reduce porosity in thick-tothin section transition areas as well. HIPped castings can replace forgings for some high stress applications due to the elimination of porosity and other defects. The elimination of porosity with HIPing increases fatigue life and raises tensile properties. It also significantly reduces scrap, rework and weld repair requirements. Further, mold design can be simplified to save material formerly needed in complex gating and the placement of chills becomes less critical. Despite the wide application of HIPing for titanium and Ni alloys due to the enormous cost reduction potential, it has not been readily accepted for Al castings meant for aerospace and automotive applications. The reasons include: (i). Low cost of the alloy and high cost of HIPing, (ii).numerous casting techniques used including investment, die casting, permanent mold, green sand, dry sand, lost foam, thixoforming and squeeze casting, (iii).very strong tendency of Al alloys to dissolve hydrogen and form oxides, eutectics and intermetallic phases, which have a strong influence on properties and (iv). Unlike Ti and Ni alloys, the predictability of improvement in properties of Al castings is not as exact due to influence of many variables. However, in recent years, the lower cost HIPing processes such as LHIPing, Densal, PIF and quick HIP have emerged with their lower cost enabling them to be considered as a post casting treatment option for aluminium castings for niche applications[1]. Considering the importance of HIPing to high integrity aluminium alloy castings, this paper brings out the current status of this potent post cast treatment tool. 219/2

3 Evolution of HIPing The very first patent on HIPing dates back to Table 1[1,2] lists other significant milestones in the evolution of HIPing. In view of the potential improvement realized in castings subjected to HIPing (application of a high inert gas pressure at high temperature), the foundrymen had shown interest in the utilization in the sixties for aerospace and high performance components for racing engines and other niche market. High cost due to high investments and long cycle times (up to 10 hrs) is still the main barrier for extending the use of HIPing for high volume production. In addition, the gas pressurization used in conventional HIPing process is also very dangerous. A variant and recent innovation of Metal Casting Technology Inc and GM, USA termed Liquid HIPing, wherein a molten mixture of salts is used as the fluid for applying isostatic pressure on the components, achieves results similar to those obtained by the much longer gas HIPing cycle time. Basics of HIPing Liquid aluminium is prone to both hydrogen absorption and oxidation. Aluminium castings exhibit gas porosity and oxide inclusions inevitably. Further, improper feeding can result in shrinkage porosity. Typically gas and shrinkage pores are spherical and irregular in shape respectively and can associate with aluminium oxides as well. All these significantly deteriorate the fatigue properties of Al castings by shortening both fatigue crack propagation and the initiation period. Figure 1 shows the SEM micrographs revealing the origination of fatque cracks in Sr modified A356 castings from pores and oxide films[3]. The decrease in fatigue life is directly related to the increasing defect size. Porosity is more detrimental to fatigue life than oxide films. A defective casting shows at least an order of magnitude lower fatigue life than sound casting. In HIPing, the castings placed in a chamber are slowly heated while the pressure of the surrounding inert gas is simultaneously increased. Castings/components are subjected to the simultaneous application of both heat and high pressure in an inert gas atmosphere. This pressure while acting on the casting isostatically enables collapse of any internal porosity left in the castings. Later, the castings are cooled to the room temperature. The simultaneous application of heat and pressure converts the material in to a plastic state leading to the collapse of voids and porosity. It is also to be borne in mind that the collapsed voids do not change either the shape or dimensions of the parts in general. The clean surfaces of the voids enable diffusion bonding together and making a stronger part. HIPing is invaluable in the precision casting, power metallurgical, metal bonding and ceramic industries. It improves the performance and yield of precision castings. The isostatic nature of the applied gas pressure is well suited for defects healing in castings. HIPing of complex shapes parts can be done without complex or expensive tooling. 219/3

4 Relationship between benefits and casting quality HIPing provides significant improvements in mechanical properties, such as higher strength, enhanced toughness, improved fatigue resistance and longer creep life. Well documented benefits of HIPing include (i) An approximately 50% improvement in ductitity (ii) 3-10 times improvement in fatigue life (iii) Definite improvement in ultimate tensile strength (UTS) (iv). No change in yield strength (YS), (v). Reduction in porosity and thus minimizing the scatter in mechanical properties and (vi). Salvaging the scrapped castings due to porosity and hence improving casting yield especially in castings subjected to radiographic inspection. However, actual benefits to be exhibited in the castings depend on its quality. For example, in test bars cast without serious porosity, HIPing does not affect the tensile properties. If porosity is fine, ductility may be controlled by silicon particles or intermetallics. If the porosity is below a critical value, HIPing will not be effective. However, HIPing results in significant improvement in fatigue properties. It has been shown[2] that a ten fold enhancement in fatigue life has been achieved in A356 alloys by HIPing for both fine and coarse structured castings (SDAS 30-90µm at 138MPa) due to the effective closure of interdendritic shrinkage porosities even in high quality castings with a very fine structure (30µm SDAS) and corresponding fine porosity. It has also been shown that fatigue properties are influenced dramatically by casting defects other than just porosity, which takes the lead followed by oxide particles. In the absence of these defects, fatigue failure was observed to initiate at slip band leading to significantly longer fatigue life. Variants of HIPing Alcoa 359 Process : A process, covered by US Patent No , improves the fatigue strength of aluminium sand and permanent mould castings (up to 300%) over castings without HIPing and enables the castings to meet Class A radiographic specifications[4]. Thus the process makes possible the salvaging of scrapped casting with internal porosity and enhancing he casting yield of a foundry diesel engine pistons, permanent mould cast and treated with Alcoa 359 process, constituted a prime potential market. Any casting component benefited from superior fatigue properties could justify the additional expenditure involved in the Hiping. Densal HIPing Process : It uses a purpose built unit having a shorter cycle time and much lower acquisition and operating cost than a standard HIPing unit[2,5]. A more cost effective nickel-chromium furnace replaced the costly molybdenum furnace. The lower pressure level makes the pressure vessel less expensive. The less expensive nitrogen gas use as a cover instead of the costly argon used in standard HIPing further brings down the cost. All these brings down the cost to about one third that of a standard HIPing unit. It is also understood that an equipment operated at capacity near to that of high volume automotive foundry can reduce the 219/4

5 cost further. Moreover, solution heat treatment is also to be performed in these castings to achieve the required tensile properties. Because of the similarities of the processing temperatures in HIPing and solution heat treatment, attempts have been made to combine these two for greater process efficiency by Boydcote NA, Inc USA, the developer of Densal HIPing process. It is a low cost process specially developed for aluminium alloys in general and the Al-Si alloys in particular. Combining T6 heat treatment with Densal HIPing has been reported to reduce the total processing time and cost even further. Two case studies revealing action of Densal in improving the quality of a high performance air frame casting and an automotive steering knuckle casting are give below: A large air frame casting, with long, thin interconnected branches of thin and thick sections complicating the feeding of liquid metal is designated as a replacement of a complex sheet metal counterpart. The Densal treatment did improve the ductility of poor quality material by about 60% but not the good quality one. However, the fatigue life of both good and poor quality materials is improved by 3 and 7 folds respectively. Further, the fatigue life of the latter is 2-3 times better than a good quality material in T6 condition without Densal treatment. An automotive steering knuckle is also a high performance component requiring high quality casting route for its production. Densal treatment of a sand cast steering knuckle has closed internal porosities thus enhancing its quality equivalent to that of a high quality casting route but at a lower cost than the latter. LHIPing : Instead of inert gas, a molten mixture of salts is used as the liquid / fluid to apply the isostatic pressure on the cast components. A few minutes cycle time is sufficient to achieve the results equivalent to those obtained with the traditional gas HIPing with longer cycle time. Effect of HIPing on mechanical properties and microstructure Table 2 summarises some of the researchers findings on the improvement of mechanical properties of aluminium alloys[6-8]. US Naval Air systems command in its attempt to study the effects of HIPing on dynamic properties of castings has shown that fatigue properties of A356 aluminium alloy is significantly enhanced as shown in Figure 2 on HIPing due to closure of the voids[9]. The closure of microshrinkage porosities is clearly reveled in Figure 3 showing the SEM micrographs of fracture A356 alloy with and without HIPing[10]. Discussion Al castings are finding increasing use in applications warranting fatigue failure resistance. HIPing is paramount in the production of these castings since it eliminates inherent porosity, densifies the castings and thus results in improved fatigue properties. Conventional HIPing using argon gas to apply high pressure for sealing internal porosities is very 219/5

6 effective, but costly ( longer cycle time and high investment) and dangerous. On the other hand, liquid HIPing, a recent innovation, where fluid is used to apply the isostatic pressure, achieves similar results of conventional HIPing in a shorter cycle time. A marked reduction in the statistical spread or scatter usually associated with casting properties is of high significance Minimum observed values are usually increased resulting in improved reliability and efficiency of materials utilization. HIPing can also render castings fit for applications requiring more expensive forged or wrought and machined parts. In addition, alloys prone for hot tears or the formation of deleterious phases during solidification and once considered uncastable can now be redissolved by HIPing. Poor gating system can introduce surface turbulence and incorporate both oxide films and air in the castings. In the process, the oxide films get folded forming an oxide-to-oxide interface in the liquid and entraining varying quantity of air between them. These double films called bifilms, do not have any significant bonding across the interface and hence act as cracks in the liquid melt as well as in solidified casting. Despite the HIPing of critically stressed Al alloy castings meant for aerospace and automotive applications to achieve closure of shrinkage and gas pores as well as improved mechanical properties and reliability, cracks of bifilms are some what resistant to closure[10]. Effects of HIPing on both fatigue and tensile properties often depend on the as cast quality of the casting surface connected porosity, alloy chemistry, grain size, dendrite arm spacing and the presence of oxide inclusions. A critical defect size exists for the initiation of fatigue crack, which initiates from other competing initiators like eutectic particles and slip bands below this critical defect size. The critical defect is in the range of 25-50µm for Sr modified A356 alloy. The fatigue life of sound A356/357 castings depends on the microstructural fineness (SDAS), composition, eutectic modification and heat treatment. Sr modification results in longer fatigue life in these alloys than unmodified ones. Significant decrease in fatigue life results in both unmodified and Sr modified alloys when Mg increases from 0.4 to 0.7%. Increasing Fe content too decreases fatigue life particularly in alloys with longer SDAS values. Further, an adequate solution treatment too is beneficial because of dissolution and segmentation of large Fe intermetallic [3] particles. Friction stir welding[6], an emerging metal joining route for aerospace applications, can also be used to embed wrought microstructure in casting by localized modification leading to dramatic improvement in ductility and strength of A356 alloy (Table II) which can be further improved by HIPing. This approach is aimed at achieving the best combination, low overall cost due to casting and higher performance in localized areas due to the introduction of wrought microstructure by friction stir processing. Further, very significant improvement in Weibull modulus and quality index, the 219/6

7 often used parameters for casting quality has also been observed. LHIPing has also been utilized for Al castings made by thixoforming[11,12] and lost foam or evaporative casting process[13]. Conclusions A defective casting exhibits atleast an order of magnitude lower fatigue life than a sound casting. Now-a-days, foundrymen have been focusing on the production of high integrity aluminium alloy castings meant for critically stressed applications in aerospace and automotive sectors both by improving casting techniques and adopting post casting treatments, namely heat treatment and hot isostatic pressing (HIPing). Variants of conventional or traditional gas HIPing with reduced cycle time are available now making this post casting treatment more economical and safer. Although the development of HIPing is pertinent to the broad range of premium quality Al castings, it is of special relevance to the more difficult to cast Al-Cu series. In a nut shell, a concept of cast-to-fill and HIPing-to-density can be utilized to take full advantages of HIPing. Casting exhibiting superior fatique properties on HIPing could justify the additional expenditure involved. A well designed bottom gating with filters along with a controlled melt quality, wherein oxide films are eliminated or at least greatly minimized in size and number can enable the production of reliable Al alloy castings without hipping. However, foundryman has to go a long way in achieving the above. Acknowledgements The author thanks the Director Regional Research Laboratory (CSIR), Thiruvananthapuram, India for his interest and all the authors and publishers of the articles cited. References 1. Hebeisen JC, Cox BM and Rampulla B, HIP of aluminum castings, Advanced Materials & Processes, April 2004, pp Charles Barre, Hot Isostatic pressing, Advanced Materials & Processes, March 1999, pp Want QG, Apelian D and Lados DA, Fatigue behaviour of A356/357 aluminium cast alloys. Effect of microstructural constituents Part II, J. of light Metals, 1(1) 2001, pp Casting densification process, TMD Report N.5, Alcoa technology Marketing Dn. 5. http// MDSite/viewSelectedArticle.asp 6. Friction stir casting modification, 7. Premium quality Aluminum castings, Technical update 3D5, Hitchiner Manufacturing Co., Inc. 8. Sergio Gallo and Claudio Mus, Current quality needs for casting in automotive, M.C.Flemings Symposium on solidification and materials processing, 2001, pp , edrs. Abbaschian R, Brody H and Mortensen A, pub. TMS 219/7

8 9. Dale Moore, Naval aircraft materials and processes, Advanced Materials & Processes, March 1999, pp Nyahumwa C, Green NR and Campbell J, Influence of casting technique and hot isostatic pressing on the fatigue of an Al7SiMg alloy, Met. & Matls. Trans. A, 32A, Feb. 2001, pp Rosso,M, Mus,C and Chiarmetta,G, Liquid hot isostatic pressing process to improve properties of thixoformed parts, Metallurgical Science and Technology,18,2,Dec 2000,pp Rosso,M, Romano,E and Barone,S, Properties of thixoformed parts by liquid hot isostatic pressing process, Metallurgical Science and Technology,19,1,June 2001,pp Molina,R, Leghissa,M and Mastrogiacomo,L, new developments in high performance cylinder heads : application of LHIP and split cylinder head concept, Metallurgical Science and Technology,22,2,Dec 2004,pp3-8 (a) (b) Figure 1: SEM micrographs showing the origination of fatique cracks from (a). Oxide films and (b). Pore [3] 219/8

9 Table 1 : Evolution of Hot Isostatic Pressing(HIPing)[4,6] YEAR EVENT 1913 First patent 1950s Development by Battelle Columbus Laboratories 1960s Initially developed for cladding nuclear fuel elements[2] Immense interest by Foundryman in utilizing the potential quality improvement of castings by HIPing with high inert gas pressure at high temperature by eliminating defects like shrinkage, porosity and hydrogen pin-holes and using increasing mechanical properties. Many Hiping units put in to operation to treat high performance aerospace and racing engines components and other niche markets. High cast factor due to huge investments and long cycle time (up to 10hrs) restricted its use for high volume production castings viz. automotive. Applied to investment castings for gas turbine engines[1]-ti and Ni base super alloys 1980s Hitchiner Manufacturing Corpn. and General Motors, USA patented a low cast version of Hiping coined as Liquid Hot Isostatic Pressing (LHIP). Demonstration of innovative liquid HIPing (LHIPing) requiring only few minutes against hours for the same and results by HIPing by Hitchiner Mfg. Co. Inc and General Motors (GM) on a small scale pilot plant level Teksid, Italy and Idra Presse was authorized by Hitchner and GM to develop an industrial LHIPing for high volume automotive sector with process cost competitiveness tag. Figure 2 : Effect of HIPing on fatique properties of A356 aluminium Castings. P- premium quality and U-lower quality[9] 219/9

10 (a) (b) Figure 3 : SEM micrographs of fractured surface of A356 alloy (a). without and (b). with LHIPing[12] Table 2: Mechanical properties of aluminium alloys without and with HIPing treatment Alloy Condition UTS MPa A356 [6] As Cast As FSP* As FSP+T6 169±10 251±5 301±7 YS MPa 132±5 171±14 216±14 Elonga tion, % 3±1 31±1 28±3 Fatigue strength MPa Remarks Localized property improved by localized modification 356[7] Sand cast, unchilled, quenched directly after liquid HIPing and aged A356 [8] Sand cast +T6 sand cast+ LHIP +T FSP- friction Stir Processed 219/10