Development of Tilt Casting Technology For High Performance Sport Wheels. K K Tong*, M S Yong*, Y W Tham*, R Chang** and K W Wee**

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1 Development of Tilt Casting Technology For High Performance Sport Wheels K K Tong*, M S Yong*, Y W Tham*, R Chang** and K W Wee** * Singapore Institute of Manufacturing Technology, Agency for Science, Technology and Research, Singapore, ** Stamford Tyres Corporation Ltd, Singapore. Abstract Stamford Tyres, a Singapore-based regional distributor for a complete range of tyres, wheels and supporting services has collaborated with SIMTech s metal forming technology group to design and instal its new casting plant in Thailand. The three-year partnership has enabled Stamford Tyres to build new capabilities for manufacturing its proprietary brand of lightweight aluminium alloy wheels. SIMTech has supported Stamford Tyres with: a joint survey and evaluation of manufacturing facilities; technical training; plant installation and commissioning; computer aided design; melt treatment; experimentation; and defect and process analysis through advanced CAE and diagnostic and processing facilities. The successful collaboration has provided Stamford Tyres with the confidence to accept the demanding challenge to design, develop and manufacture high performance sport wheels from its Singapore base. The paper describes how the lightweight, high performance sport wheels are produced using the latest tilt casting technology and identifies the practices necessary to ensure that the highest product quality is achieved. Key words Aluminium wheels, tilt casting, light alloy wheels, quality control 1. Introduction Aluminium alloy wheels produced using A356 (Al Mg) are one of the most difficult automotive castings to produce because of the stringent cast surface and mechanical property requirements. Tilt casting technology utilizes the latest state of the art programmable logic controls, hydraulic systems and precision metal parts to ensure rigidity, stiffness and smooth transition of tilting with a significant reduction in vibration. The technology optimizes wheel quality with respect to time to market of new designs, productivity, and efficiency and cost benefits. The successful manufacture of high quality aluminium alloy wheels requires an understanding and application of casting technology knowledge. In order to deliver the required mechanical properties and fatigue performance, the alloy is typically modified and heat-treated. Alloy 190/1

2 modification is used to alter the acicular morphology of the Al-Si eutectic into a finer, more interconnected fibrous phase that improves toughness and ductility [1]. Although both sodium and strontium modification have been used historically, the trend in industry is toward the use of strontium, as it has a tendency to fade less, is less toxic, and less corrosive to refractories [2]. This paper describes the various processing steps that are taken to manufacture high quality aluminium alloy wheels that are light and have a good surface appearance. The tilt casting technology produces wheels with diameters of 13 (330mm) to 26 (660mm). The paper also reports a case study of a competitor s wheel that had failed in service due to poor microstructure control and casting defects. 2. Manufacturing Steps for High Quality Wheels The process flow diagram for wheel manufacture is shown in Figure 1. The quality of a wheel is affected by each step of the production process, namely the melt preparation, casting process, heat treatment, machining and painting (surface finishing). Every production step in wheel manufacture is closely monitored and controlled to deliver the best possible quality that can be achieved in the final product. The following briefly describe the processes required to deliver the optimum combination of strength and ductility while maintaining an excellent surface finish. The alloy used in the plant is a strontium pre-modified primary A356 ingot with the chemical composition shown in Table 1. Careful preparation of the aluminium melt is the basis of high quality wheel production. Tight control is exercised over melting procedures to ensure the use of clean scrap, good quality raw aluminium ingots and chemical treatments or fluxing that ensure the cleanliness of every melt of aluminium poured into the wheel moulds. The main factors that affect melt quality are: a) charge material, b) melt holding, c) melt transfer operations, d) gas content and e) modification and grain refinement [3]. The reduction of hydrogen gas pick-up was achieved with a continous N 2 degassing by rotary impeller. The typical conventional gravity casting process produces turbulence in the metal stream during pouring. This tilt casting technology provides quiescent mould filling to minimize inclusion generation and entrapment. Tilt pouring is analogous to pouring beer into a tilted mug, where the formation of bubbles and turbulence is greatly reduced. The benefit can also be demonstrated by computer simulation, as shown in Figure 3. The tilt casting line includes a complete 6-axis industrial robotic arm able to ladle the melt into the mould with consistent melt volume control and little temperature loss (Figure 4), precision hydraulic tilt control system and mould opening (Figure 5), automated pick-up of castings and a linear 190/2

3 control quenching mechanism. This automation ensures product consistency and quality. The heat treatment process is used to obtain the optimum combination of strength and ductility in the wheels. Either a T4 or T6 heat treatment procedure can be applied to achieve the required mechanical property specifications. Upon completion of the heat treatment, the wheels were machined to its correct dimensions. All wheels are finally painted or coated to specified paint colours including customized colours with high quality finish and visual aesthetics. The wheels that leave the plant are tested and certified to very strict international specifications such as VIA, JIS and JWL. Testings such as salt spray for corrosion resistance, tensile testing for mechanical strength, radial endurance testing, cornering bending endurance testing, leak testing and impact testing were conducted to ensure wheels are of a highest standards. 3. Experiments to characterize the effects of cooling rate on structure and mechanical properties It is essential to understand the relationship between cooling rate, microstructure and mechanical properties if an effective process and quality control system is to be implemented. This requirement was highlighted by the failure study conducted on a competitor s product as part of a bench-marking exercise. An experimental programme was initiated and comprised of the following activities: Microstructural Examination Metallographic techniques were employed to determine the general microstructure of the wheel casting. Microstructure examinations were taken from the sample areas indicated in Figure 6. Dendrite Arm Spacings (DAS) measurement DAS was measured on the micrographs using the random linear intercept technique [4] and image analyzer software. Failure Investigation of broken wheel in service Metallurgical analysis comprising of visual examination, optical microscopy and scanning electron microscopy was conducted. Impact testing (Drop test) The wheels were oriented at 13 0 and 30 0 to the horizontal. A static load of up to 1 tonne was dropped and the wheels must meet the required international standards against permanent strain in a repeated test. 190/3

4 4. Results and Discussion Microstructure and DAS Measurements The microstructure at points 1 and 6 (as indicated in Figure 6) are shown in Figures 7 and 8 respectively. In general, the microstructures of all the points indicated in Figure 6 revealed no visible porosity. DAS is controlled by solidification rate and the solidification time is given by equation 1 [5]: t s = (DAS / 7.5) 2.56 Equation (1) Where: DAS = Dendrite Arm Spacing: and t s = solidification time. The average DAS and solidification time for each point on the sample was calculated and plotted as shown in Figure 9. In a wheel casting, the solidification is complicated by the spoke/rim intersection, which has a larger mass (volume) than the adjacent rim and spoke areas. The DAS of a casting is directly affected by solidification rate and this in turn will affect mechanical properties. With reference to Figure 6, liquid metal at zone A, the end of the spoke, (points 1, 2 and 3) freezes first. Zone C (points 5-10), which is located at the intersection of the rim and spoke and is connected to the riser, is a much thicker section in comparison to either zone A or B. Thus the solidification time for zone C is much longer than either zone A and B and consequently the DAS is much greater. The shorter solidification time/higher solidification rate in zones A and B will result in a finer structure (measured by the DAS) and better mechanical properties. Figure 10 illustrates how a finer DAS (faster solidification rate) will improve tensile strength and elongation. The mechanical properties of the casting at zone C (Figure 6) are inferior when compared with those at zones A & B. A faster solidification rate at zone C can be achieved by extracting heat using a cooling mechanism to provide a smaller DAS, thereby resulting in better mechanical properties. CASE STUDIES A. Failure of wheels in service A cracked A356 aluminum alloy sport wheel from a competitor was subjected to a metallurgical failure investigation to determine the possible cause of the premature failure (shown in Figure 11). The cracked wheel was reported to have failed after about 1.5 years of use under normal road service conditions. Figure 12 shows the presence of beach marks at the fracture surface, which are typical characteristics of fatigue failure. SEM was employed to study the fracture surface and evidence of fatigue striations confirmed the cause of failure (illustrated in Figure 13). Microstructural examination of the cracked spoke showed significant porosity in the structure as shown in Figure 14. Porosity can arise from either shrinkage or the presence of gas 190/4

5 or, indeed, a combination of the two causes. Hydrogen gas is easily soluble in liquid aluminum particularly at high temperatures above C. Therefore, upon solidification any dissolved hydrogen will come out of solution forming pores in the casting. The presence of porosity reduces mechanical properties and will especially reduce fatigue performance if the pore is located at the surface. Hydrogen levels less than 0.20cc/100g are typically required to minimize the detrimental effects of the porosity [3]. The microstructure also exhibited acicular eutectic silicon suggesting the absence or ineffectiveness of modification and the primary dendrite arm spacing was observed to be large. These microstructural features lower the ability of the wheel to carry a higher load during its normal service condition. The results indicated that the fractured sport wheel cracked as a result of fatigue failure under cyclic loading. The crack originated from the inner side of the spoke at the sharp corner in that region. The casting defects and poor quality microstructures inherent in the wheel are not acceptable and were the likely causes of the premature failure. B. Failure during Impact / Drop Test Wheels were subjected to stringent reliability tests and impact testing is one such test. The shrinkage porosity is the weakest point where the fracture or cracking occurs during the drop load test. One of the wheels that failed the impact test was sent for failure investigation. Figure 15 shows the fracture surface of a wheel at the spoke area after impact testing. Microstructural examination of the fracture surface shown in Figure 15 consisted of shrinkage porosity (as shown in Figure 16) and a coarse DAS (Figure 17). As was observed in the previous section, these inherent casting defects combined with a coarse structure will lead to poor mechanical properties. The shrinkage porosity is due to poor directional solidification and insufficient provision for feed metal. As the liquid aluminium alloy begins to solidify, the volumetric shrinkage occurring due to the change of physical state is not compensated by sufficient liquid metal. This problem can be overcome by the provision of a feeder head or riser [6]. The feeder must be designed to solidify after the casting proper so that whilst the latter is solidifying liquid metal runs from it to compensate for the shrinkage. When the casting and riser are cool, and have been removed from the mould, the feeder that will contain all the shrinkage can be cut off from the casting. 190/5

6 5. Recommendations The following recommendations should be followed to ensure the production of high quality wheel castings. The melting practice should minimise the presence of oxide particles or films because melt cleanliness is very important for high quality wheel castings. Effective degassing must be practiced since hydrogen gas is readily soluble in liquid aluminum and any dissolved hydrogen will come out of solution to form porosity in the castings. Modification, preferably with strontium, should be practiced to avoid acicular eutectic silicon, reduce the presence of stress concentrators and thus improve ductility. The molten alloy must be handled and transferred in a nonturbulent manner to prevent the formation and entrapment of inclusions in the castings. Directional solidification, heat transfer management and effective feeding must be implemented to reduce shrinkage porosity and promote the preferred DAS. 6. Summary and Conclusion The following summary and conclusions based on the reported work can be drawn. The fractured wheel failed as a result of fatique failure under cyclic loading. The failure was initiated from porosities in the casting and at sharp corners, which acted as stress concentrations points for cracking to start and propagate along a weak microstructure. The microstructure examined in the broken wheels consisted of larger DAS, resulting from poor solidification patterns and insufficient grain refinement, which lowered the ability of the wheels to sustain higher loads in service. The shrinkage porosity seen in the castings were caused by incorrect solidification patterns and poor thermal gradient along the rims which has restricted feeding to the spoke areas, which are generally thicker in sections. References 1. Thompson S, Cockcroft S L and Wells M A, Advanced light metals casting development, Materials Science and Technology, Vol 20, Feb 2004, p Colmaco Aluminium Ltd, Modification of foundry Al-Si alloys, Technical Report No.4, Dec Colmaco Aluminium Ltd, Factors affecting the quality of Al wheels, Australian Die Casting Association Report, 2004, p D.G. Eskin, J. Zuidema Jr., V.I. Savran, L. Katgerman., Structure formation and macrosegregation under different process 190/6

7 conditions during DC casting, Mat. Sci. Eng., A384: , (2004). 5. ASM Handbook, Volume 9, Metallography and Microstructures, Ninth Edition, American Society of Metals, pp. 629, (1998). 6. Webster P D, Fundamentals of foundry technology, Portcullis Press, Redhill, pp , (1980). Acknowledgements The author wishes to acknowledge the great support given by Stamford Tyres, particularly Mr Wee Kok Wah (President) and Mr Roger Chang (Senior Vice President) for this collaborative partnership. Table 1: Chemical composition of A356 ingots Si Fe Mg Sr Zn Ti Mn Cu Na Al Rem Melt Preparation Tilt Casting Heat Treatment Machining Painting Finished wheel In-Process Testing Figure 1: Brief schematic of the process flow of wheel manufacturing. Figure 2: Tilt casting machine will ensure smooth liquid metal filling into mould. Figure 3: CAE simulation of tilt casting showing smooth flow of melt filling. 190/7

8 Figure 4: Melting furnace with robotic arm ladling for efficient melt pouring. Figure 7: Microstructure at zone A (Point 1) showing a finer DAS (100X). Figure 5: Mould opening with wheel casting ready to be extracted by pick-up mechanism Figure 6: A wheel section having different rates of solidification zones A, B and C resulting in different DAS microstructure. 11 B 8 A C Average DAS (Microns) Figure 8: Microstructure at zone C (Point 6) with a coarser DAS. (100X) Average DAS Values Points in Sample Solidification Time Figure 9: Graph illustrating DAS and solidification time vs points on wheel section in Figure 6. Solidification Time (sec) 190/8

9 Effect of solidification rate on mechanical properties for Al casting alloy Tensile strength, MPa Tensile Strength % Elongation Elongation, % Secondary dendrite arm spacing,das (um) 0 Figure 10: Graph illustrating the effect of DAS on mechanical properties for Al casting alloy [3]. Figure 13: SEM image showing fatigue striations on fracture surface as shown in Figure 12. Figure 11: General and close-up views of crack on the spoke of a sports rim. 200µm Figure 14: Microstructure taken at the crack region with the presence of porosities (MX100). Shrinkage Porosity Figure 12: Evidence of beach marks characterizing typical failure by fatique can be seen at the crack surface of the spoke shown in Figure 11. Figure 15: Fracture surface taken at spoke areas after impact tests showing shrinkage porosity that weaken the wheel strength. 190/9

10 Figure 16: Cross-sectional microstructure of fracture surface taken after drop test showing shrinkage porosity. Figure 17: Coarser DAS and scriptliked eutectic silicon observed which gave rise to lower impact properties during the drop test. 190/10