porosity, intermetallic phases with disagreeable morphologies such as acicular and plates, soldering of the cast metal to the die tool, alloy cost and recyclability ...
Microstructure and Uniaxial Tensile Properties of Heat Treatable Al-Zn Alloy for Structural HPDC Components Chufan Wu1, Xiaochun Zeng1, Sumanth Shankar1,*, Gabriel Birsan2, Kumar Sadayappan2, Anthony Lombardi3 and Glenn Byczynski3 1
Light Metal Casting Research Centre (LMCRC), Department of Mechanical Engineering, McMaster University, 1280 Main Street W, Hamilton, ON, Canada. 2 CanmetMaterials, Natural Resources Canada, 183 Longwood Rd S, Hamilton, ON, Canada.1 3 Nemak USA/Canada, 4655, G N Booth Drive, Windsor, ON, Canada.
ABSTRACT A newly developed heat treatable Al-Zn alloy (Nemalloy HS700) was used to produce large automotive structural components using the high-pressure die casting (HPDC) process. The casting trials were carried out in a research laboratory as well as at a production facility. Microstructural characterization and uniaxial tensile properties of these alloys will be presented and compared to Al-Si alloy (Silafont 36), which is a commonly used alloy for automotive structural components. The analysis of microstructure revealed a compact non-dendritic primary Al morphology with a fairly large skin thickness (~0.25 mm). The tensile properties of this Al-Zn alloy showed a significant improvement in both the yield and ultimate tensile strength, while the elongation was similar when compared to Silafont 36. INTRODUCTION In recent years, research on automotive lightweighting has been growing at a rapid pace to improve fuel efficiency and support environmental sustainability. Aluminum alloys are the ideal candidates for lightweight structural components due to their high strength to weight ratio, high ductility and recyclability 1. Currently, Al alloys such as Mercaloy, Silafont 36 (both Al-Si-Mg) and Castasil 37 (Al-SiMn-Mo-Zr) have been used extensively with the high pressure die casting process to produce automotive structural components This is due to their good castability and mechanical properties 2. For structural components produced using the HPDC process, there are several deterrents which prevent an alloy from being used. These factors include high susceptibility to the formation of gas and shrinkage porosity, intermetallic phases with disagreeable morphologies such as acicular and plates, soldering of the cast metal to the die tool, alloy cost and recyclability / reusability of the alloy. The Fe levels of the alloy determine the extent of detrimental intermetallic phases (Al-Fe-Si) that evolve during solidification (high Fe is bad) and the Mg and Si levels in the alloys define the evolution of the detrimental Mg2Si phase (high Mg/Si ratio is bad)2. The low Fe content results in high ductility, however, it also causes rapid die tool wear through soldering 3 . Alloys cast for structural components are degassed to reduce the level of dissolved hydrogen in the melt, which alleviates porosity in the casting. The shrinkage porosity is typical of components cast with primary single phase alloys such as Al wrought alloys 4.
In an attempt to solve the deficiencies of the current commercial foundry Al structural alloys, a new Al based alloy, called Nemalloy HS700, was developed by researchers at McMaster University, Canmet Materials and Nemak, and was field tested using the HPDC process. The main advantages of this alloy include: significantly higher yield and tensile strengths with a comparable ductility compared to Silafont 36. This paper presents the critical observations and results from casting automotive components using both the Nemalloy HS700 and Silafont 36 in the HPDC process. Two sets of casting trials were carried out: one at a research facility and the other at an industrial production site. A critical comparison between the two alloys in terms of microstructure and tensile properties in both the as-cast (F temper) and solution heat treated (T4 temper) conditions are presented in this publication. Additionally, the artificially aged (T7 temper) condition is also presented for Silafont 36 alloy castings. EXPERIMENTS The casting trials were carried out on two automotive components: one at the industry production facility and the other at the research facility. The first set of trials involved a shock-tower (Figure 1) for an automobile, which was cast using a 4000 Ton HPDC machine at the industrial production facility. The die casting machine contained a chill block and high-vacuum system to reduce the air entrapment in the component, and oil-based thermo-regulation loops in the die-tool to achieve the desired die temperature profile. The second set of trials involved a Top Hat component (Figure 2) with additional ribs as added features. This part was cast using a 1200 Ton HPDC machine with high vacuum assist at a prototype casting research facility.
Figure 1. Model of automotive shock tower component.
Both the Nemalloy HS700 and Silafont 36 were degassed with high purity argon prior to casting. The shock tower was cast with a melt superheat of approximately 80 °C above the liquidus temperature for Silafont 36 and 40 °C superheat for the Nemalloy HS700; both using automatic robot assisted pouring. The top hat was cast by manual pouring with approximately 80 °C superheat above the respective liquidus temperatures for both the Silafont 36 and Nemalloy HS700. The HPDC process parameters were individually optimized for both the alloys, respectively, prior to obtaining sound cast components for evaluation.
Figure 2. Model for the top hat component with additional rib sections as demarcated. All dimensions are in mm.
Some of the other salient casting parameters in these trials were: • • • •
Die preheat for the shock tower trials was carried out at the beginning of the process and maintained through continuous production run. In addition, hot oil thermo-regulation loops in the die tool were set to 200 °C for the Nemalloy HS700 trials and 150 °C for the Silafont 36 trials. The reduced pressure test (RPT) was carried out after degassing and prior to casting for both alloys. The shot sleeve was preheated in all casting trials. The lubricant spray was carried out manually with spray nozzles in the Top Hat trials and automatically with a distinct spray pattern for the shock tower trials.
The cast samples were sectioned to obtain representative samples from several locations in the cast component to carry out both microstructural analyses and uniaxial tensile test. The uniaxial tensile test sample was obtained as per the ASTM B557M-15 and a schematic is shown in Figure 3.
Figure 3. A schematic of the uniaxial tensile test samples as per ASTM B557M-15. All dimensions are in mm.
Solution heat treatment was carried out for all the uniaxial tensile test samples using an electric convection furnace and the samples were quenched in both forced air flow. The castings with Silafont 36 underwent a T7 artificial ageing treatment, as proposed by the alloy data sheet 5. The samples for optical microscopy were prepared using standard metallographic techniques including mounting, polishing and etching with Keller’s reagent. The imaging was carried out using a Nikon Eclipse LV100 in bright field mode. The uniaxial tensile tests were carried out in a MTS ZZZ machine with a 50 kN load cell. A loading rate of 1 mm/min coupled with an on-line digital extensometer to measure transient elongation was used for the tensile tests. RESULTS The results of the microstructure analyses and tensile properties are presented in this section.
MICROSTRUCTURE
Figures 3 presents typical microstructure images of the various castings in this trial from the middle of the cast wall thickness and the edge. It can be readily observed that the castings with Nemalloy HS700 had a relatively large skin of about 250 µm in thickness while there was a small skin of about 50 to 100 µm thickness on the castings with Silafont 36 alloy. Both of the alloys showed significant amount of Externally Solidified Crystals (ESC) 6 in their respective castings, as shown by the distribution of large primary Al grains in Figure 4. The ESC phases are obtained from preliminary solidification of Al in the shot sleeve during filling of the shot sleeve and during the show shot, prior to the filling of the die tool cavity in HPDC. Typically, the grain size of primary Al solidified in the die tool, for the Nemalloy HS700, have been in the range of 10 to 20 µm, while the ESCs had a grain of about 100 µm , as shown in Figure 4. The castings with Nemalloy HS700 showed a compact non-dendritic morphology of the primary Al phase, which were typically globular and/or rosette shaped. In contrast, the Silafont 36 microstructure consisted of large primary Al dendrites, which were ESCs, in combination with smaller primary Al dendrites and Al-Si eutectic, which were solidified in the die tool.
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Figure 4. Typical microstructure images from optical microscopy. (a) and (b) Nemalloy HS700 from Shock tower casting obtained at the edge and middle of the wall thickness, respectively. (c) and (d) Nemalloy HS700 from Top Hat casting obtained at the edge and middle of the wall thickness, respectively. (e) and (f) Silafont 36 from Shock Tower casting obtained at the edge and middle of the wall thickness, respectively.
UNIAXIAL TENSILE TEST Figure 5 show the results of the uniaxial tensile test for the as-cast (F temper) condition; Figure 5(a) is a graph of the Ultimate Tensile Strength (UTS) and percent elongation and Figure 5(b) is that of 0.2 Proof Stress or Yield Strength (YS) and percent elongation for both the Silafont 36 and Nemalloy HS700 stock tower and top hat castings. The data points shown in Figure 5 are the raw data from all the evaluated samples. Figure 5 shows that the Nemalloy HS700 exhibited better strength values (UTS and YS) than the corresponding castings of Silafont 36. With regards to the Nemalloy HS700 castings, the YS of the shock tower was lower than that of the top hat because there were intermetallic phases that evolved during solidification with facetted edges and sharp corners. It is notable that upon solution treatment, these intermetallic phases changed their morphology to a more rounded version and the yield strength of T4 solutionized samples were not adversely affected by the same. The unwanted intermetallic phase that contributed to lower YS in the shock tower was eliminated with minor compositional control in the alloy and implemented in the casting trial for the top hat component. Hence, the YS of the top hat in F temper was significantly improved from that of the shock tower, as seen in Figure 5. It can therefore be deduced that the values of YS in the top hat is typical of the Nemalloy HS700 under optimum alloy conditions. Typically, the elongation obtained for the Nemalloy HS700 was similar to those for the Silafont 36 alloy, for the respective shock tower and top hat casting samples, as shown in Figure 5. Figure 6 presents the results of the uniaxial tensile tests for the top hat castings after the T4 solution heat treatment for both the Silafont 36 and Nemalloy HS700, and after the T7 temper for the Silafont 36 alloy. Figure 6 shows that the strength of the Nemalloy HS700 is significantly higher than that for Silafont 36, specifically the YS, which was is more than double in magnitude. Furthermore, the elongation for both the alloys in the top hat trial was similar. Notably, the trends in the values of the tensile properties from various sections of the top hat castings were also similar for both alloys.
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Figure 5. Typical uniaxial tensile test data in the as-cast (F temper) condition; (a) UTS versus Elongation an (b) Yield Strength versus Elongation
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Figure 6. Comparison of tensile properties between Silafont 36 and Nemalloy HS700 from both trials after T4 and T7 heat treatment (a) Elongation vs. UTS (b) Elongation vs. yield strength
SUMMARY Some of the other salient observations recorded during these trials are below:
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The Nemalloy HS700 showed significantly higher resistance to die soldering and die tool wear than Silafont 36. The metal returns from the Nemalloy HS700 shock tower castings were all recycled and reused (100 %) and there was no appreciable change in the castability or uniaxial tensile properties. This alloy could be entirely recycled several times. There were no discernable difference in the process of melt preparation and handling for both these alloys. The alloy is predicted to be lower cost than traditional Al-Si based primary alloy alternatives – this will be confirmed in the future when compositional limits are established. Nemalloy HS700 exhibited significantly higher strength, and similar elongation compared to Silafont 36 produced under similar conditions. Hence, the Nemalloy HS700 showed a significant improvement in the strength to weight ratio compared to Silafont 36, which would enable redesign of components for appreciable light weighting. The Nemalloy HS700 has been proven to be a viable alloy for manufacturing near net shaped automotive structural components.
ACKNOWLEDGEMENTS The authors would like to thank Ontario Centres of Excellence (OCE) for funding the research of this project through the Voucher for Innovation and Productivity II (VIP II). The authors are also grateful for the NRCAN Office of energy R&D program (EIP) funding for the casting trials at CanmetMaterials. REFERENCES
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