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Effect of thermal treatment on tensile properties of vacuum die cast modified aluminum alloy AA 365. Kazi Ahmmed1 , Henry Hu1 , Yeou-li Chu2 , and Patrick Cheng2 1
Department of Mechanical, Automotive & Materials Engineering University of Windsor Windsor, Ontario, Canada N9B 3P4 e-mail:
[email protected],
[email protected] 2
Department of Research and Development Ryobi Die Casting (USA), Inc. Shelbyville, IN 46176-9720 e-mail:
[email protected], and
[email protected]
ABSTRACT In the past three decades, aluminum have increasingly been used in automobiles and are projected to continue doing so reaching 300 pounds per vehicle worldwide by 2020 due primarily to rising demand on vehicle weight reduction and fuel economy. Recently, vacuum assisted high pressure die casting processes (HPDC) appear to gain popularity in the automotive industry to manufacture aluminum components with high requirement on engineering performance since they are capable of offering high component integrity with enhanced quality, and high productivity. However, HPDC processes often result in residual stresses in castings which could cause distortion and warping in components with relatively thin walls and long geometry. To release residual stresses, a thermal treatment is applied to as-cast components. In this study a number of thermal treatment schemes over a wide range of temperatures between 120˚ to 350˚ C have been experimented in an effort to understand the effect of thermal treatment on tensile properties of vacuum die cast modified AA365 alloy. It was found from this study that, the morphology of eutectic silicon has a sound effect on the tensile properties of the tested alloy. The content of magnesium-based intermetallic phase, their morphology and dis tribution throughout the matrix affect on the mechanical properties as well. The reduction in the strengths of the alloy treated at 350˚C for two hours should be at least attributed partly to the absence of the magnesium-based intermetallic phase. However the presence of sufficient amount o f magnesium intermetallic phase had played important role in strengthening the alloy thermally treated at 200˚C.
INTRODUCTION High pressure die casting is a near-net shape manufacturing process in which molten metal is injected into a metal mould at high speeds and allowed to solidify under high pressures [1]. This technique becomes very popular and cost-effective for mass producing metal components where physical dimensions must be accurately replicated and surface finish is important [2]. However, this process creates inherent defects, typified by gas porosity in the produced castings which is mainly due to the entrapment of air in the molten metal as a consequence of the high speed injection of the molten metal into the die cavit y. The presence of gas porosity adversely affect on the mechanical properties of the cast components. In addition these formed pores, specifically which are located at the casting surface may expand in size during heat treatment and thus causes blister s. Entrapped gas is always a major source of porosity in conventional die castings and minimizat ion of this problem can improve the mechanical properties in some extent. A vacuum system in die casting process is thus a logical necessity to extract the gases from the die cavities, runner system and shot sleeve during processing [3]. Vacuum die casting is such a process which meets this logical requirement and thus makes die casting process much more meaningful. By creating a pressure lower than atmospheric pressure in the injection chamber and die cavity, the relative absence of air results in casting
of better quality. Back pressures encountered by metal trying to fill the die cavity are also reduced which ensures a smooth liquid flow through die cavity. Niu et al. [4] studied the effect of vacuum assistance on aluminum alloys and found that the use of pressure level of 18×10-3 -28×10-3 MPa during casting, significantly prevents the cast components from porosity in comparison with conventional die casting. The die casting was even possible to improve the mechanical properties by applying solution treatment. Schneider and Feikus et al. [5] studied the effect of solution treatment parameters on tensile properties of vacuum die cast test bars of GD-AlSi9Cu 3 (German standard) alloy which is equivalent to A380 in North America. They found that both solution treatment temperature and time affect the tensile properties of vacuum die cast alloy GD-AlSi9Cu 3. According to Hu et al. [6] high pressure vacuum die casting alloy A380 responds to solution treatments. It was shown that solution temperature has a strong affect on tensile properties of solution treated A380. Moreover the treatment also changes the morphology of silicon phases which enhance the ductility and UTS as well. The work by Lumley et al. [7] indicated that blistering is substantially reduced and eventually completely eliminated as the temperature and time of solution treatment are decreased. They have studied on aluminum alloy 360 (Al-9Si-0.7Fe-0.6Mg-0.3Cu-0.2Zn-0.1Mn) and found at temperature below 525˚C for 15 minutes have no blistering or dimensional instability. Timelli et al. [8] studied the effect of solution treatments on microstructure and mechanical properties of a die-cast AlSi7MgMn alloy. According to his study a solution heat treatment of 15 minutes at 475˚C, or a heat treatment involving even more time at 525˚C, c auses spheroidization, coarsening, and an increase in the interparticle distance of the eutectic silicon particles, leading to substantial changes in the microstructure and mechanical properties. Moreover the distribution of the silicon particles (eutectic Si) is significantly affected by a short solution heat treatment time. Most of these studies were carried out by solution treatment where precipitation of different strengthening phase has done on matrix to improve the mechanical properties. A proper die sealing and vacuum level is always required for approaching high temperature treatment like solution treatment (T6), in die casting components [4]. Ryobi Die Casting Co. experienced that the gas content of 10cc/100 gm (in final filling area) is not even suitable for T6 heat treatment. However they are currently using a process called HV2 (high vacuum and high velocity) which can be introduced to those components where the fast shot velocity is required very high (3 m/sec) in order to fill thin sections (less than 2 mm). NADCA [9] also recommended that it is not always essential to apply solution heat treatment on die castings as because of severe chill rate and ultra fine grain size in die casting their as cast structure approaches almo st similar microstructural condition of solution heat treatment. So, low temperature aging treatment (T2 or T5) may be good enough for stress relief or dimensional stability. High pressure die cast components always suffers residual stress problem due to the involvement of high pressure and differential solidification rate in die cavity. Therefore, a proper thermal treatment is required to get relief from this residual stress and to provide a dimensionally stable component . The present study was carried out to investigate the effect of thermal treatment on tensile properties of a vacuum die cast modified aluminum alloy AA365. The influence of different thermal treatment on microstructure as well as in tensile properties has conducted in this study and an optimum condition has been confirmed by analysing the mechanical properties obtained at different thermal treatment. EXPERIMENTAL PROCEDURES THERMAL TREATMENT Low silicon and strontium modified AA365 (RYOBI HD-3SF) alloy component used in the present study was cast at RYOBI DIE Casting (USA) Inc. Chemical composition of the as -received alloy is shown in Table 1. For isothermal heat treatments, the components were initially cut into rectangular shapes and then were thermal treated in the temperature range of 120350˚C. The duration for thermal treatment was initially selected for 2 hours and varied as 0.5, 1 and 1.5 hours at optimized temperature. All the treatments were carried out in a muffle furnace of which temperature could be controlled within ±5˚C. All the samples were cooled in air after the thermal treatments. Table 1. Chemical composition of modified AA365 alloy Alloy
Element (wt.%)
Ryobi HD3SF
Si
Fe
Mn
Mg
Zn
Ni
Sn
Cr
Sr
7.75-8.25
0.05-0.15
0.325-
0.225-
