Embodiments of the present invention relate generally to aluminum alloy high pressure die castings and particularly to methods of enhancing mechanical properties of aluminum alloy high pressure die castings and to methods of manufacturing aluminum alloy high pressure die castings in high pressure die casting and heat treatment processes.
High pressure die casting (HPDC) processes are widely used for mass production of metal components because of the processes' low cost and the close dimensional tolerances (near-net-shape) and smooth surface finishes they provide to the castings formed therefrom. For example, manufacturers in the car industry use HPDC to produce near-net-shape aluminum alloy castings for engine and, in particular, transmission applications.
One disadvantage of conventional HPDC processes, however, is that the HPDC castings generally are not amenable to solution treatment (T4) at high temperatures, such as about 500° C., for most high pressure die cast aluminum alloys. This significantly reduces the potential for precipitation hardening in the castings through a full T6 and/or T7 (=T4+T5, see detailed description below) heat treatment. The castings generally are not amenable to solution treatment (T4) due to a high quantity of porosity and voids in the components. The porosity and voids generally are attributable to shrinkage of the alloy from a low density liquid metal to a high density solid casting during solidification and, in particular, to gases, such as air, hydrogen or vapors formed from the decomposition of die wall lubricants, entrapped while filling the die with the molten metal. As such, virtually all HPDC castings have large gas bubbles formed therein. Further, internal pores containing gases or gas forming compounds within HPDC castings typically expand during conventional solution treatment at elevated temperatures, thereby, forming surface blisters on the castings. The presence of these blisters affects not only the appearance of castings, but the dimensional stability and, in particular, the mechanical properties of the castings as well.
Therefore, to avoid the potential for blister formation, conventional aluminum alloy HPDC castings generally are used in as-cast and/or, to a lesser extent, in aged conditions, such as T5. Even with subjecting HPDC castings to a conventional T5 aging, however, the increase of yield strength, and other mechanical properties, is still very limited, since, in conventional as-cast aluminum alloy high pressure die castings, the concentrations of solutes available for strengthening in artificial aging (T5) are very low due to slow cooling after solidification. Additionally, the conventional single step isothermal aging (T5) at an intermediate temperature in many cases cannot maximize the mechanical properties for given concentrations of solutes in the material prior to aging. As a result, the mechanical properties of the conventional HPDC castings are usually low for a given composition of the aluminum alloy in comparison with other casting processes since the aluminum alloy castings made by other casting processes generally may be heat treated in full T6 or T7 conditions.
Developed technologies, such as the use of vacuum to remove air in mold cavities during die filling, improve the quality of HPDC castings and their solution treat-ability. Use of these technologies, however, is still limited due to the high cost of facility and maintenance and operational complexity. Further, it also has been disclosed that blistering can be avoided, to a certain degree, by using much shorter solution treatment times and lower temperatures. For example, experiments with strengthening aluminum alloys 360 (Al-9.5Si-0.5Mg) and 380 (Al-8.5Si-3.5Cu) have shown that significant responses to aging are still possible following such modified solution treatments ([1] R. N. Lumley, R. G. O'Donnell, D. R. Gunasegaram, M. Givord, International Patent Application PCT/2005/001909; [2] R. N. Lumley, R. G. O'Donnell, D. R. Gunasegaram, M. Givord: Mat. Sci. Forum, 2006, vols. 519-522, pp. 351-359; [3] R. N. Lumley, R. G. O'Donnell, D. R. Gunasegaram, M. Givord: Proc 13th Die Casting Conference of the Australian Die Casting Association, Melbourne, Australia, 2006, P25; and [4] R. N. Lumley, R. G. O'Donnell, D. R. Gunasegaram, M. Givord, Metall Trans. 2008 in press). It appears, however, that these experiments are limited in value not only because the data disclosed merely are based on test specimens having very low porosity, but also because the solution (T4) heat treatment process parameter window is too narrow for highly complex HPDC castings.
Conventional T6 and/or T7 heat treatment processes for aluminum alloy castings normally involve following three stages: (1) solution treatment at a relatively high temperature below the melting point of the castings (also defined as T4), often for times exceeding 5 hours to dissolve its alloying (solute) elements and homogenize or modify the microstructure; (2) rapid cooling, or quenching, such as into cold or hot water, to retain the solute elements in a supersaturated solid solution; and (3) artificial aging (T5) by holding the casting for a period of time at an intermediate temperature suitable for achieving strengthening through precipitation. Solution treatment (T4) serves generally three main purposes: (1) dissolution of elements that lead to age hardening, (2) spherodization of un-dissolved particles and/or phases, and (3) homogenization of solute concentrations in the material. Quenching after T4 solution treatment retains the hardening solutes in a supersaturated solid solution (SSS) and creates a supersaturation of vacancies that enhances the diffusion and dispersion of precipitates. To maximize the yield strength, and other mechanical properties, of the casting, the precipitation of all strengthening phases should be prevented during quenching. Aging (T5, either natural or artificial) enables a controlled dispersion of strengthening precipitates. FIG. 1 shows a typical conventional T6 and/or T7 heat treatment cycle of an aluminum alloy.
With T5 aging (FIG. 1), there generally are three types of aging conditions, which are commonly referred as under-aging, peak-aging and over-aging. At an initial stage of aging, or pre-aging, Guinier-Preston (GP) zones and fine shearable precipitates form and the casting is considered to be under-aged. In this condition, mechanical properties of the casting usually are low. Increased time at a given temperature or aging at a higher temperature further evolves the precipitate structure and increases mechanical properties, such as yield strength, to a maximum levels to achieve the peak-aging/strength condition. Further aging decreases the mechanical properties and the casting becomes over-aged due to precipitate coarsening and its transformation of crystallographic incoherency. Simply for exemplary purposes, FIG. 2 shows an example of aging responses of cast aluminum alloys A356/357 manufactured under conventional sand casting processes and aged at a temperature of 170° C. For the period of aging time tested at a given aging temperature, the castings, whether sand castings or high pressure die castings referred to herein, undergo under-aged, peak-aged, and over-aged conditions.
Considering that conventional aluminum alloy HPDC castings generally inevitably contain internal porosity, artificial aging (T5) may be one of the ideal means (solutions) to help to achieve the desired mechanical properties in the castings without creating blisters. The strengthening resulting from aging occurs because the retained hardening solutes in the supersaturated solid solution form precipitates that are finely dispersed throughout the grains and that increase the ability of the casting to resist deformation by slip and plastic flow. Maximum strengthening may occur when the aging treatment leads to the formation of a critical dispersion of at least one type of these fine precipitates.
In addition, in conventional HPDC casting processes, the castings often are slowly cooled to a low temperature, such as below 200° C., prior to removal from the die to quench. This slow cooling to a low temperature significantly diminishes the subsequent aging potential of the casting since the hardening solute solubility decreases dramatically with the decrease in temperature, i.e., the lower the temperature, the lower the solubility. For example, the solubility of magnesium (Mg) in HPDC aluminum alloy A380 is about 0.34% at about 500° C. and decreases to nearly zero at about 200° C. Therefore, the conventional aluminum alloy high pressure die casting processes are ineffective in terms of energy consumption and achievable mechanical properties.