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Magnesium alloys are becoming more commonly used for large castings with sections of varying thicknesses. During subsequent processing at elevated temperatures, residual stresses may relax and become a potential mechanism for part distortion. This study was conducted to quantify the effects of thermal exposure on residual stresses and relaxation in a high pressure die cast magnesium (AM60) alloy. The goal was to characterize relaxation of residual stresses at temperatures that are commonly experienced by body components during a typical paint bake cycle. A residual stress test sample design and quench technique developed for relaxation were used and a relaxation study was conducted at two exposure temperatures (140°C and 200°C) over a range of exposure times (0.25 to 24 hours). The results indicate that a significant amount of residual stress relaxation occurred very rapidly during exposure at both exposure temperatures.

Magnesium die-cast alloys are known to have a layered microstructure composed of: (1) An outer skin layer characterized by a refined microstructure that is relatively defect-free; and (2) A “core” (interior) layer with a coarser microstructure having a higher concentration of features such as porosity and externally solidified grains (ESGs). Because of the difference in microstructural features, it has been long suggested that removal of the surface layer by machining could result in reduced mechanical properties in tested tensile samples. To examine the influence of the skin layer on the mechanical properties, a series of round tensile bars of varying diameters were die-cast in a specially-designed mold using the AM60 Mg alloy. A select number of the samples were machined to different final diameters. Subsequently, all of the samples (as-cast as well as machined) were tested in tension.

A demonstration structure was cast in AM60. The structure, known as the Generic Frame Casting or GFC, was designed specifically to mimic features seen in castings for closure applications. Excised samples were subsequently removed from different areas of the casting and tested under axial loading conditions. Component level tests were also conducted. Comparison of the excised sample results and the component level testing indicated the influence of local properties on the component level deformation. It was shown that varying the casting processing conditions could change the local ductility and yield strength in different areas of casting with the same geometry. Lowering the local ductility decreased the total displacement in a component level test and lowered the amount of energy absorption. Therefore, understanding the processing conditions and their influence on the local properties is important for predicting behavior in a component level test.

A study was conducted to examine the effect of solidification time and solution treatment time on the tensile properties of a 319-type aluminum alloy. Tensile samples with solidification times ranging from 0.3 to 35.5 minutes were solution-treated at 495 C for 8 hours and for 240 hours. All samples were then water-quenched and aged at 260 C for 4 hours. The tensile results show that solidification time and solution treatment time can have significant effects on the tensile properties. In general, as the solidification time increased, the ultimate strength, yield strength, and ductility decreased; increasing the solution-treatment time from 8 to 240 hours improved only the tensile strengths. The amount of Cu available in solid solution to precipitate during aging is found to be a key factor. Additionally, coarse microstructures require very long (and commercially-impractical) solution-treatment times to significantly improve the tensile strengths.

Quantification of residual stresses is an important engineering problem impacting manufacturabilty and durability of metallic components. An area of particular concern is residual stresses that can develop during heat treatment of metallic components. Many heat treatments, especially in heat treatable cast aluminum alloys, involve a water-quenching step immediately after a solution-treatment cycle. This rapid water quench has the potential to induce high residual stresses in regions of the castings that experience large thermal gradients. These stresses may be partially relaxed during the aging portion of the heat treatment. The goal of this research was to develop a test sample and quench technique to quantify the stresses created by steep thermal gradients during rapid quenching of cast aluminum. The development and relaxation of residual stresses during the aging cycle was studied experimentally with the use of strain gauges.

Previous studies on the effect of Mg on the hardness of 319-type alloys are contradictory. The present study was conducted in an attempt to resolve this confusion and allow for a more rational choice of Mg concentration specifications. Four 319-type alloys were prepared with the following target Mg concentrations: 0.00, 0.15, 0.35 and 0.45 wt%. The addition of only 0.15 wt% Mg had a significant effect on the hardness of the alloy but further incremental additions of Mg did not produce the expected trends in hardness. Two hypotheses for this unexplained behavior are presented. This work suggests that the Mg concentration can be allowed to vary between 0.15 wt% and 0.45 wt% without significantly impacting the aging response (hardness) of the alloy.