The effect of solidification time on the mechanical properties of a cast 319 aluminum alloy

The increased use of cast aluminum for automotive components has dictated the need for more detailed information relating the effects of processing on the mechanical behavior of cast aluminum. It is known that solidification time can have a large effect on the microstructure, especially on the secondary dendrite arm spacing (SDAS), the size, and the distribution of porosity. A study was conducted to determine how the mechanical properties of a high-Si version of the 319 aluminum alloy (designated W319 Al) are affected by solidification time. A wedge-shaped casting was developed to provides samples with a SDAS of 10--100 mum and a porosity range of 0.1--2%; both porosity-containing (non-HIP) and porosity-free (HIP) samples were produced. The samples were heat-treated to either a T6B ("Peak-Aged") or T7B ("Overaged") condition. Multiple tensile and stress-controlled fatigue tests were performed so that the results could be statistically compared.; The results show that solidification time has a large effect on the tensile and stress-controlled fatigue properties of W319; in general, all of the mechanical properties decreased as solidification time increased. Heat-treatment also had a dramatic effect on tensile properties and the fatigue properties of the HIP samples. However, the fatigue life of the non-HIP samples remained unaffected by heat-treatment; in these samples, microshrinkage porosity was associated with all of the fatigue failures and was located at or near the specimen surface. Analysis revealed that as solidification time increased, the average initiating pore diameter increased and fatigue life decreased. Quantitative measurements of porosity determined that conventional metallographic techniques underreport the pore sizes present in the W319 alloy; measurements from tensile surfaces were found to correlate well with the sizes of the pores that initiated cracks in the failed fatigue specimens.; The data on initiating pore size was used to evaluate a short crack-based fatigue life prediction model. The model provides reasonable predictions of S-N behavior and run-out stresses in the W319-T7B alloy for a broad range of stress amplitudes and solidification times. The model yields close agreements between predicted and experimental lifetimes when porosity is the dominant fatigue-initiating feature in the microstructure of the W319-T7B samples.