EFFECT OF SUPERIMPOSED PRESSURE ON DAMAGE EVOLUTION AND FRACTURE BEHAVIOUR OF AUTOMOTIVE ALUMINUM ALLOYS

Abstract

This study examined the damage evolution and fracture behaviour oftwo aluminum automotive alloys as a function ofvarious material and testing variables. When applicable, the observations were plotted in stress-strain space in order to construct fracture mechanism maps as functions ofpressure and temperature. The influence ofiron content was examined through the use oftwo different iron contents for each ofthe two alloy systems. For the solution hardened 5754 alloy, it was found that the presence ofincreasing iron content led to more rapid void nucleation, growth and coalescence which produced a decrease in ductility, and change in fracture mode from cup and cone to shear MVC. No change in fracture mode was observed with increasing iron content for the age hardenable 6111 alloy in the T4 temper. However, a decrease in ductility was observed. The Iow-Fe 6111 T6 alloy was found to fail intergranularly, while the high-Fe variant was observed to fail by shear MVC. It was proposed that this was the result ofthe grain refining effects ofthe increased iron content. The evolution of damage was examined by performing uniaxial tensile test under superimposed hydrostatic pressure. In this way, the amount ofdamage generated within the sample during straining could be controlled and its effect on the fracture process examined. The application ofpressure was found to decrease the amount ofdamage present in all samples tested and universally increased the observed ductility. When tested under sufficient pressure, it was found that the 5754 and 6111 T4 alloys would change fracture modes and fail by ductile rupture, with the damage processes suppressedor completely eliminated. Pressure was found to suppress the intergranular fracture mode in the Iow-Fe 6111 alloy, causing a transition to the shear MVC mode offailure. The effect oftemperature on the flow behaviour ofthe 5754 was also examined, as this alloy was found to exhibit the temperature sensitive Portevin Le Chatelier effect. The serrated yielding associated with the effect was found to be greatly reduced testing at 77 K, and grain scale and large scale shear processes within the material were observed to be almost completely eliminated. This was found to lead to large increases in uniform elongation, and a change in fracture mode from cup and cone to shear MVC for the IowFe variant.