Lifetime evaluation of two different hot work tool steels in aluminium extrusion

by C. Sommitsch, R. Sievert, T. Wlanis and C. Redl

During aluminium extrusion, the die experience cyclic thermo-mechanical loads that can lead to materials degradation and failure. For a process optimization and a comparison of different hot work tool steels, the finite element method is an appropriate means. Local inelastic strains result from the interaction of the applied temperature and stress loading and can be computed by suitable inelastic constitutive equations. Stress amplitudes and dwell times during extrusion result in creep-fatigue damage. A lifetime consumption model sums increments of a damage variable over time and defines materials failure as the accumulation of the resulting damage variable to a critical value. The procedure for the identification of the material parameters for both the constitutive and the damage model is described in detail, including the material parameters for the description of time-effects, and applied to the hot work tool steel Böhler W300 ISOBLOC (EN 1.2343). The lifetime consumption for two different hot work tool steels is compared on the basis of an example in aluminium extrusion.

Computational Materials Science

doi:10.1016/j.commatsci.2007.07.054

Keywords: Extrusion; Creep; Fatigue; Viscoplasticity; Aging; Failure

Effect of casting imperfections on the fatigue life of 319-F and A356-T6 Al–Si casting alloys

Effect of casting imperfections on the fatigue life of 319-F and A356-T6 Al–Si casting alloys

H.R. Ammara, A.M. Samuela and F.H. Samuel, a, aDépartement des Sciences Appliquées, Université du Québec à Chicoutimi, 555 Boulevard de l’Université, Chicoutimi, Québec G7H 2B1, Canada

Abstract

Casting imperfections, such as porosity, in cast aluminum components greatly influence their fatigue properties. The effect of porosity on the fatigue life of 319-F and A356-T6 aluminum alloys was studied, where the porosity characteristics on the fracture surfaces of fatigue-tested samples were examined using SEM and image analysis. The results show that porosity has the greatest detrimental effect on fatigue life: 92% of all tested samples fractured as a result of porosity which acted as the main crack initiation site. In the absence of casting imperfections, other microstructural aspects such as slip bands may be held responsible (4%). Porosity was investigated in terms of the pore size at the sample fracture surface. It was found that fatigue life decreases as the size of the surface pore increases. A comparison was made between the fatigue behavior of low-pressure-permanent mold-cast 319 alloy and lost foam-cast A356-T6 alloy. The results show that the 319 alloy provides greater fatigue strength compared to the 356 alloy, which may be explained by taking into consideration the nature of the surface porosity (single pore versus multiple shrinkage pores) that initiated the fatigue crack in the two alloys. The microstructural characteristics are of secondary importance in this regard.

doi:10.1016/j.msea.2007.03.112