STRESS-STRAIN MODELLING INFLUENCED BY POROSITIES IN CAST ALUMINIUM ALLOYS

摘要

Due to the enhancing utilization of aluminium, especially in the automotive and telecom industries, it is of significance to enlighten the influence of defects on the tensile behaviour of aluminium cast alloys. Typical defects can be entrapped gas and or shrinkage porosities, oxide films or unwanted brittle phases as the iron rich β-phase. Recent research show that cracks can be initiated and propagated at these defects and a lot of comprehensive models on how these defects control the final mechanical properties have been developed. Most researchers agree on which parameters that affect the strength of a material, but there is a need for more research when it comes to the questions on how the parameters affect the material and which parameters that have the largest influence. This research paper will focus on the effect of microporosity on the tensile behaviour of Al-Si based cast alloys. A three-dimensional solid FE-model, in the commercial FE-code ABAQUS, is developed in order to predict how the volume fraction, location and the relative size of the spherical pores affect the tensile properties. The model is parametrisized regarding adjustable material data such as the strain hardening exponent n and strength coefficient K, and adjustable porosity data such as volume fraction, distribution and relative size of the pores. As for the outcome of the result i.e. the result variable, the model uses the elongation when necking first occurs in the material during a tensile test. The result shows that the strain-hardening exponent has the largest effect on the result. An increased value of n provides a more porosity-tolerant material. The highest concentrations of plastic-strain and stress occur in the vicinity of the largest pores in the material throughout the tensile test. The volume fraction of porosity has the largest effect on the result variable. A higher volume fraction of porosity gives a reduced value of the result variable. According to the model, as well as to literature, the most porosity-tolerant material is achieved when the strain-hardening exponent n is as high as possible, the volume fraction of porosity is as low as possible, and the pores are distributed in the tensile direction.