Toughness characteristics of as-die-cast am60b alloy

Abstract

The present paper report on an experimental study on the toughness characteristics of the Mg-Al-Mn high purity alloy AM60B, that combines rather good ductility, casting properties and corrosion resistance. Today large thin section components for the automotive industry made of this alloy are industrially produced on large scale. In this context, the need of experimental data on toughness properties of this alloy in its specific microstructtural conditions with which the parts are serviced, i.e. in thin-section as die-cast components, is clear.

A set of experimental fracture toughness tests was carried out at room temperature on specimens machined directly from component produced on large scale in a high pressure cold-chamber machine. During each test the crack propagation was monitored via a potential drop method combined by the classical Johnson calibration equation, after checking its suitability by means of fractographic measurements of crack length at the end of tests. Crack growth resistance curves (R-curves) were obtained for several specimens of thickness in 2.5-3.1 mm range. No effect of specimen thickness was observed. The stress intensity factor leading to unstable crack propagation was determined for each specimen, being of about 20 MPam1/2.

This crack extension properties of die-cast components are reasonably related not only to the chemical composition and microstructure of the alloy (this latter severely affected by cooling conditions), but also to the presence of defects such as inclusions or porosities and to temperature. The results obtained in the present experimental research, in particular the critical stress intensity factor, will thus be useful to compare the effects of different process parameters or of heat treatments or to check the effect of temperature, not as an absolute values of toughness easily extendable to thicker components.