Development of novel heat-treatable magnesium alloys for low-pressure twin-roll casting

Abstract

Magnesium alloys are ideal for light weighting, especially given the continued pressure on the automotive industry to reduce CO2 emissions while increasing the fuel efficiencies. Major contribution to weight savings in automotive applications can be made using sheet product. However, due to hexagonal crystal structure magnesium is inherently difficult to deform and a strong basal texture forms during large scale deformation such as rolling or extrusion. Twin-roll casting (TRC) process provides a pathway to producing sheet material by casting directly into thickness close to final thickness, opening a path to produce sheet magnesium. The conventional sheet production routes, such as rolling, requires many homogenisation steps and small rolling passes which is energy-intensive, time consuming and ultimately not economical. Currently, majority of TRC research is based on magnesium alloy AZ31 (Mg-3Al-1Zn (wt%)) which can only be strengthened by grain size control. The new alloy development for conventional twin-roll casting considers alloys with very dilute concentrations that relies on grain size strengthening as the main strengthening mechanism. However, the low-pressure TRC developed within BCAST, relies on solidification to control the microstructure. This process minimises the severe segregation issues associated with conventional TRC and produce a thin strip that may be directly stamped. This requires a different approach to alloy design as the constraints associated with the conventional TRC process no longer controls the microstructure evolution. The approach used in the design of alloy compositions considers both the availability of the additions as well as individual and combined effect of these additions on both mechanical properties’ enhancement and texture modification of the TRC strip. Alloying elements and compositions that lends to precipitation hardening following final forming are chosen to provide enhanced strengthening. The thesis reports the development of novel thermo-mechanically processed low pressure TRC magnesium alloys based on Mg-1Al-1Zn-0.3Mn, Mg-1Al-4Zn-0.3Mn and Mg-4Al-1Zn-0.3Mn, with and without Ca additions. Alloys show enhancement of age hardening response following small scale deformation at room temperature which allows enhancement of the mechanical properties of the low-pressure TRC alloys. The addition of calcium to Mg-1Al-1Zn-0.3Mn led to rapid peak ageing response within 1 hour and significant grain refinement of 48%, this resulted in grain size reduction from 238±9μm to 114±7μm. The greatest increment in age-hardening was observed through increasing concentration of zinc to 4%wt with calcium containing alloy. At 48hrs, a hardness value of 71HV was achieved, an increment of 25HV from as-quenched condition. TEM micrographs of peak-aged Mg-1Al-4Zn- 0.3Mn and Mg-1Al-4Zn-0.3Mn-0.3Ca showed precipitates are mainly rod like precipitates that form parallel to the <0001> direction. In low-pressure TRC Mg-1Al- 1Zn-0.3Mn-0.3Ca, the columnar grains measured to approximately 153±13μm with minimal sign of centre line segregation. T6 treatment of low-pressure TRC Mg-1Al- 1Zn-0.3Mn-0.3Ca drastically improved yield strength from 93.5MPa to 140.3MPa. As for low-pressure TRC Mg-1Al-4Zn-0.3Mn-0.3Ca it was greater as the yield strength increased from 85.1MPa to 171.4MPa.