Solidification processing, thermo - mechanical treatment and structure - property correlation of an Al-Zn-Mg-TiO2 particulate composite

Abstract

Metal-Matrix Composites(MMCs) have been investigated since the early 1960s, with the main emphasis then being the high potential structural properties engineered to specific applications. Both the fibre and particulate reinforced MMCs exhibited substantial improvements in stiffness and strength over the un-reinforced alloys. However, due to high cost of fibres coupled with the complication of dispersing them in the alloy, attention has been diverted to more cost effective reinforcements and near net shape technology. This has resulted in the development of MMCs with inexpensive particulates as the effective dispersoids. Furthermore, these MMCs have demonstrated isotropic properties and hence significant improvements in performance. However, scant attention has been paid in the literature to the methodology and process parameters related to the optimum use of the available techniques, in the synthesis and characterisation of ceramic and particulate reinforced wrought Al-Zn-Mg alloys.

The present research involves a study of: 1. Synthesis of wrought Al-Zn-Mg-TiO2 particulate composite with special emphasis on wettability, casting routes and the suitability of incorporation of TiO2 in wrought aluminium alloy. 2. Various parameters affecting the secondary processing of composites such as forging, extrusion and rolling. 3. Room temperature and elevated temperature mechanical properties and other physical properties. Both Rheocasting (RC) and Liquid Metallurgy (LM) casting routes were used to synthesise the composites. During synthesis by either process, it was observed that, there was an exothermic reaction between Mg and TiO2. This exothermic reaction raised the temperature of the melt by 50 K in RC route and 80 K in LM route, keeping the melt temperature sufficiently high for a longer time to facilitate simultaneous addition of other dispersoids as well. Reaction zones were observed around the dispersoids and the thickness of the zones were measured as 3 µm in LM cast billets and 1 µm in RC billets. The reaction products were analysed to be rutile grade TiO2 and Magnesium dititanate (MgTi2O5).

Application of RC route was found to be better than LM route since RC route resulted in better distribution of the dispersoids with refined and equiaxed cells. The cross section of the RC billet exhibited 85% of the area to be of chill zone with very fine grains and 15% of the area to be of equiaxed grains with no columnar grains in the structure. Whereas, LM billets exhibited 64% chill zone, 27% columnar grains and 9% equiaxed grains. About 94% of the particle agglomerates in RC billets were of size below 50 µm, with average particle size 86.6 µm.

The secondary process involving forging resulted in opening up of the particle clusters and subsequent pull out, ending up with surface cracks. This suggested that the particles are poorly bonded in the matrix. This observation is in good agreement with the reported data on Al / ceramic composites. However, extrusion process appeared to be more promising with practically no defects in the extruded products. The tensile strength of the solid extruded rods in T6 heat treated condition, was upto 300 MPa, comparable to the theoretical prediction of 340 MPa. The percentage elongation was 14% and comparable to that of the base alloy. Both hot rolling and cold rolling experiments were conducted on the composite and the base alloy. Composite exhibited good hot workability comparable to that of the base alloy. Cold workability was found to be much superior to the composite. Composite has been cold rolled to a foil of thickness 0.1 mm without even an intermittent annealing. This was possible by the continuous de-agglomeration of the weakly bonded particulates and the reduction in the flow stress beyond 75% reduction in thickness. The particulates got aligned parallel to the direction of rolling. Maximum strength level of 540 MPa was observed at 75% cold reduction. This was 60% higher than the theoretical prediction and about 22% higher than that of the base alloy in the same condition. This increment in strength was primarily due to the particle-dislocation interactions, formation of dislocation tangles and formation of sub-grains. Further cold work beyond 75% however decreased the strength level due to relaxation of dislocation tangles. Mechanical properties of the cold rolled sheets in heat treated condition (333 MPa) was very close to the theoretical prediction (340 MPa). The high temperature tensile strength of the composite at the test temperature of 473 K was significantly high showing about 210 MPa, more than 100% above that of the base alloy at the same test temperature. Young’s Modulus and density of the composite and the base alloy are comparable.