Processing of Commercial Purity Titanium, Aluminium and Al-5Zn-1Mg Alloy by Equal Channel Angular Pressing

Abstract

Severe plastic deformation (SPD) is a metal forming process in which a very large plastic strain is imposed on a billet in order to make an ultra-fine grained metal. Among all SPD processes, equal channel angular extrusion or pressing (ECAE/P) is an attractive processing method because of its simplicity and the possibility to scale up the technique for use in industrial applications. ECAE/P was originally developed by Segal et al. in the beginning of the 1980s’ to introduce a homogeneous simple shear deformation into billets without any change in its dimensions. There are various types of ECAP that have been developed and applied in the production of fine grained structures like ECAP of rods, bars, tubes etc. Many industrially important materials such as commercially pure (CP)-Ti and its alloys, CP-Al and its alloys, etc. have been processed by ECAP. In the present study, ECAP of CP-Ti rod in the wrought form, CP aluminium rod and tube in cast and wrought forms and Al-5Zn-1Mg alloy rod and tube in cast form using a die channel angle of 150˚ were investigated for various passes using four fundamental processing routes: route A where the sample is pressed repetitively without any rotation, route BA where the sample is rotated by 90˚ in alternate directions between consecutive passes, route BC where the sample is rotated in the same sense by 90˚ between each pass and route C where the sample is rotated by180˚ between passes. Mechanical properties of as-received and as-pressed billets after each pass were determined by Vickers hardness and tensile tests. Frictional property was determined by ring compression testing. The as-received microstructure and its evolution due to ECAP were characterized by optical microscopy. Failure analysis of fractured billets was characterized by scanning electron microscope (SEM). Compression testing was carried out for as-received and as-pressed billets to determine the flow properties before and after ECAP. Peak extrusion pressure was estimated for each pass for various routes by carefully recording the peak force for each pass.Processing of CP-Ti at room temperature resulted in improved strength with reduction in ductility. In spite of increased strength the peak punch pressure decreased as the number of passes was increased to three. Processing of CP-Al rod in the cast form at room temperature resulted in improved strength. Flow properties like strength coefficient (K) and strain hardening exponent (n) were also found to increase as number of passes was increased to two. The peak punch pressure increased for second pass compared to the first pass. CP-Al rod in the cast form failed in the third pass. A failure analysis of the same was carried out with the help of SEM. Processing of CPAl rod in the wrought form at room temperature resulted in improved strength with reduction in ductility. The peak punch pressure increased as the number of passes was increased to four. Al-5Zn-1Mg rod in the cast form failed during the second pass for all the temperatures selected for the study (303-673K). Friction factor (m) for Al-5Zn- 1Mg was determined in the temperature range of 303-673K. The maximum plasticity was observed in the temperature range of 373-573K where a hardness improvement is also seen. Processing of CP-Al tube in the wrought form at room temperature resulted in improved hardness. The peak punch pressure increased as the number of passes was increased to three. CP-Al and Al-5Zn-1Mg tube in cast form could not withstand even a pass. Failure analysis was carried out and crack propagation was observed clearly. Out of the four routes employed in the study route B (BA or BC) showed superior mechanical properties in all the chosen metals and alloy. As far as peak punch pressure is concerned route A showed the least punch pressure for CP-Ti after three passes, whereas for CP-Al, route C showed lowest punch pressure except in tubular form where route A showed the minimum punch pressure. From the present study it is clear that the different microstructural parameters for grain refinement criterion may give rise to different conclusions on the effectiveness of deformation route.