Abstract
Titanium’s remarkable physical properties of high strength, low density, and a high level of corrosion resistance that is only matched by a small number of naturally occurring materials, makes it suitable for use in a wide range of industrial applications. However, the metal is difficult and costly to extract from its ore minerals due to titanium’s high affinity for oxygen, and to fabricate into semi-finished or finished products. Powder metallurgy is understood to be a good alternative to ingot metallurgy, cost-wise, in forming net shaped titanium products. In ideal instances, powder metallurgy is a three-stage process i.e., powder production, powder compaction, and sintering. Sintering is an energy intensive process and sometimes it is not economically wise, and/or metallurgically wise to prolong the process to reach full densification because of massive grain growth that is bound to occur under prolonged sintering conditions.
The aim of this study was to investigate cold rolling as a post-sintering process that can be utilized to achieve densification. The experimented material was titanium brown compacts. The brown compacts were formed from a commercially pure grade 2 titanium powder (100 mesh) formed by the hydride-dehydride process and roll compacted to green strips with an average relative density of 85.6 %, and then sintered under an inert argon atmosphere to an average relative density of 89.3 %.
The different cold rolling parameters that were investigated were the roll speed, percentage reduction, and the number of roll passes. It was found that cold rolling significantly increases the density of titanium brown compacts that have a starting relative density of 89.3 %. The maximum relative density obtained in this study was 97.7 %. The 97.7 % was an average across the width of the rolled strips and it was obtained without any annealing. The roll speed, number of roll passes, and percentage reduction played a significant role on how the rise in density was achieved. The rate of densification increases with a decrease in roll speed, and with an increase in the percentage reduction. The lowest number of roll passes needed to reach the maximum average relative density of 97.7 % were observed at slow roll speeds. About 7 roll passes are sufficient to reach the relative density of 97.7 % at the roll speed of 1 rpm and at 50 % reductions. Faster rolling at the speed of 10 rpm resulted in reduced
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densification and a higher level of thickness springback. It was observed that brown compacts that were cold rolled at high percentage reductions were much more susceptible to center splitting and severe edge cracking than those rolled at lesser percentage reductions.
The results that were obtained from this research study improves the understanding of plastic deformation via cold rolling as a means of achieving post-sintering densification. This research adds to the availability of more manufacturing technology on powder metallurgy which will assist in improved decision making when it comes to the economics and process consideration in the manufacturing of titanium products.