Development of a titanium sheet manufacturing process via direct powder rolling and spark plasma sintering.

Abstract

The intention of this research was the improvement of titanium processing in South African industry. South Africa currently has limited ability to efficiently process its titanium reserves and the results of this research, combined with other work in the associated research consortium, has the potential to lead to significant positive economic impact. A novel method of titanium processing, combining the processes of direct powder rolling (DPR) and spark plasma sintering (SPS), was explored in the course of this research. A rolling mill was designed using modelling and simulation techniques, manufactured based on the resulting design, and the DPR-SPS process parametrically tested on the rolling mill using commercially pure titanium powder. The mechanical aspects of this project included experimental testing on a range of titanium powder samples to determine the properties of the powder, modelling of the rolling mill behaviour using MATLAB, 3D modelling of the proposed and iterated rolling mill frame design and its components using Siemens NX and OnShape, finite element analysis of the rolling mill frame and auxiliary components using Siemens NX, and manufacture and parametric testing of the mill for titanium powder compaction. The electrical aspects of this project included connecting and programming a variable speed drive and AC motor to control the speed of the mill rolls, simulating the behaviour of the integrated SPS-type sintering circuit using Simulink, designing a suitable method for safely and effectively transmitting the large SPS current to the rotating rolls, and building and testing the circuit for titanium compact spark plasma sintering. The manufactured direct powder rolling mill compacts titanium powder into strip through a pair of rolls, measuring 350mm diameter and 50mm width. Each of the rolls is mounted on the same shaft as a worm-driven gear. A 5.5kW three phase AC motor drives the worm shafts, which have opposing threads to ensure the rolls rotate in opposing directions. The worm and gear arrangement serves to both evenly transmit the drive power to both rolls, and to increase the torque from the motor to the rolls. The motor is controlled using a variable speed drive – this allows the roll speed to be adjusted as necessary, to optimise the consolidation process. The bearings used on the roll and worm shafts were designed and manufactured using an insulating bearing material to ensure the current used for sintering is not instead transmitted through the steel rolling mill frame. Flexible couplings with polymeric inserts isolate this current from the motor and between each worm gear. The minimum density of the green compact required for further handling was determined empirically as being greater than 65 % of the theoretical density of titanium. The mill was designed for an optimal density of 81%. The density range achieved by varying the parameters of roll speed, number of passes through the mill, and roll gap, was 55 – 84%. It is expected that the strip density may be greatly increased with implementation of the improvements and parameter changes identified. The spark plasma sintering circuit sinters the titanium either during the direct powder compaction process (simultaneous DPR-SPS), or following it (sequential DPR-SPS) during re-rolling through the same set of rolls used for compaction. A number of electrical circuits with different output types were designed for the SPS process; one of these was built and tested towards proof of concept of the DPR-SPS process. The tested circuit uses a DC source to apply a sintering current through the titanium sample. Application of the SPS current to the titanium resulted in a theoretical density increase of 11 –14% compared to DPR only, depending on whether the processes were performed sequentially orsimultaneously This multidisciplinary project employed a broad range of combined materials, mechanical and electrical design and analysis methods. The resulting mill can be used as is for further parametric testing, improved as will be discussed for the same process, or adapted to a range of different applications.