Abstract
Titanium forming processes are often limited by severe adhesive wear as a result of poor friction conditions. This can be partially remedied with careful selection of lubricant, billet preparation and tool coating, but the optimal combination of these factors is not known. A full factorial ring compression experiment, with grade 2 commercially pure titanium rings deformed at 300 °C, was conducted to study the effect of each of these factors over three levels. The change in internal diameter was compared to a set of calibration curves generated by an FEA simulation of the process in order to determine the friction coefficient during each trial. A robust statistical analysis methodology was used to isolate and evaluate the effect of varying each factor. The choice of lubricant was found to be the most statistically significant factor by a considerable margin, followed by the method of billet preparation, with tool coating found to be insignificant. Of the lubricants tested, the graphite-based lubricant resulted in the lowest friction, followed by the WS2- and MoS2-based lubricants. Sandblasted billet surfaces resulted in similar friction to as-machined surfaces, whereas those subjected to micro-arc oxidation performed notably worse. For reducing friction during warm forming of titanium, a graphite-based lubricant is therefore recommended, with tool coating and billet surface preparation unlikely to provide significant further improvement.
Highlights
One of the foremost limiting factors in titanium forming is friction, which leads to severe adhesive wear, resulting in poor surface finish of formed parts or, worse, an interrupted process due to damaged tools
The ring measurements from the experimental trials have been superimposed onto the graphs, allowing for the friction coefficient to be approximated to 3 decimal places for each trial
While the current study provides an indication of which factors are significant when attempting to reduce friction during the warm forming of titanium, it is recommended that further study is carried out to test a wider variety of lubricants, preparation methods and tool coatings for a more complete understanding of the optimum processing conditions
Summary
One of the foremost limiting factors in titanium forming is friction, which leads to severe adhesive wear, resulting in poor surface finish of formed parts or, worse, an interrupted process due to damaged tools. One method of determining friction and the effectiveness of lubricants is the ring compression test. For warm forming of commercially pure titanium (CP Ti), there appears to be no ideal lubricant, but those used are often based on graphite or molybdenum disulphide (MoS2) [2,3,4]. Another option, less prevalent in the academic literature but similar in nature to MoS2 and with arguably better tribological benefits, is tungsten disulphide (WS2) [5,6]. One possible method is sandblasting, which roughens the surface using a stream of abrasive particles [7] Another proposed method is Micro-Arc Oxidation (MAO), sometimes known as Plasma Electrolytic Oxidation. MAO is an electro-chemical oxidation process which, through the creation of plasma
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