Waterjet Turning of Titanium Alloys

Abstract

Titanium alloys offer outstanding properties with regard to its strength to density ratio and a good corrosive resistance in air atmospheres. Substantial advancements could be made by using titanium alloys, in particular for applications in the aerospace industry and medical engineering. However, no product innovation is possible without an appropriate machining technology. For example, low thermal conductivity and hot hardness lead to limitations regarding the applicable machining parameters, particularly for continuous cutting operations. Turning of high performance materials sets high demands on machine tools and especially on the used cutting tools. For conventional continuous cutting of titanium alloys the tool life time and therefore the tool life volume is limited due to the thermal mechanical behaviour. Depending on the chemical and structural composition of the alloy, conventional cutting operations can rarely be regarded as an economic solution. The Abrasive Waterjet Turning process (AWJT) represents a promising alternative manufacturing method to produce rotation-symmetrically or helical parts made of difficult to machine materials. The AWJT process combines the kinematics of conventional turning methods with process-specific advantages of the abrasive waterjet machining. The main advantages are the high variety of machinable materials, the long life time T of the focus nozzles of at least 300 minutes and its independence of the material to be processed. Furthermore, material-inhomogeneity or the initial geometrical contour of the workpiece cannot result in tool failures. An interaction of workpiece and tool known from conventional cutting processes cannot occur. An investigation on hyper eutectic aluminium alloys has shown that AWJT is an economic manufacturing process regarding the resulted material removal rates Qw and tool life volumes. The resulting roughnesses and roundnesses are comparable to a rough turning operation. In addition, AWJT results in a lower hardness penetration depth tw in comparison to conventional turning. Machining of titanium alloys with cylindrical and external turning operations as well as grooving is the next step in the experimental investigation of the machinability of difficult to machine materials with AWJT. Therefore, the objective of the presented work is to provide a model for predicting the material removal rate, the cylindrical roundness and the surface roughness of waterjet turning of the titanium alloy Ti6Al4V. In a screening experiment the significant setting parameters were identified and an adequate range of parameter settings for the response surface study was determined. The tested parameters were the feed rate vf, the abrasive flow rate m and particle size dp, the depth of cut dc and the rotational speed n of the workpiece. It is shown that in relation to the material removal rate Qw linear main effects as well as interaction effects are significant. The developed second-order-regression-model includes these linear main and interaction effects and the quadratic effects of the relevant setting parameters. Furthermore, the achieved material removal rates, tool life volumes, cylindrical roundness and surface quality are used as target values. Additionally the changes like plastic deformations and grain damages in the rim zone were compared to conventional machined parts. Relating to the material removal rate Qw, up to 2.5 cm³/min could be achieved for AWJT at a maximum height of profile Rz below 100 microns. Furthermore, the investigation resulted in a maximum tool life volume of 750 cm³ at a given nozzle life time. The results show that AWJT can be used as an economic alternative manufacturing process for rough turning of titanium alloys.