Abrasive water-jet : controlled depth milling of titanium alloys
Abrasive waterjet (AWJ) technology is used in a routine manner in manufacturing industry to cut materials that are difficult to cut by other methods. Whilst the technology for through cutting of materials is mature, the process is also being developed for controlled depth milling (CDM) of materials. The aerospace industry have a requirement to remove redundant material from components manufactured from difficult to machine Ti6Al4V and titanium aluminide alloys and thus reduce component weight. The two main processes available to facilitate this are chemical milling and AWJ-CDM. The two processes have the advantage that they impose negligible forces, thus allowing flexible structures to be processed. However, the process of chemical milling is under threat due to the high costs associated with the disposal of the spent acids. Thus, this research seeks to evaluate the A WJ-CDM process as a replacement for chemical milling for Ti6A14V and titanium aluminide alloys. The magnitude and effect of the process characteristics of chemical milling on fatigue life are well established; however, this is not the case for AJW-CDM. The aerospace industry considers the characteristics of surface roughness, grit embedment and surface morphology to be significant parameters in determining the fatigue life of components manufactured using AJW-CDM. Therefore, before AJW-CDM can be considered a viable alternative, the effect of the process variables on the workpiece characteristics have first to be established. The current research has determined the role of a number of process parameters on the material removal rate, roughness and waviness, grit embedment and surface morphology in the AJW-CDM of Ti6AI4V and titanium aluminide. Nozzle traverse speed and jet impingement angle are shown to govern the operative mechanism of material removal and thus the material removal rate. It is also shown that the surface waviness can be reduced as the traverse speed is increased and as the jet impingement angle is decreased, but it should be noted that waviness increases with number of passes of the jet over the workpiece. The surface roughness is not strongly dependent on traverse speed. Surface waviness and roughness are strongly dependent on jet impingement angle; significant reductions are possible by employing low angle milling techniques. Smaller sized grit leads to a reduction in material removal rate but also to a decrease in both waviness and roughness. It has been demonstrated that grit embedment can be minimised either by milling with a high jet traverse speed at low impingement angles or by low speed milling at jet impingement angles up to 45°in the backward direction only. However, even in the best cases, 5% of the area of a milled surface comprised of embedded grit. Surface morphology can either exhibit directional grooving or cratering, depending upon complex interactions of the various processing parameters. The understanding of the role of various process parameters on the workpiece characteristics will allow the process parameters to be optimised for given requirements. Future work needs to examine the fatigue performance of the AJW-CDM structures, and again optimisation of the processing parameters to maximise fatigue life can be performed. Masking has been employed to provide an economic manufacturing solution for the AJW-CDM process for a specific component. Thus, AJW-CDM has been established as a potential replacement process for chemical milling.