Machining of Titanium Alloys

This chapter reviews the main difficulties impairing the machinability of titanium alloys. The overview of machinability of titanium alloys is presented with respect to the following performance criteria: cutting tool wear/tool life, cutting forces, chip formation, and surface integrity attributes, mainly surface roughness. Thereafter, the effects of various lubrication and cooling methods in machining titanium alloys is also discussed. Furthermore, a case study on the metallic particle emission when machining Ti-6A1-4V is also presented.

The chip formation in machining operations is commonly accomplished by a combination of several elements working together to complete the job. Among these components, cutting tool is the key element that serves in the front line of cutting action. Cutting action becomes a challenge when it comes to machining difficult-to-cut materials. Titanium and its alloys are among the most difficult-to-cut materials which are widely used in diverse industrial sectors. This chapter aims to provide a historical background and application of different cutting tools in machining industry with a main focus on the applicable cutting tools in machining titanium and titanium alloys. Selection of appropriate tool material for a certain application is directly influenced by the characteristics of material to be machined. In this context, a brief overview of the metallurgy of titanium and its alloys is also presented. Recent progresses in tool materials, appropriate tools for cutting titanium alloys, and their dominant wear mechanisms will also be covered in this chapter.

Titanium is widely used material in advanced industrial applications such as in aeronautics and power generation systems because of the distinguished properties such as high strength and corrosion resistance at elevated temperatures. On the other hand, the machinability of this material is poor. Relatively low thermal conductivity of Titanium contributes to rapid tool wear, and as a result, high amounts of consumable costs occur in production. Therefore, understanding the mechanics of titanium machining via mathematical modeling and using the models in process optimization are very important when machining Titanium both in macro and micro scales. In this chapter, mechanical effect of process parameters in five axis milling and micro milling are analyzed. Thus, different cutting conditions were tested in dry conditions and the effects of tool orientation on cutting forces in five axis macro milling was investigated. For five-axis ball end milling operation, a series of experiments with constant removal rate and different tool orientation (different lead and tilt angle) were conducted to investigate the effect of tool orientation on cutting forces. The aim of the tests was finding the optimum orientation of the cutter in which the normal cutting force applying on machined surface is minimum. Moreover, a new method to predict cutting forces for micro ball end mill is presented. The model is validated through sets of experiments for different engagement angles. The experiment and the simulation indicated that the tool orientation has a critical effect on the resultant cutting force and the component that is normal to the machined surface. It also possible to predict the tool orientation in which the cutting torque and dissipated energy is minimum. In micro milling case, the force model for ball end mill is able to estimate the cutting forces for different cutting conditions with an acceptable accuracy.

The current research deals with the analysis of physical cutting mechanisms involved during the machining process of titanium alloys: Ti-6Al-4V and Ti-55531. The objective is to understand the effect of all cutting parameters on the tool wear behavior and stability of the cutting process. The investigations have been focused on the mechanisms of chip formation and their interaction with tool wear. At the microstructure scale, the analysis confirms the intense deformation of the machined surface and shows a texture modification. As the cutting speed increases, cutting forces and temperature show different progressions depending on the considered microstructure Ti-6Al-4V or Ti-55531 alloy. Results show for both materials that the wear process is facilitated by the high cutting temperature and the generation of high stresses. The analysis at the chip-tool interface of friction and contact nature (sliding or sticking contact) shows that the machining Ti-55531 often exhibits an abrasion wear process on the tool surface, while the adhesion and diffusion modes followed by coating delamination process are the main wear modes when machining the usual Ti-6Al-4V alloy. Moreover, the proposed study describe the real effect on machining of the tool geometry, coating and lubrication. Finally, the investigations allow to identify some ways to improve the machinability of these alloys, particularly the Ti-55531 alloy.

Although many studies have been done on the MQL applications in different machining process, there are few of studies on the influences about selections of MQL parameters. Different parameters of MQL system have different influences on the milling force and milling temperature, which are closely connected to the lubrication and the coolant. The cutting force and temperature play significant roles in the improving/reducing cutting quality of workpiece and extend/shorten tool life, by changing different parameters of MQL system. This chapter presents an experiment of end-milling titanium alloy with MQL system, discussing the influences of different parameters. One object of the experiment is to investigate the influences of Ti-6Al-4V. The results of experiment will help to understand the influences of selecting different parameters on the end-milling process. Another object is to apply the selected optimal parameters in the former object to milling Ti-6Al-4V.

In this chapter we discuss the nuances of a non-conventional machining technique known as ultrasonically assisted machining, which has been used to demonstrate tractable benefits in the machining of titanium alloys. We also demonstrate how further improvements may be achieved by combining this machining technique with the well known advantages of hot machining in metals and alloys.