Abstract
All fabrication techniques utilized to manufacture metallic parts modifies the surface integrity of the part. Complementary machining is a relatively recent machining strategy which is characterized by combining metal cutting and mechanical surface treatment. Typically, it implies that after conventional machining the cutting insert is used in a reverse direction to modify the surface by local plastic deformation.
Mechanical surface treatment is an additional process step in the process chain of part manufacturing to enhance part performance but typically increasing production time and costs. Hence, different hybrid processes have been developed including complementary machining, which has the benefit of using conventional machine tools and their associated cutting tools.
The machining of titanium has long piqued the curiosity of both the manufacturing sector and the global scientific community. Due to their extraordinary strength-to-density ratios when compared to other materials, titanium alloys find extensive use across a variety of industries. Titanium alloys, in particularly Ti6Al4V (Grade 5 Ti-alloy) has several mechanical advantages, including low density, a high strength-to-weight ratio, exceptional corrosion resistance, thermal stability, and the capacity to maintain high strength at high temperatures. However, these same properties make titanium a difficult-to-machine material. In addition to its low thermal conductivity, titanium is chemically reactive in air and with cutting tools, as a result affecting tool life and the rate at which material is removed which ultimately leads to an increase in machining costs. Titanium alloys are considered metals that exhibits poor machinability, where machinability rate is dependent on tool life, surface integrity and the power consumed during the machining process. These major issues pose significant productivity concerns, particularly in the aerospace and automobile industries.
Various research publications have investigated these challenges. However, it is not currently clear if complementary machining of titanium alloys, and more specifically Ti6Al4V, is viable for modifying the surface to enhance the properties. No evidence of this has been reported.
Friction plays a significant role during complementary machining. Minimum quanty lubrication (MQL) is a cooling/lubrication strategy that may have significant benefits as far as surface integrity is concerned along with good perceived environmental aspects. There is currently little available literature that documents/describe the use and the possible benefits of using MQL during the complementary machining of titanium alloys.
The main aim of the current study is therefore to investigate the viability of complementary machining to modify the surface integrity of Ti6Al4 and assess the possible beneficial effects of MQL.
An extensive literature review was conducted with the focus on the machining of titanium and its alloys, surface modification, cooling, and lubrication during machining and complementary machining. This was followed by an experimental investigation where the viability of complimentary machining to modify selected surface integrity descriptors, when machining Ti6Al4V, was assessed. This was done by evaluating its effects on surface roughness, tool wear, microhardness, and residual stress state, when subjected to three different cooling/lubricating strategies i.e., dry cutting, flood cooling and, MQL. The effect of two different tool types i.e., coated, and uncoated, were also investigated.
iii ABSTRACT | Complementary Machining of Ti6Al4V
The results of this investigation clearly demonstrates that complementary machining may be employed to modify and/or improve selected mechanical properties of the workpiece. The surface finish may be improved along with an increase in hardness (surface) by utilizing conventional cutting tools without subjecting them to increased wear (when compared to conventional machining). The resultant residual stress field may also be modified and improved as far as the perceived fatigue life is concerned by an increase in the resultant stress levels and a beneficial spatial reorientation of its direction. In general, the main conclusions hold true for all the cooling techniques investigated although flood and especially MQL machining seems more suited to obtain the most beneficial results.
In summary, MQL machining with uncoated carbide tools was found most suited for extracting the maximum demonstrated benefit from complimentary machining of Ti6Al4V.
Key words: Complementary Machining, Conventional Machining, Titanium Alloys (Ti6Al4V), Minimum Quantity Lubrication.