Investigation of the Machining of Titanium Components for Lightweight Vehicles

Abstract

Due to titanium’s excellent strength-to-weight ratio and high corrosion resistance, titanium and its alloys have great potential to reduce energy usage in vehicles through a reduction in vehicle mass. The mass of a road vehicle is directly related to its energy consumption through inertial requirements and tire rolling resistance losses. However, when considering the manufacture of titanium automotive components, the machinability is poor, thus increasing processing cost through a trade-off between extended cycle time (labor cost) or increased tool wear (tooling cost). This fact has classified titanium as a “difficult-to-machine” material and consequently, titanium has been traditionally used for application areas having a comparatively higher end product cost such as in aerospace applications, the automotive racing segment, etc., as opposed to the consumer automotive segment. Herein, the problems associated with machining titanium are discussed, and a review of cutting tool technologies is presented that contributes to improving the machinability of titanium alloys. Additionally, nonconventional machining techniques such as High Speed Machining and Ultrasonic Machining are also reviewed. Also discussed are additional factors that need to be considered especially pertaining to the machining of titanium alloys, a crucial one being the non-conformity with standard tool wear models. Subsequently, the results of a controlled milling experiment on Ti-6Al-4V is presented, to evaluate the relationship between certain tool preparation/process parameters and tool wear for a comparison with traditional wear models. INTRODUCTION Titanium is the seventh most abundant metal and the fourth most abundant structural metal in earth’s crust behind aluminum, iron and magnesium. Titanium and its alloys are considered as alternatives in many engineering applications due to their superior properties such as retained strength at elevated temperatures, high chemical inertness and resistance to oxidation. Titanium has traditionally been utilized as a lightweight, very strong and exceedingly corrosion resistant material in the aerospace industry, electric power plants, seawater desalination plants, and heat exchanges. Also, it has been used in industrial applications such as petroleum refining, nuclear waste storage, food processing, pulp and paper plants, and marine applications [1]. Nevertheless, when considering the use of titanium as an automotive component material, there are several conflicting aspects that must be addressed. First of all, the cost of titanium is relatively high in comparison to other common engineering materials such as aluminum, magnesium, and steel. For this reason, it specifically calls for implementation and use only when extreme conditions are to be met, such as in the aerospace industry. The main reason for the increased cost is due to the limited demand from other market segments, thus making the extraction of the titanium ore expensive. Also, the processing costs for converting the ore into commercially usable titanium and its alloys is extensive and requires special processing procedures and involves vast batch production and careful process control, making them difficult to automate. Second, the difficulty in efficiently manufacturing titanium components has a significant adverse effect on processing cost which is mainly due to its low modulus of elasticity and high yield stress. Another manufacturing concern that arises during the machining of titanium is its susceptibility to work hardening during the cutting process and its tendency to react with many cutting tool materials causing substantial tool wear. Additionally, titanium has poor thermal conductivity properties, making heat dissipation a problem, again contributing to higher tool wear. Of primary concern however is the lack of material grade development outside the aerospace industry in which most of the alloys are developed for extreme conditions. This severely limits the currently available grades suited for automotive applications. Thus, a suite of lower strength alloys with properties specially catered for commercial automotive use needs to be developed. This paper examines most of the issues traditionally associated with the machinability of titanium and titanium alloys. As mentioned before, some methodologies and techniques are recommended for mitigating the non-desirable effects during titanium processing and analyzed in more detail, is its unique tool wear characteristics especially in light of manufacturing automotive components. Thus, this study is expected to primarily assist in the reduction of the processing cost of titanium and its alloys for automotive component manufacture. This will help reduce the operating cost of a road vehicle in terms of better fuel economy due to the reduced mass, which in turn translates to better energy efficiency. TITANIUM IN THE AUTOMOTIVE