Influence of La-Content and Microstructure on the Corrosion Properties of a New Free Machining Titanium Alloy

摘要

Due to the combination of low density and high strength titanium alloys like Ti6A14V are widely used in mechanical engineering, especially in the fields of aerospace and medical engineering (1). In addition, titanium alloys exhibit a good corrosion resistance and are, therefore, used for applications where high corrosion resistance is needed, e.g. for heat exchangers in the chemical industry where stainless steels cannot be used anymore (2). Machining is a common operation in manufacturing titanium parts if complex geometries like blisks (= bladed disks) or parts of heat exchangers have to be produced. Machining of titanium alloys, however, involves high production costs because of their poor machinability (3). This difficulty arises from the physical, chemical and mechanical properties of titanium. Due to the relatively poor thermal conductivity of titanium, heat generated by the cutting action cannot diffuse quickly into the chip's material so that heat is concentrated in front of the rake face of the tool. Titanium's relatively low modulus of elasticity results in spring back of the workpiece during the cutting action, causing tool rubbing at the flank face, chatter and tolerance problems. The high strength of most of the titanium alloys only allows relatively low cutting speeds compared to the machining of aluminum and steels. Furthermore, the high chemical reactivity of titanium limits the number of possible materials for the cutting tools. Finally, titanium's strong alloying tendency causes galling, welding and smearing, leading to rapid destruction of the tool and decreases the quality of the finished workpiece (4). The machinability of Ti6A14V can be improved only if the chip formation process is completely understood. In machining, three different kinds of chips are known to form: continuous chips, segmented chips, and completely separated segments if extreme cutting