The 4E turning geometry from ATI Stellram, that has a micrograin carbide substrate and a super hard Nano TiAlN PVD coating, provides positive cutting action to reduce built-up edge in high-temperature titanium machining.
Machining a thin-wall part or ring — common operations — with anything but a positive-rake tool will push and deflect the part rather than cut it. This makes it difficult to cut to size. Instead of cutting the part, the wrong tool pushes it, straining the material. As the material moves away from the cutting edge it deforms plastically, instead of elastically, and that increases the material’s strength and its hardness at the point of cut. As the alloy gets harder and stronger, cutting speeds that were appropriate at the start of the cut become excessive, and the tool wears dramatically.
The alloy the workpiece is made from determines the cutting speed needed to cut it. Unalloyed titanium can be machined at speeds to 180 sfm, while tougher beta alloys require speeds as low as 30 sfm. In general, the more vanadium and chromium in a particular alloy, the lower the cutting speed that is called for. In all cases, titanium alloys demand heavy chip loads to overcome the problem of rubbing and the work-hardening that results.
The magnitude of cutting forces generated when machining titanium is only slightly higher than those developed when cutting steels with an equivalent hardness, even though machining titanium appears to be more difficult and complex.
Flank wear, notching and built-up edge are the common types of tool wear when cutting titanium. Edge notching appears as a localized abrasive wear on both the flank and rake face, along the line corresponding with the depth-of-cut parameter. This wear is caused partially by the presence of a hardened layer that typically is formed by previous casting, forging, heat treating, or prior machining operations.
Chemical reaction between the cutting tool material and the workpiece also could lead to a notching-wear mechanism. This occurs when machining temperatures exceed 800° C., and induce diffusion between the tool and the workpiece.
In contrast, during the machining process, deposits of titanium work materials tend to accumulate on the rake face of the insert. The high pressure developed in this area can weld these particles to the cutting edge, forming a built-up edge phenomenon. These particles, over successively shorter intervals, are inclined to peel off the cutting edge, pulling some carbide content from the cutting insert away with it.
The best tool substrate and coating for machining titanium alloys and super alloys is a submicron substrate that is combined with a physical vapor deposition (PVD) TiAlN coating. The thin, smooth surface of the PVD coating, together with sufficient residual stress, enhances tool resistance to chipping and notching wear, so PVD coatings provide enhanced wear resistance, chemical stability and resistance to built-up edge. Machining problems that were seen in the past that arose from earlier coatings, no longer exist with PVD coatings because of the improved adhesion techniques and the uniformity of the coatings.
Titanium and its alloys
Titanium alloys are available in four varieties: alpha, alpha/ beta, beta and the newer titanium aluminide. Because more alloying elements are being added to the particular grades, these alloys are progressively more difficult to machine.
The Alpha phase of titanium is pure titanium, relatively soft and can be machined at high speeds.
