Experimental and numerical analysis of high-throughput drilling of titanium alloys.

Abstract

Titanium (Ti) and its alloys are light-weight, corrosion resistant, and high temperature materials which have wide industrial applications. The drilling of Ti alloys is traditionally conducted at very low material removal rate to avoid short tool life due to their inherent properties, particularly the low thermal conductivity. This has hindered the productivity and increased the cost of machining Ti alloys. This research investigates the high-throughput drilling of Ti alloys through experimental exploration, finite element modeling, and metallurgical analysis. Through a comprehensive experimental analysis, the high throughput drilling of Ti-6Al-4V is demonstrated technically feasible using the spiral point drill geometry with fine-grained WC tool material and proper drilling process parameters. Supplying the cutting fluid via through-the-drill holes has proven to be critical for the drill life. At the same material removal rate, the balance of cutting speed and feed is essential to achieve long drill life and good hole surface finish. The spatial and temporal distributions of the temperature and stress of a spiral point drill are studied. The inverse heat transfer method based on finite element thermal model is applied to find the heat partition on the tool-chip contact area and the convection coefficient of cutting fluid. A thermo-mechanical finite element analysis is developed to analyze the drill stress distribution. The drilling force and torque and the chip formation are analyzed using a 3D updated-Lagrangian finite element model in which the chisel and cutting edges of a spiral point drill are treated as a series of elementary cutting tools. The influence of cutting fluid supply and the reduction in cutting speed and increase in feed at the same material removal rate on the tool and chip temperature and stress is investigated. The metallurgical analyses, including SEM, XRD, electron microprobe, and nanoindentation characterization are conducted to study the Ti-6Al-4V hole surfaces and chips under the influence of high temperature, large strain, and high strain rate deformation in high-throughput drilling. The phase transformation and property change in the subsurface layer and the chips are analyzed. The saw-tooth feature and narrow shear bands in the chips are discussed.