D. C. Van Aken, K. Doering, B. Kudlacek, G. Galecki, Missouri University of Science and Technology, Rolla, MO
Water-jet cutting is an attractive process for the machining of titanium components, but little is known regarding how this cutting operation affects performance. In this paper, Ti-6Al-4V sheet was water-jet cut and the cyclic life determined by load control fatigue testing. Titanium sheet with nominal thickness 0.0415 inches was obtained in the annealed condition as MIL-T-9046J (AB-1). Fatigue specimens were water-jet cut from the Ti-6Al-4V sheet at a pressure of 40,000 psi, using a 80 mesh garnet abrasive of less than 180 ?m diameter, fed at a rate of 0.62 pounds per minute with linear cutting speeds of 5 and15 inches per minute. Specimens cut at the faster speed were randomly selected and the edges were polished using 600 and 800 grit metallographic papers. Fatigue tests were performed in load control with R(Smin/Smax) = 0.1 and using a sinusoidal waveform. All fatigue cracks initiated at the specimen edge and propagated across the gage section. Polishing the water jet surface improved the overall fatigue life and increased the fatigue strength at 10^6 by up to 40%. A modest 8% increase in the fatigue strength at 10^6 cycles was observed for the lower cutting speed. Post mortem analysis revealed that fatigue crack nucleation occurred at surface scars produced during water jet cutting.
Summary: Water-jet cutting is an attractive process for the machining of titanium components, but little is known regarding how this cutting operation affects performance. In this paper, Ti-6Al-4V sheet was water-jet cut and the cyclic life determined by load control fatigue testing. Titanium sheet with nominal thickness 0.0415 inches was obtained in the annealed condition as MIL-T-9046J (AB-1). The microstructure consisted of an equiax 1 to 2 µm diameter alpha-Ti grain structure with the beta-phase at grain corners and grain edges. The 0.2% offset yield strength and ultimate tensile strength were 1110±67 MPa and 1140±64 MPa, respectively. The elongation to failure was 12.1± 1.3% in a 38 mm gage.
Fatigue specimens were water-jet cut from the Ti-6Al-4V sheet at a pressure of 40,000 psi, using a 80 mesh garnet abrasive of less than 180 µm diameter, fed at a rate of 0.62 pounds per minute with a linear cutting speed of 15 inches per minute. Fatigue specimens had an hour glass shape with radii of 2 inches and gage width of 0.25 inches that resulted in an edge stress concentration factor of 1.04. Fatigue tests were performed in load control with R(Smin/Smax) = 0.1 and using a sinusoidal waveform. Low cycle fatigue was performed at 2 Hz and 8 Hz was used for high cycle tests. All fatigue cracks initiated at the specimen edge and propagated across the gage section. A slight taper was produced during the water-jet cutting process, but crack initiation was located at the minimum section area and randomly along the gage edge. The taper was eliminated in subsequent tests by polishing the specimen edge with 600 and 800 grit SiC metallographic papers. Polishing the water jet surface improved the overall fatigue life and increased the fatigue strength at 10^6 by up to 40%. This led to an investigation of the cutting parameters related to water-jet cutting and specifically with respect to cutting speed.
A second set of fatigue specimens were cut at a pressure of 40,000 psi, using a 80 mesh garnet abrasive of less than 180 µm diameter, fed at a rate of 0.62 pounds per minute with a linear cutting speed of 5 inches per minute. The figure is a compilation of all of the fatigue tests performed on the Ti-6Al-4V sheet. A modest 8% increase in the fatigue strength at 10^6 cycles was observed for the lower cutting speed. Post mortem analysis revealed that fatigue crack nucleation occurred at surface scars produced during water jet cutting.
