Cold Rolling Machinability of TC6 Titanium Alloy Plate
Mar 29, 2024
Alloy Identity and Designation
TC6 is a martensitic Ti-Al-Mo-Cr-Fe-Si alpha-beta alloy with the nominal composition Ti-6Al-2.5Mo-1.5Cr-0.5Fe-0.3Si, and the designation is listed in GB/T 3620.1, the Chinese standard for titanium and titanium alloy designations and chemical composition. It is a heat-treatable, two-phase alloy with the beta transus reported between 960 degC and 1000 degC depending on interstitial content; a measured value of 975 to 985 degC is typical of production ingot with oxygen around 0.10%.
The alloy combines the usual titanium advantages of high specific strength and corrosion resistance with good room-temperature and elevated-temperature mechanical properties, and it is rated for service up to about 450 degC. Forging, casting, tube and bar processing routes are well established; cold rolling of plate is less commonly documented and is the subject of the trial data below.
Mill Route for TC6 Plate
The trial stock was triple vacuum arc remelted, opened in the beta region and forged to slab in the alpha-beta region. Slabs were hot rolled above the beta transus to break down the cast structure, then rolled to 3.5 mm in the alpha-beta field, annealed and pickled. Cold rolling was carried out in increments on the annealed and pickled coil, with a 200 mm test coupon cut from the head of the plate after every 5% of additional reduction so that the remaining plate could continue to be rolled. Rolling stopped when edge or surface cracks appeared.
Inter-pass annealing is the practical requirement of this route. Without recrystallisation between passes the accumulated dislocation density raises flow stress until the mill load limit or the plate cracking limit is reached, so published reduction limits only apply to a specified annealing schedule.
Work-Hardening Data
| Cold reduction | Observed response |
|---|---|
| Up to about 15% | Tensile and yield strength rise, elongation falls |
| About 27% | Strength curves flatten; entry into the difficult deformation range |
| 30.3% (maximum achieved) | Elongation 5.8%; surface ripples and convexity; edge cracking; 20 mm width spread |
The strength increase with reduction is caused by rising dislocation density and the associated work hardening, and the flattening of the curve at about 27% is the point at which further reduction no longer buys proportionate strength. The maximum cold reduction reached before cracking was 30.3%, and at that point the material had lost most of its ductility and the plate showed visible surface and edge defects.
Specifications for Qualifying the Finished Plate
Finished plate is ordered against ASTM B265, which fixes chemistry, mechanical property minima and thickness tolerances for titanium sheet, strip and plate. Tensile testing follows ASTM E8/E8M; the 200 mm coupons used in the trial are convenient because they satisfy the grip length requirement of a standard flat specimen without further cutting. Where the plate is subsequently machined into bar-type parts, ASTM B348 applies to the bar product. Interstitial elements, in particular oxygen, must be held within specification, because oxygen raises strength and lowers ductility and therefore shifts both the beta transus and the cold rolling limit.
Practical Guidance for Fabricators
Plan cold reduction per pass from the measured work-hardening curve rather than from handbook values for Ti-6Al-4V, since the Mo and Cr additions in TC6 change the response. Anneal and pickle before the accumulated reduction approaches the flattening point, cut a coupon at each stage to track properties, and stop rolling at the first sign of edge cracking. Where the formed part requires a flat surface, allow for the 20 mm width spread observed at the rolling limit and size the starting plate accordingly.
Frequently Asked Questions
Q: What is the nominal composition of TC6?
A: Ti-6Al-2.5Mo-1.5Cr-0.5Fe-0.3Si, as listed in GB/T 3620.1.
Q: What is the beta transus of TC6?
A: The reported range is 960 to 1000 degC depending on interstitial content; a measured value of 975 to 985 degC is typical.
Q: How much cold reduction can TC6 plate take?
A: In the rolling trials the maximum achieved was 30.3%, with a 20 mm width spread, elongation down to 5.8% and visible edge cracking.
Q: Why does the work-hardening curve flatten?
A: Dislocation density saturates the strengthening contribution at around 27% reduction, so additional reduction adds little strength while consuming remaining ductility.
Q: Which specification applies to the finished plate?
A: ASTM B265 for titanium sheet, strip and plate, with tensile testing to ASTM E8/E8M.
