In the last article we discussed what’s the high temp alloy, we know nickel-based alloy is the most commonly used high temperature alloy used in aerospace, aviation and other fields at about or above 1000℃. Because it contains many high-melting-point alloy elements such as Fe, Ti, Cr, Ni, V, W, Mo, etc., which forms austenitic alloy with high purity and dense structure, and some elements from metal and non-metallic compounds with high hardness, small specific gravity and high melting point with non-metallic elements such as C, B, N, making its poor machinability. Its relative machinability is only 5 ~ 20% of that of plain carbon steel. The superalloys are characterized by difficulty cutting, do you know why they are difficult to cut? There are some features you should know before answer the question.
- High content of fortified elements
In the process of cutting, a large number of abrasive metal carbide, intermetallic compound and other hardpoints are formed, which has a strong scratch on the knife, and is easy to produce deposition and clipping, affecting the quality of the processed surface.
- High temp strength and work hardening tendency
The cutting process produces great plastic deformation resistance and cutting load, and the cutting temperature is high. The unit cutting force of nickel-based superalloy is 50% higher than that of medium carbon steel. The working hardening and residual stress of the surface layer after processing are large, and the hardening degree can reach 200% ~ 500%, leading to serious edge and edge wear, groove wear is also easy to occur.
Now that we know that Nickel-based high temp alloys are difficult to cut, here we will discuss 3 tips should pay attention to in cutting.
- Metal Cutting Tools
Superalloys must have special tool materials for cutting. The most commonly used are carbide cutters, or high speed steel for machining complex surfaces with very low cutting speed. Hot-working grades with better performance are most commonly used in practice. In addition, Si3N4 ceramics tools are the best option for high temp alloy due to its higher resistance to adhesion, heat resistance and hardness than cemented carbides and are also suitable for semi-finishing and finishing of superalloy.
- Tool Feeds and Speeds
The cutting of superalloy materials also requires the geometric parameters of cutting tools, such as forging, hot rolling and cold drawing. The rake angle of the tool （γ0） is about 10 degrees while the casting superalloy is about 0 degrees. The cutter’s back angle (a) = 10 ~15. The cutter inclination angle (λs) is – 5 10 in rough machining and lambda s = O 3 in finish machining. The main deviation angle (κr) is 45 ~75. The arc radius (r) of the tool tip is 0.5-2 mm, which is large in rough machining.
- Cutting Parameters and Conditions
The choice of cutting amount is basically the same as stainless steel, the most important is the cutting speed. For carbide tools, cutting speed (Vc) =20 ~ 50m/min; Feed quantity (f) should be small, generally f=0.1 ~ 0.5mm/r, the large value should be taken for coarse turning, the small value should be taken for fine turning. Backdraft (ap) should not be too small. For coarse turning, ap=2 ~ 4mm, and for fine turning, ap=0.2 ~ 0.5mm.Vc=5 ~ 10m/min for high temperature alloy machining with HSS endmill; Fn =0.05 ~ 0.12mm/r, ap+1 ~ 3mm.The carbide face milling cutter is Vc=20 ~ 45m/min. Fn =0.05 ~ 0.1mm/r, ap=1 ~ 4mm.