Based on the understanding of the machining mechanism of titanium alloy and the past experience, the main process know-how for machining titanium alloy is as follows:
(1) Use inserts with positive angular geometry to reduce cutting forces, cutting heat and workpiece deformation.
(2) Maintain constant feed to avoid hardening of the workpiece, the tool should always be in the feed state during the cutting process, and the radial draft ae should be 30% of the radius when milling.
(3) Use high-pressure high-flow cutting fluid to ensure the thermal stability of the machining process, to prevent the workpiece surface denaturation and tool damage due to high temperature.
(4) Keep the insert edge sharp, blunt tools are the cause of heat collection and wear, which can easily lead to tool failure.
(5) Machine titanium in its softest state possible, as the material becomes more difficult to machine after hardening, and heat treatment improves the strength of the material and increases wear on the insert.
(6) Use a large tip radius or chamfer cut to put as much of the cutting edge into the cut as possible. This reduces cutting forces and heat at each point and prevents localised breakage. When milling titanium alloys, the cutting speed of each cutting parameter has the greatest impact on tool life, followed by radial draft (milling depth).
From the blade to solve the titanium processing problems
Titanium alloy machining insert groove wear is the back and front in the direction of the depth of cut in the local wear, it is often due to the hardened layer left by the previous processing. Chemical reaction and diffusion between the tool and the workpiece material at a machining temperature of more than 800°C is also one of the reasons for the formation of groove wear. Because in the machining process, the titanium molecules of the workpiece in front of the insert accumulation, in the high pressure and high temperature "welding" to the cutting edge, the formation of chip tumour. When the chip veneer peels away from the cutting edge, it carries away the carbide coating of the insert, so titanium machining requires special insert materials and geometries.
Tool construction for titanium machining
The focal point of titanium machining is heat, and large quantities of high-pressure cutting fluid need to be sprayed onto the cutting edge in a timely and accurate manner to remove the heat quickly. There are unique milling cutter configurations on the market specifically for titanium machining.
Starting with specific machining methods
- Turning
Titanium alloy products turning, easy to obtain a good surface roughness, work hardening is not serious, but the cutting temperature is high, fast tool wear. For these characteristics, the main tool, cutting parameters to take the following measures:
Tool material: YG6, YG8, YG10HT are selected according to the existing conditions of the factory.
Tool geometry parameters: suitable tool front and rear angles, tip grinding round.
Lower cutting speed, moderate feed, deeper depth of cut, adequate cooling, the tip of the tool can not be higher than the centre of the workpiece when turning cylindrical, otherwise it is easy to tie the tool, fine turning and turning thin-walled parts, the main deflection angle of the tool should be large, generally 75 ~ 90 degrees.
- Milling
Milling of titanium alloy products is more difficult than turning, because milling is intermittent cutting, and chips are easy to bond with the cutting edge, when the teeth of the sticky chips are cut into the workpiece again, the sticky chips are touched off and take away a small piece of cutting tool material, forming a chipping edge, which greatly reduces the durability of the tool.
Milling mode: generally adopt smooth milling.
Tool material: high-speed steel M42.
General alloy steel processing are not used smooth milling, due to the machine tool screw, nut clearance, smooth milling, the milling cutter acting on the workpiece, in the direction of the feed direction of the force and the same direction of the feed, easy to make the workpiece table to produce clearance fluctuations, resulting in a knife. For forward milling, the cutter teeth start to cut into the hard skin and lead to tool breakage. However, due to the reverse milling chip is from thin to thick, in the initial cut into the tool is easy to dry friction with the workpiece, aggravated by the tool of sticky chips and chipping. In order to make titanium alloy milling smoothly, it should also be noted that relative to the general standard milling cutter, the front angle should be reduced, and the rear angle should be increased. Milling speed should be low, as far as possible, the use of pointed tooth milling cutter, avoid the use of spade tooth milling cutter.
- Tapping
Titanium alloy products tapping, because the chips are fine, easy to bond with the cutting edge and the workpiece, resulting in a large surface roughness value of the processed surface, the torque is large. Tapping tap selection and improper operation is very easy to cause processing hardening, processing efficiency is extremely low and the tap breakage phenomenon.
Need to prioritise the use of a jump tap in place, the number of teeth should be less than the standard tap, generally 2 to 3 teeth. Cutting cone angle should be large, taper part of the general 3 to 4 buckle thread length. In order to facilitate chip removal, can also be ground in the cutting cone part of the negative inclination. Try to use a short tap to increase the rigidity of the tap. The inverted taper part of the tap should be appropriately larger than the standard one to reduce the friction between the tap and the workpiece.
- Reaming
Titanium alloy reaming tool wear is not serious, the use of carbide and high-speed steel reamer can be. When using carbide reamers, take a process similar to drilling system stiffness to prevent the reamer from chipping. Titanium alloy reaming holes when the main problem is reamed hole finish is not good, must use oil stone repair narrow reamer edge band width, so as to avoid the edge of the band and the hole wall bonding, but to ensure sufficient strength, the general edge width of 0.1 ~ 0.15mm is good.
Cutting edge and calibration part of the transfer should be a smooth arc, wear and tear should be timely grinding, and require the teeth arc size consistent; if necessary, can increase the calibration part of the inverted cone.
- Drilling
Titanium alloy drilling is more difficult, often in the process of burning and broken drill phenomenon. This is mainly due to poor drill sharpening, chip removal is not timely, poor cooling and process system rigidity and other reasons. Therefore, in the titanium alloy drilling process must pay attention to reasonable drill sharpening, large top angle, reduce the outer edge of the front angle, increase the outer edge of the back angle, the inverted cone added to the standard drill 2 to 3 times. Return the tool diligently and remove the chips in time, pay attention to the shape and colour of the chips. If the chips appear feathery or the colour changes in the drilling process, it indicates that the drill bit has been blunt and should be sharpened in time by changing the cutter.
The drilling mould should be fixed on the working table, and the guiding surface of the drilling mould should be close to the processing surface, and the short drill bit should be used as far as possible. Another noteworthy issue is that when taking manual feeding, the drill should not be in the hole, otherwise the drill edge rubbing the processing surface, resulting in processing hardening, so that the drill becomes blunt.
- Grinding
A common problem in grinding titanium alloy parts is the clogging of the grinding wheel due to sticky chips and burns on the surface of the parts. The reason for this is that the poor thermal conductivity of titanium alloys generates high temperatures in the grinding zone, which leads to bonding, diffusion and strong chemical reactions between titanium alloys and abrasives. Sticky chips and wheel clogging lead to a significant decrease in the grinding ratio, and as a result of diffusion and chemical reaction, the surface of the workpiece being ground is burned, leading to a reduction in the fatigue strength of the parts, which is more obvious when grinding titanium alloy castings.
The measures taken to solve this problem are:
Selection of suitable grinding wheel material: green silicon carbide TL. slightly lower grinding wheel hardness: ZR1.
The cutting of titanium alloy materials must be controlled in terms of tool materials, cutting fluids, and machining process parameters in order to improve the comprehensive efficiency of titanium alloy material processing.
