Since the discovery of the element titanium in 1790, mankind has carried out a hundred years of arduous exploration to obtain its extraordinary performance. In 1910, mankind produced the metal titanium for the first time, but the application of titanium alloys was a long and arduous road, and it was not until 40 years later, in 1951, that industrial production was finally realized.
Titanium alloys are characterized by high specific strength, corrosion resistance, high-temperature resistance, and fatigue resistance. A titanium alloy of the same size weighs only 60% of steel but is stronger than alloy steel. Due to its good characteristics, titanium alloy has been increasingly used in the fields of aviation, aerospace, power generation equipment, nuclear energy, shipbuilding, the chemical industry, medical equipment, and so on.
Reasons for complex machining of titanium alloys
The four characteristics of titanium alloy, such as low thermal conductivity, severe work hardening, high affinity with the tool, and small plastic deformation, are the essential reasons why titanium alloy is rugged to process. Its cut index is only 20% of that of free-cutting steel.
Low thermal conductivity
Titanium alloy has a thermal conductivity of only about 16% of steel. Processing heat can not be conducted in time, resulting in cutting-edge local high temperatures (machining the tip temperature is more than one times that of steel), which easily triggers tool diffusion wear.
Severe work hardening
The work-hardening phenomenon of titanium alloy is obvious. The surface hardening layer is more serious compared to stainless steel, which will cause some difficulties for subsequent processing, for example, tool boundary damage increases.
High affinity to tools
Severe bonding with titanium-containing carbide.
Low plastic deformation
The modulus of elasticity is about half that of steel, so the elastic recovery is large, and friction is severe. At the same time, the workpiece is also prone to clamping deformation.
Process know-how for machining titanium alloys
Based on the understanding of the machining mechanism of titanium alloys and the experience, the primary process know-how for machining titanium alloys is as follows:
(1) Use inserts with positive angle geometry to minimize cutting forces, cutting heat, and workpiece deformation.
(2) Maintain a constant feed to avoid hardening the workpiece. The tool should constantly be fed during the cutting process, and the radial draft should be 30% of the radius when milling.
(3) High-pressure, high-flow cutting fluid is used to ensure the thermal stability of the machining process and to prevent denaturation of the surface of the workpiece and damage to the cutting tool due to excessively high temperatures.
(4) Keep blade edges sharp. Dull tools are the cause of heat buildup and wear, which can easily lead to tool failure.
(5) Machining of titanium alloys in the softest state possible, as the material becomes more challenging to process after hardening, and heat treatment increases the strength of the material and increases wear on the insert.
(6) Use a large tip radius or chamfer cut to bring as much of the blade into the cut as possible. This reduces the cutting force and heat at each point and prevents localized breakage. When milling titanium alloys, cutting speed has the most significant impact on tool life among the various cutting parameters, with a radial draft (milling depth) coming in second.
Solving Titanium Processing Problems from the Blade's Perspective
Insert groove wear that occurs during titanium machining is localized wear at the back and front in the direction of the depth of cut, and it is often caused by the hardened layer left by the previous machining. Chemical reaction and diffusion between the tool and the workpiece material at a machining temperature of more than 800 ℃ is also one of the reasons for the formation of groove wear. In the machining process, the titanium molecules of the workpiece in the front of the blade accumulate in the high pressure and high temperature “welding” to the cutting edge, causing the formation of chip tumors. When the chip-accumulator peels away from the cutting edge, it carries away the carbide coating of the insert, so titanium machining requires unique insert materials and geometries.
The focal point of titanium machining is heat, and large quantities of high-pressure cutting fluid must be sprayed onto the cutting edge in a timely and accurate manner to remove the heat quickly. Unique configurations of milling cutters are on the market specifically for titanium machining.
turning
Titanium alloy products turn out to have easy-to-obtain, good surface roughness, work hardening is not severe, but the cutting temperature is high, and tool wear is fast. For these characteristics, the primary tool, cutting parameters to take the following measures:
Tool material: YG6, YG8, YG10HT according to the existing conditions of the factory.
Tool geometry parameters: suitable tool front and rear angles, tip rounding.
With lower cutting speed, moderate feed, more profound depth of cut, and adequate cooling, the tip of the tool can not be higher than the center of the workpiece when turning cylindrical. Otherwise, it is easy to tie the knife, precision turning and turning thin-walled parts; the central deflection angle of the tool should be large, generally 75 ~ 90 degrees.
milling
Titanium alloy product milling is more complex than turning because milling is intermittent cutting, and chips are easy to bond with the cutting edge; when the sticky chips of the teeth of the cutter cut into the workpiece again, the sticky chips are touched off and take away a tiny piece of tooling material, the formation of chipping, which significantly reduces the durability of the tool.
Milling method: generally use smooth milling.
Tool material: HSS M42.
General alloy steel processing is not used in smooth milling due to the machine tool screw, nut clearance, smooth milling, the milling cutter’s role in the workpiece, in the direction of the feed direction of the force, and the feed direction of the same, easy to make the workpiece table to produce clearance fluctuations, resulting in hit the knife. For forward milling, the cutter teeth start to cut into the hard skin, which leads to tool breakage. However, due to the reverse milling chip being from thin to thick, in the initial cut into the tool, it is easy to dry friction with the workpiece, aggravating the tool stick chip 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 a pointed tooth milling cutter, avoid the use of a spade tooth milling cutter.
tapping
Titanium alloy products tapping, because the chip is small, easy to bond with the blade and the workpiece, resulting in processing surface roughness value is large, the torque is ample. Improper selection of taps and improper operation when tapping can very easily cause hardening, and processing efficiency could be much higher with broken taps.
We need to prioritize the use of a jump tap in place; the number of teeth should be less than the standard tap, generally 2 to 3 teeth. The cutting cone angle should be significant, and the taper part of the general 3 to 4 buckle thread length. In order to facilitate chip removal, but also in the cutting cone part of the grinding negative inclination. Try to use a short tap to increase the rigidity of the tap. The inverted cone part of the tap should be appropriately more significant than the standard to reduce the friction between the tap and the workpiece.
Reaming
Tool wear is not severe when reaming titanium, and both carbide and high-speed steel reamers can be used. When using cemented carbide reamers, it is necessary to take a similar drilling process system stiffness to prevent the reamer from chipping. Titanium alloy reaming when the main problem is reamed hole finish is not good. Must use oil stone repair narrow reamer edge bandwidth so as to avoid the edge band and the hole wall bonding, but to ensure sufficient strength, general edge width in 0.1 ~ 0.15mm is suitable.
The cutting edge and calibration part of the transfer should have a smooth arc, wear and tear should be grinding in a timely manner, and the teeth arc size should be consistent; if necessary, it can increase the calibration part of the inverted cone.
drilling
Titanium alloy drilling is relatively tricky, often in the process of burning and broken drill phenomenon. This is mainly due to poor drill sharpening, chip removal needs to be timely, inadequate cooling and process system rigidity, and other reasons. Therefore, in the titanium alloy drilling process, we must pay attention to reasonable drill sharpening, large top angle, reducing the outer edge of the front angle, increasing the outer edge of the back angle, and 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 color of the chips. If the chips appear feathery or the color changes during the drilling process, it indicates that the drill bit has been blunt and should be sharpened in time.
The drilling mold should be fixed on the working table, the guiding surface of the drilling mold should be close to the machining surface, and the short drill bit should be used as much as possible. Another noteworthy issue is that when taking manual feed, the drill should not be in the hole. Otherwise, the drill edge rubs the machining surface, resulting in hardening so that the drill is dull.
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 the poor thermal conductivity of titanium alloys, which generates high temperatures in the grinding zone, resulting in bonding, diffusion, and strong chemical reactions between the titanium alloy and the abrasive. 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 part, which is more apparent when grinding titanium alloy castings.
Measures taken to address this issue:
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 machining.
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