Titanium alloy processing and experience discussion

2021-09-16
Titanium alloy has excellent mechanical properties but poor process performance, which leads to the contradiction that its application prospects are promising but processing is more difficult. This article analyzes
The metal cutting performance of alloy materials, combined with many years of practical work experience, has discussed the selection of titanium alloy tools, the determination of cutting speed, the characteristics of different cutting methods, machining allowances and processing precautions, etc., and clarified my opinion Insights and suggestions on titanium alloy machining.
Keywords: titanium alloy; mechanical properties; process method; materil
                                                                                                                                                                                                                                                                 
Titanium alloy has low density, high specific strength (strength/density), good corrosion resistance, high heat resistance, toughness, plasticity, and weldability. It is currently used in aerospace, automotive, medical, sporting goods and electrolysis industries, etc. Titanium alloys have been widely used in many fields. However, poor thermal conductivity, high hardness, and low modulus of elasticity have also led to titanium alloys becoming more difficult to process metal materials. This article summarizes some technological measures in the cutting of titanium alloys based on its technological characteristics.
1 The main advantages of titanium alloy materials
(1) Titanium alloy has high strength, low density (4.4kg/dm3) and light weight, which provides a solution for reducing the weight of some large structural parts.
(2) High thermal strength. Titanium alloys can still maintain high strength and work stably under the conditions of 400-500°C, while the working temperature of aluminum alloys can only be below 200°C.
(3) Compared with steel, the inherent high corrosion resistance of titanium alloy can save the cost of daily operation and maintenance of aircraft.

2 Analysis of processing characteristics of titanium alloy
(1) Low thermal conductivity. The thermal conductivity of TC4 at 200℃ is l=16.8W/m?℃, and the thermal conductivity is 0.036 calories/cm?sec?℃, which is only 1/4 of steel, 1/13 of aluminum, and 1/25 of copper. In the cutting process, the heat dissipation and cooling effect is poor, which shortens the tool life.
(2) The modulus of elasticity is low, and the machined surface of the part has a large springback, which leads to an increase in the contact area of ​​the machined surface and the flank surface of the tool, which not only affects the dimensional accuracy of the part but also reduces the durability of the tool.
(3) Hardness factor. Titanium alloys with low hardness values ​​will be sticky during machining, and chips will stick to the cutting edge of the tool rake face to form built-up edge, which affects the processing effect; titanium alloys with high hardness values ​​are prone to chipping and abrasion of the tool when machining. These characteristics result in a low removal rate of titanium alloy metal, which is only 1/4 of that of steel parts, and the processing time is much longer than that of steel parts of the same size.
(4) Strong chemical affinity. Titanium can not only chemically react with the main components of nitrogen, oxygen, carbon monoxide and other substances in the air to form TiC and TiN hardened layers on the surface of the alloy, but also react with the tool material under the high temperature conditions generated by the cutting process, reducing the tool The durability.
(5) Poor safety performance during cutting. Titanium is a flammable metal. The high temperature and sparks generated during micro-cutting may cause titanium chips to burn.

3 Titanium alloy processing technology
(1) Use cemented carbide tools as much as possible,
Cobalt-based cemented carbide has the characteristics of high strength and good thermal conductivity, and is not prone to chemical reaction with titanium at high temperatures, so it is suitable for processing titanium alloys.
(2) Reasonably choose the geometric parameters of the tool. In order to reduce the cutting temperature and reduce the phenomenon of tool adhesion, the rake angle of the tool can be appropriately reduced, and the contact area between the chip and the rake surface can be increased to dissipate heat; at the same time, the tool clearance angle can be increased to reduce the springback of the machined surface and the tool flank. Surface friction contact causes tool adhesion and reduced accuracy of the machined surface; the tip of the tool should adopt a circular arc transition to enhance the strength of the tool. When processing titanium alloys, tools must be sharpened frequently to ensure that the blade is sharp and chip removal is smooth.
(3) Appropriate cutting parameters. To determine the cutting parameters, you can refer to the following solutions: lower cutting speed-high cutting speed will lead to a sharp increase in cutting temperature; moderate feed rate-high feed rate means higher cutting temperature, and small feed rate means that the cutting edge In the hardened layer, the cutting time is long and the wear is accelerated; the larger cutting depth-cutting the tool tip over the hardened layer on the surface of the titanium alloy can increase the tool life.
(4) The flow and pressure of the cutting fluid during processing should be large, and the processing area should be fully and continuously cooled to reduce the cutting temperature.
(5) When selecting a machine tool, attention must always be paid to improving stability to avoid vibration trends. Vibration will cause the blade to shatter and damage the blade. At the same time, the rigidity of the titanium alloy processing system should be good to ensure that a larger cutting depth is used during cutting. However, titanium alloy processing has a large springback and a large clamping force will aggravate the deformation of the workpiece. Therefore, you can consider using auxiliary supports such as assembly fixtures during finishing. Meet the rigidity requirements of the process system.
(6) Down milling is generally used for milling. The chip sticking and chipping of the milling cutter caused by up-milling in titanium alloy processing are much more serious than the damage of the milling cutter caused by down-milling.
(7) The common problems in grinding are sticky debris causing blockage of the grinding wheel and burns on the surface of the parts. Therefore, green silicon carbide grinding wheels with sharp abrasive grains, high hardness and good thermal conductivity should be used when grinding; F36~F80 can be used according to the different surface finish of the processed surface; the hardness of the grinding wheel should be soft to reduce abrasive particles and debris Adhesion to reduce the grinding heat; the grinding feed rate should be small, the speed is low, and the emulsion is sufficient.
(8) When drilling titanium alloy, it is necessary to grind the standard drill bit to reduce the phenomenon of burning knife and drill bit breaking. Grinding method: appropriately increase the apex angle, reduce the rake angle of the cutting part, increase the back angle of the cutting part, and double the inverted taper of the cylindrical edge. The number of tool withdrawals should be increased during processing. The drill bit must not stay in the hole, and the chips should be removed in time. The sufficient amount of emulsion should be cooled. Pay attention to observe the dullness of the drill bit and remove the chips in time, and the sufficient amount of emulsion should be cooled. Replace and dress.
(9) Titanium alloy reaming also needs to be modified to the standard reamer: the width of the reamer blade should be less than 0.15mm, and the cutting part and the calibration part should be arc transitioned to avoid sharp points. When reaming, the reaming tool can be used for multiple reaming, and the diameter of the reamer should be increased by 0.1mm or less each time. The spindle speed should be slightly slower, and the tool will not stop when retracting. Reaming in this way can achieve higher finish requirements.
(10) Thread tapping is the most difficult part in titanium alloy processing. Due to excessive torque, the tap cutter teeth will wear out quickly, and the rebound of the processed part can even make the tap break in the hole. When selecting ordinary taps for processing, the number of teeth should be appropriately reduced according to the diameter to increase the chip space. After leaving a 0.15mm width on the calibration teeth, the clearance angle should be increased to about 30°, and 1/2 to 1 removed. /3 tooth back, the calibrated tooth retains 3 buckles and increases the inverted taper. It is recommended to use skipping taps, which can effectively reduce the contact area between the tool and the workpiece, and the processing effect is also better.

4 Processing considerations
(1) The cutting tools are frequently ground and kept sharp to ensure that the cutting heat is generated as little as possible during the machining process.
(2) Equipment, knives, tools and fixtures should be kept clean and tidy, and chips should be removed in time.
(3) Use non-combustible or non-combustible tools to transfer titanium scraps. Store the discarded debris in a non-flammable container and cover it well.
(4) Wear clean gloves when handling cleaned titanium alloy parts to avoid sodium chloride stress corrosion in the future.
(5) Fire prevention facilities are provided in the cutting area.
(6) In the case of micro-cutting, once the cut titanium chips catch fire, they can be extinguished with dry powder fire extinguishing agent or dry soil and dry sand. 


 
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