Titanium alloy is an important metal structural material that emerged in the 1950s. In recent years, titanium will be widely used in the civilian field. The developed titanium alloy products have almost penetrated into all aspects of modern life.
1 Common fault analysis
It was found in the production that the common faults of the surface quality of titanium alloy machining include over-corrosion, ash hang, unoxidized scale and stripe pattern.
- 1.1 Over-corrosion
Over-corrosion refers to the appearance of pits or uneven uneven defects on the surface of the titanium alloy after pickling, which is different from the material structure. Generally, the cause of over-corrosion defects is the imbalance of the ratio of hydrofluoric acid and nitric acid. Too high hydrofluoric acid concentration or insufficient nitric acid concentration can cause this defect. Another reason is that the pickling time is too long, generally the pickling t is 1mm ~ 4min, the process parameters can be adjusted according to the operation site to shorten the pickling time appropriately. - 1.2 hanging gray
Hanging dust refers to the oxides attached to the surface of the titanium alloy after pickling. During pickling, the dry titanium alloy and the acid solution undergo a chemical reaction, and the generated oxides accumulate on the surface, preventing further reaction. The defects of hanging ash are generally caused by excessive deposition of hanging ash during pickling and insufficient flushing after pickling. During pickling, the parts should be shaken continuously to make the products after the reaction fall off the surface of the titanium alloy. After pickling, the spraying or washing method should be strengthened to remove the dust. Domestically, the high-speed water flow mixed with compressed air and tap water is generally used for washing parts, and the effect is good. - 1.3 The scale is not removed
There are many reasons for this defect, and each process is possible. It is possible that the oil removal is not good, or the molten salt treatment time is insufficient, or the pickling solution fails. When this defect occurs, various possible factors should be ruled out one by one, and when necessary, the blasting process can be added in the pre-treatment. - 1.4 Striped piebald
The cause of this defect is generally due to uneven reaction. It can be eliminated by shaking the parts during pickling and lowering the temperature of the pickling solution. In addition to the above defects, sometimes products that pass inspection after pickling are found, and after a period of time, mottling appears on the surface. For this phenomenon, there are few studies now. It may be due to the presence of residual acid liquid on the surface after pickling or the presence of corrosive media brought by subsequent production, which is generated under the joint action of stress. The difference generally does not affect its performance, and can be removed by pickling again, but the stressed parts should strengthen the hydrogen removal treatment after the second pickling.
2. Factors affecting the machining performance of titanium alloy
Thermal conductivity, elastic modulus, chemical activity, alloy type and microstructure are the main factors that affect the machining performance of titanium alloys. The thermal conductivity of titanium alloy is small, about 1/3 of iron, and the heat generated during machining is difficult to be released through the workpiece. At the same time, because the specific heat of titanium alloy is small, the local temperature rises quickly during processing. It is easy to cause the tool temperature to be very high, which causes sharp wear of the tool tip and reduces the service life. Experiments show that the temperature of the tip of the tool for cutting titanium alloy is 2-3 times higher than the temperature of cutting steel. The low elastic modulus of titanium alloy makes the processed surface prone to springback, especially the processing of thin-walled parts is more serious, and it is easy to cause strong friction between the flank surface and the processed surface, thereby wearing the tool and chipping. Titanium alloys are very chemically active, and easily interact with oxygen, hydrogen, and nitrogen at high temperatures, increasing their hardness and decreasing plasticity. It is difficult to machine the oxygen-rich layer formed during heating and forging. Titanium alloys have different alloy compositions and different processing properties. In the annealed state, the a-type titanium alloy has better mechanical processing performance; the a + β-type titanium alloy is second; the β-type titanium alloy has high strength and good hardenability. But the machining performance is the worst.
In view of the above, in order to carry out high-efficiency and high-precision machining of titanium alloys, corresponding measures should be taken to avoid the occurrence of defects in processing.
3. Research on various machining of titanium alloy
There are many methods for machining titanium alloy, including: turning, milling, boring, drilling, grinding, tapping, sawing, EDM, etc. ,
- 3.1 Turning and boring of titanium alloy
The main problems of turning titanium alloy are: high cutting temperature; severe tool wear; large cutting springback. Under suitable machining conditions. Turning and boring are not particularly difficult processes. For continuous cutting, mass production, or cutting with a large amount of metal removal, cemented carbide tools are generally used. When forming cutting, grooving or cutting, suitable steel tools are used, and cermet tools are also used. As with other machining operations, a constant forced feed is always used to avoid cutting interruptions. Do not stop or slow down during cutting. Generally do not cut, but should be fully cooled; coolant can use 5% sodium nitrate aqueous solution or 1/20 soluble oil emulsion solution. Before forging, the cemented carbide tool is used for turning the oxygen-rich layer on the surface of the original bar. The cutting depth should be greater than the thickness of the oxygen-rich layer. The cutting speed is 20-30m / min and the feed rate is 0.1-0.2mm / r. Boring is a finishing process, especially for thin-walled titanium alloy products in the boring process, to prevent burns and parts deformation. - 3.2 Drilling of titanium alloy
It is easy to generate long and thin curled chips when drilling titanium alloy. At the same time, the drilling heat is large, which is easy to cause the chips to accumulate excessively or adhere to the drilling edge. Drilling should use short and sharp bits and low-speed forced feed, the support bracket should be fastened, and repeated and sufficient cooling should be given, especially for deep hole drilling. During the drilling process, the drill bit should maintain the drilling state in the hole and not allow idling in the drill hole, and should maintain a low and constant speed drilling speed. Drill through holes carefully. When drilling soon, in order to clean the drill bit and the drill hole, and to remove the drill cuttings, it is best to return the drill bit. When the hole is finally broken, forced feed is used to obtain a smooth hole. - 3.3 Tapping of titanium alloyTapping of titanium alloy is probably the most difficult machining process. When tapping, the removal of titanium chips is restricted and the serious tendency to bite will lead to a poor thread fit, causing the tap to jam or break. When the tapping is completed, the titanium alloy tends to shrink tightly on the tap. Therefore, it should try to avoid processing blind holes or too long through holes to prevent the surface roughness of the internal thread from becoming larger or the phenomenon of broken cones. At the same time, the tapping method should be continuously improved, for example, the trailing edge of the tap can be ground off. Along the length of the tooth edge, the axial chip removal groove is ground on the tooth tip. On the other hand, taps with oxidized, oxidized or chrome-plated surfaces are used to reduce bite and wear.
- 3.4 Sawing processing of titanium alloy
When sawing a titanium alloy, low surface speed and continuous forced feeding are required. The experiment proves that the coarse tooth high-speed steel saw blade with a tooth pitch of 4.2mm to 8.5mm is suitable for sawing titanium alloy. If a band saw is used to saw titanium alloy, the pitch of the saw blade is determined by the thickness of the workpiece, which is generally 2.5 mm to 25.4 mm. The thicker the material, the greater the pitch. At the same time, the mandatory feed capacity and the required coolant must be maintained. - 3.5 EDM of titanium titanium
EDM of titanium alloy requires an operating gap between the tool and the workpiece. The range of the gap is preferably 0.005mm0.4mm. The smaller gap is often used for finishing with smooth surface, and the larger gap is used for roughing that requires rapid metal removal. The electrode material is preferably copper and zinc.
Through the above analysis and research, the reason of titanium alloy machining surface quality failure was obtained, and various methods in the machining process were analyzed, so as to find a practical method for solving the titanium alloy machining surface quality problem.
