1、 Common fault analysis

During production, it was found that common faults in the surface quality of titanium alloy machining include corrosion, ash accumulation, incomplete removal of oxide skin, and striped spots.

1. Overcorrosion

Overcorrosion refers to the uneven defects such as pits or bumps on the surface of titanium alloy after acid washing, which are different from the exposed material structure. The general reason for the occurrence of over corrosion defects is the imbalance of the ratio of hydrofluoric acid and nitric acid. Excessive concentration of hydrofluoric acid or insufficient concentration of nitric acid can lead to the occurrence of this defect. Another reason is that the acid washing time is too long. Generally, the acid washing time is 1mm to 4 minutes, and the process parameters can be adjusted according to the operation site, Shorten the pickling time appropriately.

2. Hanging ash

Hanging ash refers to the oxide attached to the surface of titanium alloy after acid washing. During acid washing, the dry titanium alloy and acid react chemically, and the generated oxide accumulates on the surface, preventing further reaction. The defects of hanging ash are generally due to excessive deposition of hanging ash during acid washing and insufficient washing after acid washing. During acid washing, the parts should be continuously shaken to allow the reaction products to detach from the surface of the titanium alloy. After acid washing, spraying or rinsing should be strengthened to remove hanging dust. In China, high-speed water flow mixed with compressed air and tap water is generally used to wash parts, and the effect is good.

3. The oxide skin has not been completely removed

There are many reasons that can lead to this defect, and it is possible for each process. It is possible that the oil removal is poor, the molten salt treatment time is insufficient, or the pickling solution is ineffective. When this defect occurs, various possible factors should be eliminated one by one, and if necessary, sandblasting procedures can be added to the pre-treatment.

4. Striped flower spots

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-mentioned defects, sometimes products that pass the inspection after acid washing may also show spots on the surface after a period of time. There is currently relatively little research on this phenomenon, which may be due to the presence of residual acid on the surface after pickling or corrosive media brought in by subsequent production under the combined action of stress. It is different from the general corrosion form under microscopic inspection and generally does not affect its performance. It can be removed by re pickling, but the load-bearing parts need to strengthen the hydrogen removal treatment after secondary pickling.

2、 Factors affecting the machining performance of titanium alloys

The thermal conductivity, elastic modulus, chemical activity, alloy type, and microstructure are the main factors affecting the mechanical processing performance of titanium alloys. The thermal conductivity of titanium alloy is small, about one-third of that of iron, and the heat generated during mechanical processing is difficult to release through the workpiece; Meanwhile, due to the small specific heat of titanium alloy, the local temperature rises rapidly during processing. Easy to cause high tool temperature, resulting in sharp wear of the tool tip and reduced service life. Experiments have shown that the temperature at the front end of the tool for cutting titanium alloy is 2-3 times higher than that of cutting steel. Titanium alloy has a low elastic modulus, which makes the processed surface prone to springback, especially for thin-walled parts, which can cause strong friction between the back cutting surface and the processed surface, leading to tool wear and blade breakage. Titanium alloy has strong chemical activity and is prone to react with oxygen, hydrogen, and nitrogen at high temperatures, resulting in an increase in hardness and a decrease in plasticity. The mechanical processing of the oxygen rich layer formed during heating and forging is difficult. The alloy composition of titanium alloy is different, and its processing performance is also different. In the annealed state, A-type titanium alloy has better mechanical processing performance; A+ β Type titanium alloy comes second; β Titanium alloy has high strength and good hardenability, but its mechanical processing performance is the worst.

In view of the above situation, in order to carry out efficient and high-precision mechanical processing of titanium alloys, corresponding measures should be taken to avoid the occurrence of defects during processing.

3、 Research on Various Mechanical Processing of Titanium Alloy

There are many methods for machining titanium alloys, mainly including turning, milling, boring, drilling, grinding, tapping, sawing, and electrical discharge machining.

1. Turning and boring of titanium alloy

The main problem with turning titanium alloys is: high cutting temperature; Severe tool wear; Cutting rebound is large. Under suitable machining conditions. Turning and boring are not particularly difficult processes. For continuous cutting, mass production, or cutting with high metal removal, hard alloy cutting tools are generally used. When performing forming cutting, grooving, or cutting, adjusting steel cutting tools are suitable, and metal ceramic cutting tools are also applied. Like other machining operations, constant forced feed is always used to avoid cutting interruptions. Do not stop or slow down during the cutting process. Generally, cutting should not be carried out, but sufficient cooling should be carried out; The coolant can be a 5% sodium nitrate aqueous solution or a 1/20 soluble oil emulsion aqueous solution. Before forging, hard alloy cutting tools are used to turn the oxygen rich layer on the surface of the original bar material, and 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 precision machining, especially for thin-walled titanium alloy products. During boring, burns and deformation of parts during clamping should be prevented.

2. Drilling and machining of titanium alloys

When drilling titanium alloys, it is easy to produce long and thin curled chips, and the high drilling heat can cause excessive accumulation or adhesion of chips to the drilling edge, which is the main reason for the difficulty in drilling titanium alloys. Short and sharp drill bits and low-speed forced feed should be used for drilling, the support bracket should be tightened, and repeated and sufficient cooling should be provided, especially for deep hole drilling. During the drilling process, the drill bit should remain in the drilling state inside the hole and not be allowed to idle inside the hole, and should maintain a low and constant drilling speed. When drilling through holes, it is important to be careful. When the hole is about to be drilled, in order to clean the drill bit, drill hole, and remove drilling debris, it is best to retract the drill bit. When the hole is finally broken, a forced feed is used to obtain a smooth hole.

3. Tapping of titanium alloy

Tapping titanium alloy may be the most difficult machining process. When tapping, the removal of titanium shavings due to limitations and severe biting tendencies can lead to poor thread fit, resulting in the tap getting stuck or breaking. When the tapping is completed, the titanium alloy tends to dry tighten on the tap. Therefore, it is advisable to avoid machining blind holes or excessively long through holes as much as possible to prevent the occurrence of increased surface roughness or cone breakage of internal threads. At the same time, continuous improvement should be made in the tapping method, such as grinding off the trailing edge of the tap. The chip removal groove along the length of the tooth edge in the axial direction of grinding at the tooth top. On the other hand, a tap with a surface treated with oxidation, oxidation, or chrome plating is used to reduce biting and wear.

4. Sawing processing of titanium alloy

When sawing titanium alloy, low surface speed and continuous forced feeding should be used. Experimental results have shown that coarse toothed high-speed steel saw blades with a tooth pitch of 4.2mm to 8.5mm are suitable for sawing titanium alloys. If titanium alloy is sawed with a band saw, the tooth pitch of the saw blade is determined by the thickness of the workpiece, generally ranging from 2.5mm to 25.4mm. The thicker the material, the larger the tooth pitch. At the same time, it is necessary to maintain the forced feeding capacity and the required coolant.

5. Electrical discharge machining of titanium titanium

The electrical discharge machining of titanium alloy requires an operating gap between the tool and the workpiece. The best gap range is 0.005mm 0.4mm. Smaller gaps are commonly used for precision machining that requires a smooth surface, while larger gaps are used for rough machining that requires rapid removal of metal. It is best to use copper and zinc as electrode materials.

Through the above analysis and research, the causes of surface quality faults in titanium alloy machining have been identified, and various methods during the machining process have been analyzed, thus finding practical and feasible solutions to solve the surface quality problems in titanium alloy machining.