1. Classification of titanium alloys

In 1956, McGovern made a strict distinction between titanium alloys based on the difference of annealing state, and divided them into three types: α、β and α+β And other titanium alloys.

TC4 (Ti-6Al-4V) belongs to α+β Phase structure, belonging to the largest amount of titanium alloy and the most complete performance data of titanium alloy, aluminum and vanadium are the main alloying elements contained in TC4 (Ti-6Al-4V), and aluminum is α Stabilized element, vanadium is β Stabilize elements.

2. Processing characteristics

TC4 titanium alloy is very difficult to process. The comprehensive process of titanium and titanium alloy is very different from steel, aluminum alloy and many heavy metals in terms of crystal structure, physical properties and chemical properties. The following three factors determine that titanium alloy is a kind of metal that is difficult to process.

(1) Because of its unstable chemical composition. TC4 titanium alloy will react with oxygen and nitrogen under thermal deformation, and even with some oxygen-containing gases. The reaction will produce oxide skin attached to the surface of the workpiece. If the temperature is higher than 900 ℃, the oxide skin attached to the surface of the workpiece will produce scales. In this way, oxygen and nitrogen elements are likely to infiltrate and diffuse into the metal, and eventually form a surface getter layer. High hardness and low plasticity are the characteristics of this kind of aspirating layer.

(2) The performance of cementite in the metallographic structure belongs to a complex Fe-C compound. The Vickers hardness can reach as high as HV1100, while the impact toughness is almost zero.

(3) Low thermal conductivity: compared with other alloys such as aluminum alloy, the thermal conductivity of titanium alloy is only about 1/15 of that of aluminum alloy and about 1/5 of that of steel. The thermal conductivity and thermal conductivity of titanium alloy are much lower than that of aluminum alloy and steel, which are only about 1/15 of that of aluminum alloy and about 2/7 of that of steel, which have a great impact on the surface processing quality of some titanium alloy parts.

3. Grinding characteristics

Because titanium alloy has material characteristics such as high strength, good thermal stability, high temperature strength, high chemical activity, low thermal conductivity, low elastic modulus, etc., it is very difficult to grind. It is one of the most difficult materials to machine. Because of this, its promotion and application range is very limited. It is because the grinding performance of titanium alloy is very poor, and there are such problems in grinding.

The main grinding characteristics of TC4 titanium alloy are as follows:

(1) The bond problem of grinding wheel is serious. The surface of the grinding wheel is adhered with titanium alloy, and the bonded surface is like smoke. The main reason is that the adhesive falls off during the grinding process, which will cause the abrasive particles to fall off along with the breaking, and eventually seriously damage the grinding wheel.

(2) The grinding force is large and the grinding temperature is high. During the single abrasive grinding test, it was found that when grinding titanium alloy, the sliding friction process accounted for a large proportion, and the contact time between the abrasive particles and the workpiece was very short. In this very short time, severe friction and severe elastic and plastic deformation occurred, and then the titanium alloy was grinded into chips, resulting in large grinding heat. At this time, the maximum grinding temperature could reach about 1500 ℃.

(3) The main reason for the formation of laminated chips during grinding is the complex deformation. The banded chips are mostly formed when grinding 45 steel with white corundum grinding wheel (WA60KV), and the laminated extruded chips are mostly formed when grinding titanium alloy with green silicon carbide grinding wheel (GC46KV).

(4) Under high temperature conditions, TC4 titanium alloy exhibits very active chemical activity, and is easy to react violently with oxygen, nitrogen, hydrogen and other elements in the air, forming brittle and hard metamorphic layers such as titanium dioxide, titanium nitride, titanium hydride, etc., resulting in the reduction of TC4 plasticity.

(5) In the process of titanium alloy grinding, the problem to be solved is mainly because the grinding heat introduced into the workpiece is difficult to guide out, which is easy to deform, burn, and even cause some cracks. Therefore, the workpiece surface will appear different degrees of roughness.

4. Grinding technology innovation

4.1 Restraining measures to solve grinding burn and crack

There are some problems when using grinding wheel to process TC4 titanium alloy. The most serious problem is adhesion. Due to the high speed, the grinding force and temperature are relatively high, which will burn the surface and produce cracks. Ren Jingxin and others have done some experimental research to reduce this kind of burn and crack phenomenon in the process of processing. They think they can use a softer grinding wheel, such as using silicon carbide or cerium silicon carbide grinding wheel instead of corundum grinding wheel. The corundum grinding wheel is bonded with resin, while the former is bonded with ceramic. And the processing parameters should also be noted, for example, the speed of the grinding wheel should not be too fast, the experimental analysis should not exceed 20 meters per second, the grinding depth should not be too much, not more than 0.02 mm, and the moving speed of the workpiece should also be required, about 12-16 meters per minute. The grinding fluid should not only have good heat dissipation, but also emphasize its lubrication effect, which can effectively inhibit the occurrence of adhesion, if it is dry grinding, The lubricant can be a solid lubricant impregnated grinding wheel.

4.2 Grinding wheel adhesion in titanium alloy grinding and its control measures

Because in the grinding process of titanium alloy, there will generally be high grinding temperature and large normal force, so severe plastic deformation will occur in the titanium alloy in the grinding area, and the bonding effect between the abrasive and the metal will occur through physical or chemical adsorption; The reason for the transfer of the ground metal to the abrasive grains is the effect of shear force, which leads to the adhesion of the grinding wheel. Finally, the abrasive grains will be broken. When the grinding force exceeds the size of the binding force between the abrasive grains, the abrasive grains and the adhesive will be stripped from the grinding wheel.

4.3 High-speed and high-efficiency grinding

On the ultra-high speed grinding machine designed and manufactured by the National High Efficiency Grinding Engineering Technology Research Center of Hunan University, some scholars carried out high-speed and efficient grinding of TC4 titanium alloy material. The influence of grinding parameters on the grinding force and specific grinding energy per unit area was analyzed. If the grinding wheel linear speed vs increases, the grinding force per unit area will decrease significantly, but if the table speed vw and the grinding depth ap increase, the grinding force per unit area will increase. If the grinding wheel linear speed vs increases, the specific grinding energy will rise, but if the workbench speed vw and the grinding depth ap increase, the specific grinding energy will decline.

The above is the introduction of titanium alloy material characteristics and grinding characteristics!