Explanation of Grinding Techniques for Precision TC4 Titanium Alloy Parts
TC4 titanium alloy processing is very difficult. 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 metal that is difficult to grind.
(1) Due to the instability of its chemical composition.
TC4 titanium alloy will chemically react with oxygen and nitrogen under thermal deformation, and even chemical reaction with some oxygen-containing gas, the reaction will produce oxide scale attached to the surface of the workpiece. If the temperature is higher than 900 ℃, the oxide scale attached to the surface of the workpiece will produce scales. In this way, oxygen and nitrogen elements are likely to permeate and diffuse into the metal, and eventually form a surface getter. Higher hardness and lower plasticity are characteristics of this gettering layer.
(2) The performance of cementite in metallographic structure is a complex Fe-C compound with a Vickers hardness of up to HV1100 and almost no impact toughness.
(3) Thermal conductivity is not high: If the thermal conductivity of titanium alloy is compared with other alloys such as aluminum alloy, The thermal conductivity is only 1/15 of the aluminum alloy and 1/5 of the steel. The temperature conductivity of titanium alloy is much lower than that of aluminum alloy and steel. The temperature conduction rate is only 1/15 of the aluminum alloy and 2/7 of the steel. These have a relatively large influence on the surface quality of some titanium alloy parts.
Main grinding characteristics of titanium alloy：
(1) The problem of bonding of the grinding wheel is serious.
The surface of the grinding wheel is adhered with titanium alloy, and the surface of the bonding surface is like smoke. The main reason is that the adhesives (titanium powder) fall off during the grinding process, which will lead to the abrasive particles falling off with the breakage, which will eventually seriously damage the grinding wheel.
(2) The grinding force is large and the grinding temperature is high.
In the single abrasive grinding test, it was found that when grinding titanium alloy, the larger proportion is the sliding process, and the contact time between the abrasive particles and the workpiece is very short. During this very short period of time, intense friction and severe elastic and plastic deformation occur, and then the titanium alloy is ground to become chips, resulting in a large grinding heat, at which the grinding temperature can be as high as almost 1500 ° C.
(3) Grinding will produce laminated cracking chips, mainly due to the complicated deformation.
When the strip-shaped chips are formed more, the 45 steel is ground with a white corundum grinding wheel (WA60KV). When the laminated chip is formed, the titanium alloy is ground with a green silicon carbide grinding wheel (GC46KV).
(4) Under high temperature conditions, the chemical activity of TC4 titanium alloy is quite active. It is easy to react with elements such as oxygen, nitrogen and hydrogen in the air to form a brittle hard metamorphic layer such as titanium dioxide, titanium nitride or titanium hydride, resulting in a decrease in plasticity of TC4.
(5) In the grinding process of titanium alloy, it is affected by the problem that is difficult to solve, mainly because the grinding heat in the incoming workpiece is difficult to be exported. It is easy to make the workpiece deform, burn, and even some cracks. Therefore, the workpiece surface will appear different degrees of roughness.
Solution
Inhibition measures to solve titanium alloy grinding burns and cracks
There are some problems when machining TC4 titanium alloy with a grinding wheel. More serious is the adhesion phenomenon. Because of the higher speed, the grinding force and temperature are relatively high, which will burn the surface and cause cracks. Kangding has done some experimental research to reduce the burns and cracks that occur during processing. They feel that they can use softer grinding wheels, such as silicon carbide or silicon carbide grinding wheels instead of corundum grinding wheels. Corundum grinding wheels are adhered with resin, while the former uses ceramic adhesion. And the processing parameters should also pay attention to, 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. The moving speed of the workpiece is also required, probably within 12-16 meters per minute. The grinding fluid not only needs to dissipate heat well, but also emphasizes its lubricating effect, which can effectively suppress the occurrence of bonding. In the case of dry grinding, the lubricant can be impregnated with a solid lubricant.
Bonding phenomenon of grinding wheel in titanium alloy grinding and its restraining measures
(1) Due to the instability of its chemical composition.
TC4 titanium alloy will chemically react with oxygen and nitrogen under thermal deformation, and even chemical reaction with some oxygen-containing gas, the reaction will produce oxide scale attached to the surface of the workpiece. If the temperature is higher than 900 ℃, the oxide scale attached to the surface of the workpiece will produce scales. In this way, oxygen and nitrogen elements are likely to permeate and diffuse into the metal, and eventually form a surface getter. Higher hardness and lower plasticity are characteristics of this gettering layer.
(2) The performance of cementite in metallographic structure is a complex Fe-C compound with a Vickers hardness of up to HV1100 and almost no impact toughness.
(3) Thermal conductivity is not high: If the thermal conductivity of titanium alloy is compared with other alloys such as aluminum alloy, The thermal conductivity is only 1/15 of the aluminum alloy and 1/5 of the steel. The temperature conductivity of titanium alloy is much lower than that of aluminum alloy and steel. The temperature conduction rate is only 1/15 of the aluminum alloy and 2/7 of the steel. These have a relatively large influence on the surface quality of some titanium alloy parts.
Main grinding characteristics of titanium alloy：
(1) The problem of bonding of the grinding wheel is serious.
The surface of the grinding wheel is adhered with titanium alloy, and the surface of the bonding surface is like smoke. The main reason is that the adhesives (titanium powder) fall off during the grinding process, which will lead to the abrasive particles falling off with the breakage, which will eventually seriously damage the grinding wheel.
(2) The grinding force is large and the grinding temperature is high.
In the single abrasive grinding test, it was found that when grinding titanium alloy, the larger proportion is the sliding process, and the contact time between the abrasive particles and the workpiece is very short. During this very short period of time, intense friction and severe elastic and plastic deformation occur, and then the titanium alloy is ground to become chips, resulting in a large grinding heat, at which the grinding temperature can be as high as almost 1500 ° C.
(3) Grinding will produce laminated cracking chips, mainly due to the complicated deformation.
When the strip-shaped chips are formed more, the 45 steel is ground with a white corundum grinding wheel (WA60KV). When the laminated chip is formed, the titanium alloy is ground with a green silicon carbide grinding wheel (GC46KV).
(4) Under high temperature conditions, the chemical activity of TC4 titanium alloy is quite active. It is easy to react with elements such as oxygen, nitrogen and hydrogen in the air to form a brittle hard metamorphic layer such as titanium dioxide, titanium nitride or titanium hydride, resulting in a decrease in plasticity of TC4.
(5) In the grinding process of titanium alloy, it is affected by the problem that is difficult to solve, mainly because the grinding heat in the incoming workpiece is difficult to be exported. It is easy to make the workpiece deform, burn, and even some cracks. Therefore, the workpiece surface will appear different degrees of roughness.
Solution
Inhibition measures to solve titanium alloy grinding burns and cracks
There are some problems when machining TC4 titanium alloy with a grinding wheel. More serious is the adhesion phenomenon. Because of the higher speed, the grinding force and temperature are relatively high, which will burn the surface and cause cracks. Kangding has done some experimental research to reduce the burns and cracks that occur during processing. They feel that they can use softer grinding wheels, such as silicon carbide or silicon carbide grinding wheels instead of corundum grinding wheels. Corundum grinding wheels are adhered with resin, while the former uses ceramic adhesion. And the processing parameters should also pay attention to, 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. The moving speed of the workpiece is also required, probably within 12-16 meters per minute. The grinding fluid not only needs to dissipate heat well, but also emphasizes its lubricating effect, which can effectively suppress the occurrence of bonding. In the case of dry grinding, the lubricant can be impregnated with a solid lubricant.
Bonding phenomenon of grinding wheel in titanium alloy grinding and its restraining measures
Since titanium alloys generally have higher grinding temperatures and larger normal forces during grinding. Such severe plastic deformation will occur in the titanium alloy in the grinding zone, and the physical or chemical adsorption between the abrasive and the metal will produce a bonding effect; The cause of the transfer of the ground metal to the abrasive particles is the effect of the shear force, which causes the grinding wheel to bond and eventually causes the abrasive particles to break. When the grinding force exceeds the bonding force between the abrasives, the abrasives and bonds will be peeled off the grinding wheel.
High speed and efficient grinding
Some scholars have carried out high-speed and high-efficiency grinding of TC4 titanium alloy materials. In the study, the influence of the grinding force per unit area and the grinding amount on the grinding amount was analyzed. If the grinding wheel line speed vs increases, the grinding force per unit area will drop significantly. However, if the table speed vw and the grinding depth ap increase, the grinding force per unit area increases. If the grinding wheel linear velocity vs is increased, it is higher than the grinding energy, but if the table speed vw and the grinding depth ap increase, it is lower than the grinding energy.
High speed and efficient grinding
Some scholars have carried out high-speed and high-efficiency grinding of TC4 titanium alloy materials. In the study, the influence of the grinding force per unit area and the grinding amount on the grinding amount was analyzed. If the grinding wheel line speed vs increases, the grinding force per unit area will drop significantly. However, if the table speed vw and the grinding depth ap increase, the grinding force per unit area increases. If the grinding wheel linear velocity vs is increased, it is higher than the grinding energy, but if the table speed vw and the grinding depth ap increase, it is lower than the grinding energy.
