At present, most parts of aero-engine use titanium alloy materials, according to the assembly requirements, precision machining surface size accuracy and surface roughness need grinding processing to ensure the quality of parts surface.Due to the physical and mechanical properties of titanium alloy materials, surface burns and surface integrity are easy to occur during grinding. At present, it is urgent to solve the problem of titanium alloy materials grinding, so it is very necessary to choose the right grinding wheel.
1. Property analysis of titanium alloy materials
Titanium alloy can be divided into three categories, alpha titanium alloy, beta titanium alloy and alpha + beta titanium alloy.It has small specific gravity, high specific strength, high temperature resistance, corrosion resistance, super memory, no magnetism, low modulus of elasticity, biocompatibility number, which makes it applied in a wide range of fields.Titanium has a melting point of 1668 ° c and a boiling point of 3400 ° c, higher than that of ferronickel. Therefore, light heat resistance becomes its excellent foundation and can work for a long time at 500 ° c.The new titanium alloy long - term working temperature is even higher, at 300-350℃ its strength is 10 times higher than aluminum alloy.Commonly used alpha beta titanium alloy strength up to 1.2GPa, specific gravity 0.44MPa, specific strength 23-27, are higher than alloy steel.The tensile strength of titanium alloy can exceed 1.5GPa, and a great force must be applied to its processing.
The thermal conductivity of titanium alloy TC11 was worse than that of titanium alloy.The elastic modulus of titanium is about 1/2 of that of steel.
Titanium alloy contains oxygen, hydrogen, nitrogen, carbon, sometimes including silicon, iron and other impurity elements, these elements have a strong reaction, in a gap type exist in the lattice, can make the titanium alloy strength increased, plastic decline, even fracture toughness, low temperature toughness, fatigue strength, corrosion resistance, cold forming and weldability bad.Titanium alloy has high chemical property in high temperature. Under a certain grinding temperature, titanium forms oxidation and nitride protective film, which makes the surface layer hardened and brittle, reduces elasticity, and increases the degree of work hardening. It is easy to stick to the surface during grinding, blocking the grinding wheel, causing grinding overheating and surface integrity reduction.
2. Selection of grinding wheel for titanium alloy grinding
2.1 titanium alloy grinding requires small grinding wheel adhesion, small wear, no clogging and low grinding temperature
This mainly includes abrasive particle size binder structure shape size.Common grinding wheel is composed of abrasive binder and blowhole.The function of abrasive is to grind the processed material to form a surface that meets the requirements.The function of the binder is to bond the abrasive together to form a certain shape and hardness, so that the abrasive particles can maintain a stable movement track in the grinding process and self-detach.Air hole is in grinding process play chip discharge, cooling and lubrication.Common abrasives include corundum series (alumina), and silicon carbide series.Silicon carbide grinding wheel should be selected for grinding titanium alloy.
2.2 selection of binders there are two kinds of binders: resin and ceramic:
1) ceramic binder particles have strong capability, good thermal and chemical stability, waterproof, heat resistant, corrosion resistant, small wear, long time maintenance grinding performance, porous, not easy to block, high productivity.Brittleness, can not withstand large impact load.
2) resin bond grinding wheel has high strength, elasticity, good impact resistance, poor thermal stability, poor corrosion resistance, and will soften and lose strength at high temperature.Ceramic bond grinding wheel should be selected for grinding titanium alloy.
2.3 selection of particle size for grinding titanium alloy, particle size 36#-80# is usually used.
2.4 the grinding wheel structure hardness for grinding titanium alloy should generally choose a large porous grinding wheel with relatively soft hardness or medium hardness and relatively loose structure.
3. Analysis of grinding force in the grinding process of titanium alloy when grinding titanium alloy with silicon carbide and alumina grinding wheel, the grinding chip is easy to stick to the surface of the grinding wheel, and the grinding force is relatively large, and the grinding wheel wears seriously, leading to the deterioration of the surface quality of the workpiece.
1) relationship between grinding force and grinding parameters.Selection of reasonable grinding parameters to control the size of the grinding force and improve the grinding quality is the key to titanium alloy grinding. The grinding force in the process of titanium alloy grinding is composed of cutting and friction, which occupy different proportions.The chip deformation force in the grinding of titanium alloy is the main one, which accounts for most of the grinding force about three times of the friction force.But compared with ordinary metal materials, the frictional force accounts for a much larger proportion.This has a great relationship with the properties of titanium alloy materials, titanium alloy elastic modulus is small, low thermal conductivity, high chemical activity, viscosity, toughness.Grinding wheel adhesion in grinding, resulting in increased friction.The friction force is less affected by the linear velocity of grinding wheel and more affected by grinding depth.Therefore, to control the grinding force, the appropriate grinding depth and grinding line speed should be used.The friction coefficient of titanium alloy increases with the increase of limit speed.In tribology, the relationship between coefficient of friction and relative sliding.Only when the load increases, the friction coefficient tends to increase with the increase of sliding velocity.In grinding, each grain is subjected to considerable stress.All friction coefficients are greatly affected by grinding parameters.
2) the linear relationship between chip deformation force and grinding depth is due to the size effect. Based on the basic principle of fracture mechanics, in fact, the grinding process is a process of crack expansion.The size effect is mainly caused by the chip deformation force in the grinding force.In fact, the relationship between metal removal rate and normal grinding force does not only depend on the dressing condition of workpiece material and grinding wheel, because size effect also exists.
3) the grinding depth has a great influence on the metal removal ratio, which is due to the size effect.The effect of workpiece speed is much smaller, so that the normal force and metal removal ratio is non-linear.Comprehensive above: grinding titanium alloy materials should choose the grinding parameters: speed 45M/S, cutting depth 0.01-0.02mm.
4. Mechanism analysis of grinding burn of titanium alloy
In the grinding process of titanium alloy, the grinding wheel adhesion is serious, the grinding force and grinding temperature are very high, so it is easy to produce grinding burns and cracks.When grinding titanium alloy with ordinary abrasive, even if the grinding depth is very small, grinding burns and cracks will occur on the grinding surface.The general appearance is yellowish brown spot, hair striate crack, its direction and grinding direction are perpendicular, with big grinding amount then the surface still can have the metal deposit of fish scale shape furrow fold and plastic deformation.What is this stuff?We look at the silicon carbide abrasive grinding titanium alloy so that the chemical reaction formula: SiC+Ti→TiC+Si.And look at the oxidation reaction formula of silicon carbide grinding particles at a certain atmospheric temperature: SiC+2O2→SiO2+CO2.
One part of the carbon atoms to be grinding workpiece surface diffusion, the sliding under the action of shear force, SiO2 oxide film stripping, cause oxidative SiC abrasive wear, and the surface oxygen and carbon diffusion layer and reaction in the weakening of abrasive surface losing carbon layer, SiC abrasive hardness strength drops, increase of grinding force, grinding wheel wear, grinding temperature rise is inevitable.From the microscopic point of view, the formation process of abrasive particles and adhesion is that the cutting edge first appears small area adhesion gradually large area adhesion, the abrasive particles are finely broken, then the abrasive particles are broken off, and the titanium alloy grinding area produces plastic deformation.Abrasive particles and workpiece bond with each other, which includes both physical adsorption and chemical adsorption. In addition to the shear force of relative sliding friction, the processed material is transferred to abrasive particles, which is the whole process of grinding wheel adhesion.
The adhesion of grinding wheel can easily cause blockage and increase the temperature of the grinding area. Distributed grinding burns occur along the surface of the workpiece, accompanied by metamorphic layer and residual stress.
In the process of grinding cooling, when the grinding depth is large, the density of oxide film produced by the grinding surface reaches over the critical temperature, and the temperature of the grinding arc area will rise sharply due to the film boiling of grinding fluid, and the cooling effect will become worse.When the temperature increases beyond the limit of normal abrasive, abrasive particles will fall off due to softening.Therefore, a new type of super-hard grinding wheel is needed to further increase the high temperature tolerance of grinding arc area.
5. Analysis of grinding surface integrity of titanium alloy
Surface integrity is an important index to evaluate the surface quality of industrial products.The fundamental task of machining is to transform the workpiece into a part that can complete a specific function under specific conditions, and the surface quality of the part is the main factor that affects its performance.Surface integrity refers to the surface roughness, surface residual stress, the state of the surface structure intact.Including parts after processing surface texture, surface metallurgical quality.Surface texture mainly includes roughness, ripple, macro crack, fold and tear.The quality of surface metallurgy mainly includes the change of microstructure, residual stress of corrosion cracks between crystal layers and lattice, alloy dilution, etc.
6. Experimental conclusion
To sum up, in the production practice, through careful observation, the change of grinding surface integrity of titanium alloy was measured, appropriate grinding parameters were adjusted, and physical and chemical methods were selected to further improve heat dissipation and lubricity in the grinding process.The final choice of silicon carbide, ceramic binder, 80# particle size, soft hardness or medium hardness, loose structure of the large hole grinding wheel is the best way to process titanium alloy grinding.
7. Procedures for safe operation in grinding
7.1 installation of grinding wheel
1) conduct a comprehensive inspection of the grinding wheel, and it shall not be used if quality, hardness and appearance cracks of the grinding wheel are found.
2) gasket shall be placed between flange plate and grinding wheel.
3) after the grinding wheel with diameter greater than or equal to 200mm is installed on the grinding wheel chuck, static balance shall be carried out first.
7.2 in the operation
1) wear protective glasses during operation.
2) check whether the grinding wheel is loose or cracked, whether the protective cover is firm and reliable, and do not start when problems are found.
3) the speed of the grinding wheel is not allowed to exceed the limit. Before feeding, a reasonable amount of knife should be selected and feeding slowly.
4) when loading and unloading the work pieces, the grinding wheel should return to a safe position.
5) the grinding wheel shall not be stopped before it moves away from the workpiece.