Grinding workability of titanium alloys
High grinding temperature and large grinding force are the remarkable characteristics of titanium alloy grinding. Under ordinary grinding conditions, the grinding force corresponding to Ti-6Al-4V is about 1.5 to 2 times that of 45 steel, and the grinding temperature is about 20% to 30% higher. The cutting temperature is also as high as 400 ℃. If the slow-feeding and the deep-cutting process are used to grind titanium alloys, it is difficult for the cutting fluid to fully cool the entire grinding arc area, and special attention should be paid to the control of grinding temperature.
Different from cutting, grinding relies on the micro-cutting action of many grinding edges to remove materials, and it is a negative rake angle cutting. Therefore, the workpiece material is deformed more severely under the action of the extrusion and cutting of abrasive particles, resulting in more serious problems on the grinding surface. scaly coating and other phenomena. Increasing the grinding speed can significantly improve this problem by reducing the individual grit thickness. Under ordinary grinding conditions, due to the high grinding temperature, the surface layer of the workpiece after grinding is mostly residual tensile stress. For example, when SiC grinding wheel is used to process Ti-6Al-4V under normal grinding conditions, the residual tensile stress on the grinding surface is as high as 500 MPa or more. If the slow-feed and deep-cut grinding processes are adopted, the temperature of the arc zone during normal grinding is only about 100 °C. At this time, the grinding force plays a leading role in the formation of residual stress, so the surface is mostly residual compressive stress.
Selection of titanium alloy grinding wheel
Among ordinary abrasives, the affinity between SiC abrasive and titanium alloy is low, so its grinding effect is better than that of corundum abrasive. If corundum abrasive is used to grind titanium alloy, to avoid large-scale material adhesion on the surface of the grinding wheel, the grinding speed must be controlled at about 10 m/s. Under the current level of research progress in grinding tool technology 2, issue 5, Xu Jiuhua: Titanium Alloy Cutting and Grinding Processing Technology, the grinding wheel wears faster when grinding titanium alloys with ordinary grinding wheels. For example, the grinding ratio for machining titanium alloys with SiC grinding wheels is only about 1 under ordinary grinding conditions. When using a super-hard grinding wheel, the increase is dozens or even hundreds of times. In addition, compared with ordinary abrasives, the thermal conductivity of super abrasives is significantly enhanced, so a higher material removal rate can be obtained. On the other hand, when grinding titanium alloy with a superhard grinding wheel, frequent dressing of the grinding wheel can be avoided, and the grinding efficiency can be further improved.
Titanium alloy grinding temperature control technology
Grinding at high temperatures is an important reason for inhibiting the efficiency of titanium alloy grinding. In this regard, researchers have carried out a series of studies on developing new abrasive tools and improving cooling methods, and have achieved remarkable results.
In terms of improving cooling, there are mainly heat pipe grinding wheel technology, low-temperature cold air technology, and radial water jet technology. For example, the heat pipe grinding wheel technology realizes the enhanced heat exchange of the grinding arc area based on the angle of internal cooling, and the heat in the arc area is quickly drained by the heat pipe, to achieve the purpose of using less or no cutting fluid; in the micro-lubrication grinding, the low-temperature cold air technology is used. , which can significantly improve the heat transfer in the grinding arc area and reduce the adhesion of the grinding wheel.
Research progress of cutting/grinding processing technology of TiAl alloy
TiAl alloy is a special kind of high-temperature titanium alloy. Its service temperature is more than 200 °C higher than that of ordinary titanium alloys, so it has broad application prospects. There are four kinds of TiAl alloys: α2-Ti3Al, γ-TiAl, TiAl3, and Ti2AlNb; among them, γ-TiAl has been widely studied due to its excellent comprehensive properties. The research on the preparation method and material properties of this alloy has been relatively mature, and there are several types of research on its cutting/grinding properties. Now combined with its main material characteristics, it is introduced as follows.
Compared with ordinary titanium alloys, γ-TiAl has poor plasticity (room temperature elongation ≤ 2%). Therefore, during its cutting process, brittle fractures of local material may occur, and material peeling and cracks are formed on the cutting surface. According to the shape of material peeling and cracks on the turning surface, it is speculated that the lamellar microstructure is the internal cause of the above surface defects: the grain boundaries between the lamellar grains initiate microcracks under the coupling of cutting force and heat and extend to a certain extent. depth, eventually causing localized material to be "pulled out" from the surface of the workpiece. The thickness of the single abrasive grain for grinding is much smaller than that of the cutting process, and there is no serious material peeling and cracking on the grinding surface. In addition, the strength of γ-TiAl is lower than that of ordinary titanium alloy, and the thermal conductivity of the material is improved due to the high content of aluminum (the proportion of atomic number is generally 42% to 48%). This is a positive factor for its cutting/grinding workability. For example, grinding γ-TiAl and Ti-6Al-4V at the same grinding dosage level, the former has a lower grinding specific energy and a significantly higher grinding ratio.
Research progress in cutting/grinding processing technology of titanium matrix composites
Titanium matrix composites refer to metal matrix composites formed by adding/in-situ generation of hard reinforcing phases in pure titanium or titanium alloys. In some applications, they are also called high-temperature titanium alloys. It has higher strength, specific modulus, and better creep resistance than ordinary titanium alloys, and is expected to be widely used in the aviation industry, automobiles, ships, and other fields in the future.
The research on cutting/grinding processing of titanium matrix composites mostly focuses on in-situ particle-reinforced (TiC, etc.) Ti–6Al–4V matrix composites with good comprehensive properties. The results show that the improvement of material strength and the existence of a reinforcing phase increase the difficulty of cutting/grinding processing of titanium matrix composites. When the PCD tool is used to cut titanium matrix composites, the tool abrasive wear is serious; when cutting Ti-6Al-4V, the tool is mainly bonded and diffused. This can lead to differences in tool durability by several or even tens of times. During grinding, the corresponding grinding force of the titanium-based composite material is 10% to 20% higher, and the grinding temperature is about 10% higher.
Different from aluminum matrix composites, limited to the development level of the current material preparation technology, the volume fraction of the reinforcement phase of the titanium matrix composites involved in the existing research is concentrated at 5% to 10%. The volume fraction of the reinforcing phase has a significant effect on the cutting/grinding properties of the material. The higher the volume fraction, the greater the difficulty of cutting/grinding. Therefore, in practical applications, reasonable material selection should be made in consideration of the service environment's requirements for material properties and the cost of cutting/grinding processing.
