Turning of magnesium alloys
Magnesium alloy, currently the lowest-density metal structural material (density 1.7-1.8 g/cm³), boasts high specific strength, excellent shock absorption, and superior thermal conductivity. It is widely used in aerospace, lightweight automotive, and consumer electronics applications. However, its turning presents unique challenges, including flammability, oxidation susceptibility, and a tendency toward work hardening. For example, magnesium alloy chips are prone to spontaneous combustion in air (ignition point approximately 520°C). Improper cooling and chip accumulation during cutting can cause fires. Furthermore, magnesium alloys have low plasticity (elongation 5%-15%), making them prone to chip breakage during turning, resulting in surface roughness. For example, when machining AZ91D magnesium alloy parts, improper cutting parameters can result in surface roughness values (Ra) of up to 6.3 μm, and tool wear is two to three times higher than when machining aluminum alloys. Therefore, magnesium alloy turning requires a dedicated safety and protection system and targeted cutting process optimization.
Safety and protection technology is paramount for magnesium alloy turning. Comprehensive protection measures must be implemented, encompassing chip control, fire extinguishing measures, and environmental management. The primary cause of chip spontaneous combustion is heat accumulation (especially in fine chips). Therefore, a high-flow cooling system (≥50 L/min) is required. This system sprays cutting fluid directly into the chip formation area to quickly remove heat and keep the chip temperature below 300°C (well below the ignition point). Non-flammable mineral oils (such as light diesel or specialized magnesium alloy cutting oils) should be used as cutting fluids. Water-based cutting fluids are strictly prohibited (magnesium reacts with water to produce hydrogen, increasing the risk of explosion). Work areas must be equipped with dry powder fire extinguishers (ABC type) and fire-resistant sand. Flammable items must not be stored, and a fire barrier must be established within 10 meters of the machine tool. Chips must be promptly removed (once every hour) and collected in dedicated metal containers (no thicker than 50 mm). These containers must be covered to prevent airflow and accelerated oxidation. By strictly enforcing safety regulations, an aviation factory has achieved 10 years of zero safety incidents in magnesium alloy turning.
The selection of tool materials and geometry must be tailored to the characteristics of magnesium alloys. Magnesium alloys have a relatively low hardness (HB30-60), but cutting tools are susceptible to adhesive wear during machining. Therefore, tool materials with good wear resistance and low affinity for magnesium are required. Carbide tools (such as WC-Co alloys with a Co content of 8%-10%) are preferred, offering superior wear resistance to high-speed steel and enabling cutting speeds of 100-300 m/min. For high-precision machining, PCD tools can be used, as they have a low coefficient of friction (0.05-0.1), effectively reducing tool adhesion and achieving surface roughnesses of up to Ra 0.02 μm. Tool geometry requires a large rake angle (15°-25°) to reduce cutting forces and frictional heat generation; a clearance angle of 10°-15° to minimize flank wear; and a lead angle of 90°-95° to direct chips toward the surface being machined, avoiding scratches on the machined surface. The tool tip radius is 0.1-0.3mm. Too small can lead to chipping, while too large increases cutting forces and chip temperatures. Experimental data shows that using a carbide tool with a 20° rake angle and a 12° relief angle reduces cutting forces on magnesium alloy by 30% and chip temperatures by 50°C.
The optimization of cutting parameters must take into account safety, efficiency, and quality. Cutting speed is crucial for magnesium alloy turning: too low a speed (<50m/min) will result in coarse chips and increased temperature (easy to spontaneously combust), while too high a speed (>400m/min) will increase tool wear. The reasonable range is: 100-200m/min for rough turning and 200-300m/min for fine turning. At this time, the chips are in the form of small fragments and are easy to cool and discharge. The feed rate must match the speed, 0.2-0.4mm/r for rough turning and 0.1-0.2mm/r for fine turning. Too large a feed rate will increase the cutting force and cause surface tearing; too small a feed rate will result in too fine chips, increasing the risk of spontaneous combustion. The back cutting depth is determined according to the allowance, 1-3mm for rough turning and 0.1-0.5mm for fine turning. For thin-walled parts, the back cutting depth must be ≤1mm to avoid deformation. Experiments show that when the cutting speed is 250m/min and the feed rate is 0.15mm/r, the surface roughness of AZ31B magnesium alloy can reach Ra0.8μm, and the chip temperature is stable at around 250℃, with good safety.
The clamping method and process route must take into account the low strength of magnesium alloys. Magnesium alloys have a low elastic modulus (approximately 45 GPa, only one-quarter that of steel), which can easily cause deformation during clamping. Therefore, a flexible clamping method is required: a three-jaw chuck with soft jaws (aluminum or copper), with a clamping force controlled at 0.5-1 MPa (monitored by a pressure sensor). For flat parts, vacuum cups are used for clamping to evenly distribute the clamping force. The process route adheres to the principle of “rapid stock removal + low-temperature finish turning”: During rough turning, high speeds and high feeds are used to quickly remove most of the stock (retaining a 0.5-1mm stock for finish turning) to reduce machining time. Stress relief treatment (100-120°C for one hour) is performed before finish turning to eliminate roughing stresses. During finish turning, low cutting force parameters are used, combined with PCD tools, to ensure dimensional accuracy (IT7-IT8 grade). For complex parts, a multi-step process of “rough turning – semi-finishing turning – aging – finishing turning” can be adopted, with natural aging (24 hours) following each step to avoid deformation caused by stress concentration. Using this process, a certain automotive wheel manufacturer reduced the machining deformation of magnesium alloy wheels from 0.5mm to less than 0.1mm.
