Research On Rare Earth Modification And Cutting Mechanism Of Die-Cast Magnesium Alloy AZ91

Magnesium alloys have been widely used in a lot of industries due to their low density, high specific strength and specific stiffness, good damping characteristics, high electromagnetic shielding and radiation resistance, excellent machinability and can be easily recycled etc. Die-cast magnesium alloy products are used in automotive industry basically, magnesium alloy die-castings reduce the weight of car, make fuel oil consumption decrease and exhaust gas discharge rate decrease, so magnesium alloys are considered as an excellent choice for weight reduction of automobile. However, common alloys are generally unsuitable for use above 120℃, the high temperature properties can't meet the requirements of many automobile applications. The poor tensile properties at elevated temperatures limit the development as the cheapest magnesium alloy in automotive industry. Machinability of magnesium alloys also gradually becomes interesting research direction along with the increasingly ingoing application of magnesium alloy. However there is rare report about the machinability of magnesium alloy. In the present work, die-cast magnesium alloys AZ91 with RE addition were prepared. The effect of RE on the microstructure and properties of AZ91 alloys, especially elevated temperature properties, are studied. The drilling test of die-cast magnesium AZ91 was carried out, established the empirical formula of drilling force. Wear maps of HSS tools drilling die-cast magnesium alloy are constructed according to principles and methods of constructing wear mechanism maps.RE are important alloying elements to magnesium alloys. With addition of RE elements La ,Ce, Nd, the microstructure of die-cast magnesium alloys AZ91 were refined obviously, γ- Mg₁₇Al₁₂ phase became discontinuous and dispersion distributing massive or bones. Dot or needle like intermetallic phases Al₄La, Al₄Ce, Al₄Nd were observed. Volume and size of γ - Mg₁₇Al₁₂ phase decreased and Al₄La, Al₄Ce, Al₄Nd became more and coursing with further La, Ce, Nd addition. New phase Al₂Nd was observed when the content of Nd reached 3%.Hardness of the die-cast magnesium alloys AZ91 had different extentincreased with the increasing of La> Ce^ Nd contents. Compared to AZ91 alloy, the ambient temperature ultimate tensile strength, yield strength and elongation of alloys with La> Ce> Nd addition have little increased. But La> Ce^ Nd greatly improved the high temperature properties of AZ91 alloy. High temperature properties of AZ91 alloys with La> Ce> Nd addition were improved effectively because of the MgnAl^ phase with low melting point decreasing and the Al-RE phases with much high melting point formation. And AZ91 alloys have the highest ultimate tensile strength and elongation when the content of La> Ce^ Nd is 1%. The effects on strength in 1^ Ce> Nd order enhanced. Form of fracture of die-cast magnesium AZ91 is cleavage fracture in ambient temperature. Fracture of alloys have characters of quasi-cleavage, tearing edge and dimple with La> Ce> Nd addition, but it is still brittle fracture. Fracture has more quasi-cleavage characters in high temperature and it is quasi-cleavage fracture.Test system of drilling force is composed of cutting tools, workpiece, machine tool, SDZ-E bi-directional piezoelectric type dynamic drilling dynamometer, CA-1 charge amplifier, PCI-7483 data acquisition card and computer. Test system software was write by VC programming language, actualized the axial force and torque data acquisition, save and processing in drilling process. Adopted orthogonal experiment to design the test, and confirmed the three main factors to effect the axial force and torque: drill tool diameter d, feed rate f, cutting speed v. Empiric formulas of drilling force and torque of die-cast magnesium alloy AZ91 are F = 6.6405dim4fojmv02216, M = 0.032J25041/09885v04266 separately. From the empiric formulas we can know that drill tool diameter d has the biggest effect on drilling force and torque, feed rate f takes second place and cutting speed v is least. Because the effect of feed rate on drilling force and torque is bigger than cutting speed when the drilling tool diameter is determinate. So we can increase the cutting speed to improve the machining efficiency and reduce the energy consumption.Uncoated HSS drill tools were used to drill die-cast magnesium alloys AZ91 under dry operations, studied the effect of machining parameters variation on drilling tools' wear in machining process. We constructed the flank wear rate mapwhich cutting speed as lateral axle and feed rate as vertical axis, every bit in the map represent the wear rate at one drilling condition. The wear rates of cutting tools are normalized into dimensionless date points, so the dots that have similar wear rates assembled to definite zone. Based on the variation of wear rates and SEM analysis, we defined the step size 0.3, and compartmentalized the map to -6.7^-7.(h -7.0--7.3, -7.3^-7.6> -7.6^-7.9 four zones.The SEM analysis of worn surface of uncoated HSS drilling tools, which drilled the die-cast magnesium AZ91 indicates that the wear mechanisms of flank wear, are different in diverse machining zone. The region whose wear rates is -7.6^-7.9 has the minimum wear rate because the steady built-up formed on the drilling tool, and built-up has the protective action on the drilling tools. The wear mechanism of the region whose wear rates is -7.3^-7.6 is adhesive wear. The wear mechanism of the region whose wear rates is -7.0^-7.3 is abrasive wear. The wear mechanism of the region whose wear rates is -6.7^-7.0 is diffusion wear. Because the formation of built-up is due to the chip adhibit on the drilling tools' surface, so the formation of built-up is also adhesive wear. There is three mechanisms of the HSS drilling tools' flank wear in present work: adhesive wear, abrasive wear and diffusion wear.The wear mechanism map is constructed according to principle and method of constructed wear maps, which cutting speed as lateral axle and feed rate as vertical axis. The wear mechanism map of uncoated HSS tools in dry drilling conditions drilling die-cast magnesium alloys AZ91 is so constructed under fixed conditions of tool material, machined material and machining parameters. The map has good comparability when analyses wear rate and wear mechanism of cutting tools. In the map there is a minor wear rate region, which is called "safety zone". The flank wear mechanism map can be used to select optimize machining operations, and be a good reference for drilling die-cast magnesium alloys.The hardness and strength of alloys, especially the high temperature strength, has improved when add La-. Ce> Nd to the die-cast magnesium alloy AZ91. The machinability of materials must change when the physical mechanical properties changed because of the changing of alloys' chemical constituent. In the presentwork, we studied the drilling force and flank wear rate of AZ91 alloys with RE elements addition in fixed drilling conditions (the cutting speed is 2000rev/min and the feed rate is O.lmm/rev). The result shown that the drilling force and wear rate of AZ91-RE alloys are all bigger than AZ91, and increase with the RE contents increasing. The hardness and strength of workpiece materials are higher and the drilling force is bigger when drilling, and the cutting power increasing followed, cutting temperature also hoist and the wear rate augmentation. So in generalized conditions, machinability decrease with the hardness and strength of workpiece increasing.