Two alloy samples, AZ91D and AZ91D-1.11Nd, were obtained by traditional die casting. Optical metallographic microscope, scanning electron microscope and X-ray diffractometer were used to analyze die-cast microstructure and phase composition, and tested its tensile mechanical properties, hardness, thermal conductivity and flow properties. Results show that after adding 1.11%Nd to AZ91D alloy, grains in die-cast state are refined, more dispersed fine granular Al2Nd and a small amount of needle-like Al11Nd3 are formed, number of β-Mg17Al12 in original semi-continuous network distribution is reduced. Die-cast AZ91D-1.11Nd alloy exhibits good comprehensive properties, tensile strength, elongation and thermal conductivity at room temperature are 272 MPa, 12.0% and 69.5 W/(m K), respectively, which are 14%, 100% and 14% higher than those of AZ91D alloy; at the same time, it exhibits excellent casting performance equivalent to that of AZ91D alloy, and flow length reaches 1 161 mm.
Foreword: Magnesium alloy has advantages of low density, high specific stiffness and specific strength, good shock absorption performance and electromagnetic shielding performance, easy processing, relatively stable component size, environmental protection, etc., and is widely used in automobiles, aerospace, communication electronics and other fields. Lightweight of new energy vehicles and key components of 5G communication puts forward higher requirements on performance and forming technology of magnesium alloy materials. However, existing magnesium alloy materials and their forming technologies cannot fully meet requirements. Therefore, it is necessary to carry out development of new die-casting rare earth magnesium alloys with high strength, high thermal conductivity and excellent casting process performance. AZ91D is the most commonly used commercial die-casting magnesium alloy. Due to its good casting performance and comprehensive mechanical properties at room temperature, it is often used to make automotive steering wheels, instrument panels, seats, gearbox casings and other components, but its relatively poor mechanical properties limit its wider application. Rare earth RE can purify alloy melt, improve microstructure of alloy, improve mechanical properties of alloy at room temperature and high temperature, and corrosion resistance. After adding an appropriate amount of rare earths such as Ce, Nd, Pr, and Y to AZ91D alloy, grains are refined, new phase of Al-RE is precipitated, size of β-Mg17Al12 is reduced, mechanical properties and corrosion resistance are improved. These studies mainly focus on effects of rare earths on mechanical properties or corrosion resistance of AZ91D alloys, while comprehensive performance studies including thermal conductivity and flow properties are rarely reported. For this reason, two alloy samples, AZ91D and AZ91D-1.11Nd, were obtained by traditional die casting under optimized process parameters in this study, analyzed microstructure of die casting state, compared die-cast tensile mechanical properties, hardness, thermal conductivity and flow properties, providing technical support for expanding application of high-strength and high-thermal conductivity die-casting magnesium alloys .

Raw materials for this test are AZ91D alloy ingots and Mg-30%Nd (mass fraction, %) intermediate alloy ingots. First, AZ91D alloy ingots were added to magnesium alloy melting furnace, mixed gas protection of CO 2 +0.2%SF6 (volume fraction, %) was passed through, and temperature was raised to 953 K. After AZ91D alloy ingot is completely melted, add Mg-30%Nd intermediate alloy ingot, heat up to 983 K after complete melting, and stir twice within 1.5 h to ensure uniform composition. Then add refining agent for refining, raise temperature to 1 013 K and let it stand for 20 min, cool down to 963 K, and die-cast on a cold chamber die-casting machine with a pressure of 32 MPa. Comparative alloy AZ91D sample was also obtained by die-casting with above process parameters, and macroscopic view of die-cast sample is shown in Figure 1a.

 

Fig.1 Comparison of die-casting samples and die-casting flow samples of two alloys
Nd content in alloy was analyzed by plasma atomic emission spectrometer (ICP) to be 1.11%. Metallographic sample was cut from same part of die-casting tensile sample, and etchant was 5% nitric acid alcohol, then microstructure and EDS analysis were performed on a Leica DMI 3000M optical metallographic microscope (OM) and a JEOL HXA-8100 scanning electron microscope (SEM); phase analysis was performed on a SmartLab type X-ray diffractometer (XRD) with a copper target. Tensile tests at room temperature and high temperature (423 K) were carried out using a DNS200 universal material testing machine. Micro Vickers hardness (HV) test was carried out with a load of 9.8 N and a loading time of 20 s; thermal conductivity was tested by a DRPL-2C thermal conductivity tester; flow properties of alloy were characterized by measuring length of flow sample.

Figure 2 is X-ray diffraction pattern of AZ91D and AZ91D-1.11Nd alloy die-casting samples. It can be seen that die-cast XRD spectrum of AZ91D alloy is mainly composed of α-Mg and β-Mg17Al12 phase peaks, while AZ91D-1.11Nd die-cast XRD spectrum not only appears above-mentioned α-Mg, β-Mg17Al12 phase peaks, but also Al2Nd and Al11Nd3 new phase peaks, but no Mg-Nd binary phase or Mg-Al-Nd ternary phase peaks.

 

Fig.2 X-ray diffraction patterns of die-cast AZ91D and AZ91D-1.11Nd alloys
Figure 3 is metallographic photograph of AZ91D and AZ91D-1.11Nd alloy die-casting samples. It can be seen that microstructure of AZ91D alloy is mainly composed of white α-Mg matrix and gray β-Mg17Al12 distributed along grain boundary in a network shape, with an average grain size of about 38 μm. After adding 1.11% Nd, grains are refined, and average grain size drops to about 23 μm.

 

Fig.3 Metallographic photographs of die-cast AZ91D and AZ91D-1.11Nd alloys
Figure 4 is SEM photos of AZ91D and AZ91D-1.11Nd alloy die-casting samples. It can be seen that microstructure of AZ91D alloy is mainly composed of black α-Mg matrix and gray β-Mg17Al12, and β-Mg17Al12 is distributed along grain boundary in a network shape. Microstructure of AZ91D-1.11Nd alloy is mainly composed of black α-Mg matrix, gray β-Mg17Al12 and a large number of bright-colored granular and a small amount of needle-like phases. Phases are diffusely distributed in grain boundaries and grains, network β-Mg17Al12 is interrupted and grain size decreases.

 

Fig.4 SEM photos of die-cast AZ91D and AZ91D-1.11Nd alloys
Figure 5 marks point-to-second phase particle analysis of different phase positions of two alloys by EDS, and results are shown in Table 1. Nd was not detected in matrix, and Nd was detected in bright hue. Content of solid solution elements in matrix of AZ91D-1.11Nd alloy was lower than that of AZ91D. Combined with XRD analysis results and related literature, it can be seen that bright granular phase is mainly Al2Nd, and needle phase is Al11 Nd3. Electronegativity of Nd, Mg, and Al are 1.14, 1.31, and 1.61, respectively, electronegativity difference between Nd, Mg, and Al is 0.17, and 0.47, respectively, so electronegativity difference between Al and Nd is larger, after adding Nd, Al-Nd phase will preferentially precipitate during solidification process of melt, which inhibits precipitation of low melting point β-Mg17Al12, at the same time volume and particle size of β-Mg17Al12 decrease.

 

Fig.5 Locations of energy spectra of die-cast AZ91D and AZ91D-1.11Nd alloys

Table 1 Energy spectrum scanning analysis results of die-cast AZ91D and AZ91D-1.11Nd alloys

Figure 6 shows room temperature and high temperature tensile engineering stress-strain curves of die-cast AZ91D and AZ91D-1.11Nd alloys, and results of their tensile mechanical properties are listed in Table 2. It can be seen that room temperature tensile strength of die-cast AZ91D alloy is 238 MPa, yield strength is 145 MPa, and elongation is 6%, while room temperature tensile strength of AZ91D-1.11Nd alloy is 272 MPa, yield strength is 149 MPa, and elongation is 12%. Compared with AZ91D alloy, tensile strength at room temperature is better than that of AZ91D alloy, but tensile strength at 423 K is lower than that of AZ91D alloy. Grain size of AZ91D-1.11Nd alloy is smaller than that of AZ91D alloy. The smaller grain, the more grain boundaries corresponding to volume, the more obstacles to dislocation movement, and the higher strength of alloy; Al 2 Nd and Al 11 Nd 3 are diffusely distributed in grain boundaries and grains, which can prevent dislocation movement and improve alloy strength. In addition, magnesium alloys with a grain size in the range of 3-30 μm usually have better plasticity, so AZ91D-1.11Nd alloy has better plasticity at room temperature and high temperature.

 

Fig.6 Tensile engineering stress-strain curves of die-cast AZ91D and AZ91D-1.11Nd alloys at room temperature and high temperature

Table 2 Mechanical properties, thermal conductivity and fluidity of die-cast AZ91D and AZ91D-1.11Nd alloys
Su et al. prepared Mg-4Al-4Nd-0.2Mn alloy by high-pressure die-casting method, which has two intermetallic phases of needle-like Al11Nd3 and granular Al2Nd, exhibits good strength at room temperature and 473 K. After Al11Nd3 is completely decomposed into Al2Nd, strength of alloy at room temperature decreases. This is because network structure formed by Al11Nd3 and Al2Nd at grain and grain boundary strengthens grain boundary, in which Al11Nd3 is main strengthening phase; after Al11Nd3 is completely decomposed into Al2Nd, network structure near grain boundary disintegrates, and strength of alloy at room temperature decreases.
In AZ91D-1.11Nd alloy cast in this test, there are Al 2 Nd and a small amount of needle-shaped Al11Nd3. Al2Nd and Al11Nd3 do not form a network structure at grain boundary, and a lot of Al2Nd is distributed inside grain, and cracking of grain boundary cannot be effectively prevented at 423 K; however, there is still a large amount of network β-Mg17Al12 in AZ91D alloy, which can prevent grain boundary cracking at 423 K, so tensile strength of AZ91D-1.11Nd alloy at 423 K is lower than that of AZ91D alloy.
Figure 7a and c are tensile fracture morphologies of die-cast AZ91D and AZ91D-1.11Nd alloys at room temperature. It can be seen that tensile fracture of AZ91D alloy is distributed with micropore-aggregated equiaxed dimples, tear edges and river-like patterns can be seen locally, which are quasi-cleavage fractures and mixed plastic-ductile fractures, number of dimples in AZ91D-1.11Nd alloy has increased, relatively deep, consistent with better plasticity results. Figure 7b and d show high-temperature tensile fracture morphology of die-casting AZ91D and AZ91D-1.11Nd alloys. The two tensile fractures are distributed with a large number of equiaxed dimples, which are micropore aggregation fractures and plastic fractures. AZ91D alloy has more dimples and more uniform distribution, showing better plasticity, which is consistent with tensile test results.

 

Fig.7 Tensile fracture morphology of die-cast AZ91D and AZ91D-1.11Nd alloys at room temperature and high temperature
Hardness of AZ91D alloy is HV57.8, and hardness of alloy slightly increases to HV59.5 after adding 1.11Nd. After adding Nd, Al2Nd and Al11Nd3 dispersedly distributed in grain boundaries and grains are formed, which reduces and refines volume of β-Mg17Al12. In addition, addition of Nd also refines grains and improves hardness of alloy.
As shown in Figure 1b, flow length of AZ91D alloy is 1 196 mm, and flow length of alloy is slightly reduced after adding 1.11Nd, but still maintains good casting process performance, and flow length can still reach 1 161 mm. Viscosity of liquid alloy is main factor affecting fluidity of alloy, increase of solid inclusions in alloy will increase viscosity and reduce fluidity of alloy. Rare earth elements react with MgO in magnesium alloy melt to remove inclusions, reduce viscosity of melt, and improve fluidity. However, with increase of rare earth elements, Al2Nd and Al11 Nd3 are formed in experimental alloy. Crystallization temperature of these phases is higher than eutectic temperature, and number of primary crystals in alloy increases to form a skeleton, which reduces fluidity of magnesium alloy.
Thermal conductivity of AZ91D alloy is 61.1 W/(m·K), while that of AZ91D-1.11Nd alloy is 69.5 W/(m·K), which is 14% higher than that of AZ91D alloy. Atoms such as Al, Zn and Mn in AZ91D alloy are dissolved in the matrix, which will cause lattice distortion, disturb periodicity of lattice, increase collision probability of electron-phonon and phonon-phonon, and strongly hinder free movement of electrons and phonons, so that mean free path of electrons and phonons is reduced. As shown in EDS scanning analysis results in Table 1, precipitated phases of AZ91D-1.11Nd alloy such as Al2Nd and Al11Nd3 consume solute elements in matrix, release lattice distortion, reduce scattering effect of solute on electron and phonon transmission. Therefore AZ91D-1.11Nd alloy has better thermal conductivity.

(1) Microstructure of die-cast AZ91D alloy is mainly composed of α-Mg matrix and β-Mg17Al12 distributed in semi-continuous network. After adding 1.11% Nd, semi-continuous network of Mg17Al12 was interrupted and particle size decreased. At the same time, more fine granular Al 2 Nd and a small amount of needle-like Al11Nd3 appeared in grain boundaries and grains, and grains had refined.
(2) Tensile strength, yield strength, elongation and hardness of die-cast AZ91D alloy at room temperature are 238 MPa, 145MPa, 6.0% and HV57.8 respectively, while tensile strength, yield strength, elongation and hardness of AZ91D-1.11Nd alloy at room temperature were 272 MPa, 149 MPa, 12.0% and HV59.5, which were increased by 14%, 3%, 100% and 3%, respectively.
(3) Thermal conductivity of die-cast AZ91D alloy is 61.1 W/(m·K), while that of AZ91D 1.11Nd alloy increases to 69.5 W/(m·K).
(4) Flow length of die-cast AZ91D alloy is 1 196 mm, while flow length of AZ91D-1.11Nd alloy is slightly reduced, but still reaches 1 161 mm, showing good casting process performance.