Effect of Process Factors on Microstructure of Semisolid Continuous Casting Billets

Semisolid continuous casting (SSCC) is a new technology to produce billets for semisolid metal forming (SSMF). The effect of process factors, such as pouring temperature, stirring rate, preheating temperature and thermal conductivity of stirring chamber, on the microstructure of SSCC billets was studied by means of the factorial experimental method. The results show that the microstructure of SSCC billets can be controlled by the above-mentioned four process factors. In order to obtain fine and rounded granular grains in an SSCC billet, the pouring temperature, preheating temperature and stirring rate should be kept in a moderate range, and the thermal conductivity of stirring chamber should be high. The regression equations with the process factors connecting the microstructure was also set up based on experimental data.

Microstructure and Thermal Conductivity of Al–Graphene Composites Fabricated by Powder Metallurgy and Hot Rolling Techniques

The objective of this research is to improve the thermal conductivity and mechanical properties of Al/GNPs（graphene nanoplatelets） nanocomposites produced by classical powder metallurgy and hot rolling techniques. The microstructural evaluation confirmed the uniform dispersion of GNPs at low content and agglomeration at higher contents of GNPs. The structure of graphene was studied before and after the mixing and the Raman spectrum proofs that the wet mixing has a great potential to be used as a dispersion method. There was no significant peak corresponding to the Al_4C_3 formation in both the differential scanning calorimetry curves and X-ray diffraction patterns. The microstructural observation in both fabrication techniques showed grain refinement as a function of the GNPs content. Moreover, the introduction of the GNPs not only improved the Vickers hardness of the composites but also decreased their density. The thermal conductivity investigations showed that in both the press-sintered and hot-rolled samples, although the thermal conductivity of composites was improved at low GNPs contents, it was negatively affected at high GNPs contents.

Mechanical Properties and Fracture Behavior of Mg-Al/AlN Composites with Different Particle Contents

In this study, magnesium matrix composites reinforced with different loading of AlN particles were fabricated by the powder metallurgy technique. The microstructure, bending strength and fracture behavior of the resulting Mg-Al/Al N composites were investigated. It showed that the 5 wt% AlN reinforcements led to the highest densification and bending strength. The total strengthening effect of AlN particles was predicted by considering the contributions of CTE mismatch between the matrix and the particles,load bearing and Hall-Petch mechanism. The results revealed that the increase of dislocation density,the change of Mg17Al12 phase morphology, and the effective load transfer were the major strengthening contributors to the composites. The fracture of the composites altered from plastic to brittle mode with increasing reinforcement content. The regions of clustered particles in the composites were easy to be damaged under external load, and the fracture occurred mainly along grain boundaries.

Characterization of Mo–Si–B Nanocomposite Powders Produced Using Mechanical Alloying and Powder Heat Treatment

Mo-Si-B nanocomposite powders with a composition of Mo-12Si-8.5B （in at.%） were processed using mechanical alloying under milling conditions for different milling time and powder-to-ball ratios. The Mo-12Si-8.5B alloy, which consists of α-Mo and intermetallic Mo3Si and T2 phases, was also synthesized by hot-pressed sintering the mechanically alloyed powders under a pressure of 50 MPa at 1600 ℃. The results demonstrated that the sizes and morphologies of the powder particles became gradually refined and uniform by both increasing the milling time and decreasing the powder-to-ball ratio. After 15 h of milling, the powders were completely homogenized at the 1：10 and the 1 ： 15 powder-to-ball weight ratios, and the homogenization was accelerated to rapidly stabilize the milling process because of their high milling energy. Annealing the Mo-Si-B milled powders could promote the growth of the intermetallic Mo3Si and the T2 phases, which formed even after low-temperature annealing at 900 ℃. Increasing the annealing temperature only improved the crystallinity of various phases. When the milled and annealed powders were hot-pressed sintered, the Mo-Si-B alloy exhibited a fine-grained microstructure, where the intermetallics Mo3Si and T2 were distributed in a continuous α-Mo matrix.