Effects of isothermal treatment parameters on the microstructure of semisolid alloys

Cold-rolled 6061 aluminium alloys are used to prepare semisolid billets by recrystallisation and partial melting. The effects of isothermal treatment parameters on the microstructures were investigated. The results indicated that the high isothermal holding temperature increases the experimental liquid fraction, average grain size and shape factor. A long isothermal holding time also increases the experimental liquid fraction and improves the spheroidisation degree of solid grains, but the average grain size also increases. Moreover, when the isothermal holding time at high temperature is increased, the size of intragranular liquid droplets increases, but their quantity decreases. The optimal isothermal holding temperature and time during semisolid isothermal treatment were 635°C and 5?–?10?min, respectively.     

1Cr18Ni9Ti激光选区熔化成形工艺参数对致密度的影响

采用正交试验,结合典型缺陷形成原因和微观组织,研究了激光选区熔化成形工艺参数(激光功率、扫描速度和扫描间距)对1Cr18Ni9Ti不锈钢致密度的影响,分析了各工艺参数对致密度的影响规律。结果表明,粉末熔化的能量输入密度主要取决于激光功率和扫描速度;在激光功率325~340 W、扫描速度1 000~1 200 mm/s、扫描间距0.12 mm的工艺参数下,SLM技术可制备致密度高于99.9%的1Cr18Ni9Ti不锈钢零件。采用优化后的SLM工艺参数成形1Cr18Ni9Ti不锈钢试棒的力学性能优于QJ501A-98标准,抗拉强度Rm≥709 MPa,屈服强度Rp0.2≥547 MPa,断后伸长率A≥41%。     

Effective Characterization of DNA Ligation Kinetics by High-Resolution Melting Analysis

High-resolution melting (HRM) analysis has been improved and applied for the first time to quantitative analysis of enzymatic reactions. By using the relative ratios of peak intensities of substrates and products, the quantitativity of conventional HRM analysis has been improved to allow detailed kinetic analysis. As an example, the ligation of sticky ends through the action of T4 DNA ligase has been kinetically analyzed, with comprehensive data on substrate specificity and other properties having been obtained. For the first time, the kinetic parameters (k_obs and apparent K_m) of sticky-end ligation were obtained for both fully matched and mismatched sticky ends. The effect of ATP concentration on sticky-end ligation was also investigated. The improved HRM method should also be applicable to versatile DNA-transforming enzymes, because the only requirement is that the products have T_m values different enough from the substrates.     

A review of particulate-reinforced aluminum matrix composites fabricated by selective laser melting

Selective laser melting (SLM) is an emerging layer-wise additive manufacturing technique that can generate complex components with high performance. Particulate-reinforced aluminum matrix composites (PAMCs) are important materials for various applications due to the combined properties of Al matrix and reinforcements. Considering the advantages of SLM technology and PAMCs, the novel SLM PAMCs have been developed and researched in recent years. Therefore, the current research progress about the SLM PAMCs is reviewed. Firstly, special attention is paid to the solidification behavior of SLM PAMCs. Secondly, the important issues about the design and fabrication of high-performance SLM PAMCs, including the selection of reinforcement, the influence of parameters on the processing and microstructure, the defect evolution and phase control, are highlighted and discussed comprehensively. Thirdly, the performance and strengthening mechanism of SLM PAMCs are systematically figured out. Finally, future directions are pointed out on the advancement of high-performance SLM PAMCs.     

Comparison of Cu–Al–Ni–Mn–Zr shape memory alloy prepared by selective laser melting and conventional powder metallurgy

This work aimed to investigate and critically analyze the differences in microstructural features and thermal stability of Cu−11.3Al−3.2Ni−3.0Mn−0.5Zr shape memory alloy processed by selective laser melting (SLM) and conventional powder metallurgy. PM specimens were produced by sintering 106−180 µm pre-alloyed powders under an argon atmosphere at 1060 °C without secondary operations. SLM specimens were consolidated through melting 32−106 µm pre-alloyed powders on a Cu−10Sn substrate. Mechanical properties were measured through Vickers hardness testing. Differential scanning calorimetry was conducted to assess the martensitic transformation temperatures. X-ray diffraction patterns were collected to identify the metallurgical phases. Optical and scanning electron microscopy was used to analyze the microstructural features. β′₁ martensite was found, irrespective of the processing route, although coarser martensitic variants were present in PM-specimens. In conventional powder metallurgy samples, intergranular eutectoid constituents and stabilized austenite also formed at room temperature. PM-specimens showed similar average hardness values to the SLM-specimens, albeit with high standard deviation linked to the porosity. The specimens processed by SLM showed reversible martensitic transformation (T₀=171 °C). PM-processed specimens did not show shape memory effects.     

Effects of WC particles on microstructure and mechanical properties of 316L steel obtained by laser melting deposition

Stainless steel has many excellent characteristics such as high strength, weldability, corrosion resistance, easy processing and surface gloss. It has been widely applied in many industrial and architectural decoration aspects such as aerospace, chemical, and automotive. However, 316L material manufactured by Laser melting deposition has poor wear resistance and tensile strength. In this paper, WC particles are used as a strengthening phase to make up for these deficiencies of 316L. In order to comprehensively evaluate the effect of WC particles on the mechanical properties of 316L formed by laser melting deposition, 316L samples with different proportions of WC particles and different fabricating directions were prepared. The microstructure of samples was observed, and the phase composition, Vickers hardness, friction and wear properties, tensile properties were analyzed. The mechanism of the influence of WC particles on the structure and the mechanism of friction, wear and tensile fracture were studied. The results show that most of the WC particles are evenly distributed in the coating. The microhardness and wear resistance of the 316L/WC composite coating have been significantly improved with the increase in the proportion of WC. The tensile strength and elongation are the best when the WC mass fraction is 6%.     

Modification of pork-skin jelly by enzymatic cross-linking: melting resistance and quality improvement

Improvements of melting resistance and quality by modification of pork-skin jelly through enzymatic cross-linking were studied, and the mechanism of quality improvement was discussed in this work. Gel strength, springiness and chewiness of modified gel increased significantly (P < 0.05). Transglutaminase (TGase) also improved the viscoelasticity, stability and melting resistance of gel system, as proved by rheological analysis. Sensory evaluation showed that increase in texture need to be moderate and the utilisation of 0.6% TGase was the most appropriate for pork-skin jelly. Significant effects of TGase on inducing protein cross-link and aggregation were confirmed by determining rheology during enzyme treatment and cross-linking extent of pork-skin soup. Correlation analysis showed TGase could improve melting temperature and texture by facilitating cross-linking. Covalent interaction based on ε-(γ-glutamyl)-lysine induced by TGase could play the main role in these improvements. This study suggested that TGase could be applied to design gelatin-based food for tailored quality properties through enzymatic cross-linking.     

Laser beam melting of aluminum alloys with hull-core build strategy by using an initial meso-scale simulation

Laser beam melting (LBM) of aluminum alloys is gaining a wide popularity in different industrial applications as an alternative technology for the production of individual and complex parts. A long build time and the high amount of experimental work for optimizing or finding new process parameters are two of the current challenges for reaching an industrial maturity. This paper proposes an efficient way to determine new process parameters for aluminum alloy aluminum-silicon10-magnesium with highest build-up rates by using a 3D finite element model on the mesoscopic level. High laser power in combination with the hull-core build strategy was used to increase the build-up rate without impairing the part accuracy. The influences of high laser power, laser diameter and scan speed on the melt pool were studied by using a thermal simulation of single laser tracks. Based on the simulation results the process window could be derived and was tested on a laser beam melting (LBM) system. The achieved reduction of the build time of up to 31 % without loss in part accuracy proved the novel approach for the prediction of the required process window as an efficient method to reduce costly and time-consuming experimental work.