Qiang Zhu

Microstructure and mechanical property of MIM 418 superalloy

In this study, the influence of hot isostatic pressing (HIP) process on the 418 alloy produced by metal injection molding (MIM) technique (named as MIM 418) was investigated based on the characteristic analysis of 418 alloy powder. And comparison analysis of the microstructure and mechanical property between the MIM 418 and as-cast 418 alloys was performed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The results show that MIM 418 alloy exhibits fine grain (~30xa0μm) and uniform microstructure. The defects existing in MIM 418 alloy formed during sintering process can be eliminated through HIP treatment, and the relative density increases from 97.0xa0% to 99.5xa0%. The mechanical property can be improved significantly because of the elimination of defects, and the tensile strength and elongation are 1,271xa0MPa and 16.8xa0%, respectively, which are increased by 34.5xa0% and 180xa0% compared with K418 alloy after solution heat treatment.

Influence of Process Parameters on Blistering during T6 Heat Treatment of Semi-Solid Castings

High pressures applied to the castings during solidification mean that semi-solid castings tend to be prone to surface blistering during subsequent T6 heat treatment. It is believed that the blistering originates from subsurface defects present in the semi-solid castings, which expand when exposed to high temperatures during the solution heat treatment. Despite the significance of blistering to the commercial development of the semi-solid casting process, there have only been limited quantitative studies of the impact of process parameters on blistering. This paper, therefore, will report on a study to examine the impact of a number of process parameters including intensification pressure, plunger velocity and solid fraction of the feed material on the blistering of semi-solid castings during T6 heat treatment. The location and average size of blisters formed at each condition have been measured and related to the casting conditions.

Evolution of Microstructure and Mechanical Properties of the Thixo-Diecast 319s Alloy during Heat Treatment

The evolution of microstructure and mechanical properties during solution and ageing heat treatment process was studied in terms of a thixo-diecast impeller of 319s aluminium alloy. The cast alloy exhibited a microstructure consisting of primary uniformly distributed in α-Al globules and the eutectics. A series of heat treatment studies were performed to determine optimum heat treatment parameters, in order to achieve fine grain structure, fine silicon particles and optimal precipitate size and distribution. Optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to study the evolution of microstructure and mechanical properties. The results demonstrate that, the full T6 heat treatments are successfully applied to thixo-diecast 319s impellers. A two-step solution heat treatment is employed to prevent porosity due to overheating. The tensile properties of thixo-diecast 319s impellers were substantially enhanced after T6 heat treatment. The plate-shaped θ′ precipitates and lath-shaped Q′ precipitates are the most effective for precipitation strengthening.

Effect of Zn Concentration on the Microstructure and Mechanical Properties of Al-Mg-Si-Zn Alloys Processed by Gravity Die Casting

The microstructure and mechanical properties of Al-8.1Mg-2.6Si-(0.08 to 4.62)Zn alloys (in wtxa0pct) have been investigated by the permanent mold casting process. X-ray diffraction analysis shows that the τ-Mg32(Al, Zn)49 phase forms when the Zn content is 1.01xa0wtxa0pct. With higher Zn contents of 2.37 and 3.59xa0wtxa0pct, the η-MgZn2 and τ-Mg32(Al, Zn)49 phases precipitate in the microstructure, and the η-MgZn2 phase forms when the Zn content is 4.62xa0wtxa0pct. Metallurgical analysis shows that the η-MgZn2 and τ-Mg32(Al, Zn)49 phases strengthen the Al-8.1Mg-2.6Si-(0.08 to 4.62)Zn alloys. After solutionizing at 510xa0°C for 180xa0minutes and aging at 180xa0°C for 90xa0minutes, the η′-MgZn2 phase precipitates in the α-Al matrix, which significantly enhances the mechanical properties. Addition of 3.59xa0wtxa0pct Zn to the Al-8.1Mg-2.6Si alloy with heat treatment increases the yield strength from 96 to 280xa0MPa, increases the ultimate tensile strength from 267 to 310xa0MPa, and decreases the elongation from 9.97 to 1.74xa0pct.

Differential Scanning Calorimetry (DSC) and Thermodynamic Prediction of Liquid Fraction vs Temperature for Two High-Performance Alloys for Semi-Solid Processing (Al-Si-Cu-Mg (319s) and Al-Cu-Ag (201))

There is a need to extend the application of semi-solid processing (SSP) to higher performance alloys such as 319s (Al-Si-Cu-Mg) and 201 (Al-Cu-Ag). The melting of these two alloys was investigated using differential scanning calorimetry (DSC) and thermodynamic prediction. The alloys had been processed by magneto-hydrodynamic (MHD) stirring before receipt to produce a microstructure suitable for SSP. The DSC results for the as-received MHD material were compared with those for material which has been taken through a complete DSC cycle and then reheated for a second DSC run. The effects of microsegregation were then analyzed. A higher liquid fraction for a particular temperature is found in the second DSC run than the first. Microstructural observations suggest this is because the intermetallics which form during the first cooling cycle tend to co-located. Quaternary and ternary reactions then occur during the second DSC heat and the co-location leads to enhanced peaks. The calculated liquid fraction is lower with 10 K/min DSC heating rate comparing with 3xa0K/min at a given temperature. The DSC scan rate must therefore be carefully considered if it is to be used to identify temperature parameters or the suitability of alloys for SSP. In addition, the starting material for DSC must represent the starting material for the SSP. With thermodynamic prediction, the equilibrium condition will provide better guidance for the thixoforming of MHD stirred starting material than the Scheil condition. The Scheil mode approximates more closely with a strongly microsegregated state.

Floating Behavior of Entrapped Gas in Semi-Solid Metal

Semi-solid die casting technology has great advantages at defects control and has been successfully used to produce high quality aluminum alloy components for several years. In this process, semi-solid metal with high apparent viscosity and low plunger velocity are used to avoid surface turbulence which is the main source of entrapped gas in conventional die casting processes. But, entrapped gas still has other sources, such as melting, pouring, surface flooding and confluence weld. Solution heat treatment is always used to strengthen semi-solid die castings. The entrapped gas leads to blister defects, which directly decreases the acceptance rate of semi-solid die castings. So, the entrapped gas is still a serious issue in semi-solid die casting process. We studied the floating behavior of entrapped gas bubble in semi-solid metal. Two floating models were established for gas bubbles with different sizes. These models were used to analyze the possibility of entrapped gas escaping from semi-solid metal in casting practice. The results showed that entrapped gas from feed billet could not escape from the semi-solid metal in the casting process of impeller, which was proved by experiment results. These results emphasized the importance of clean melt and semi-solid metal. Some advices were given at last for avoiding or removing the entrapped gas in semi-solid die casting process.

Analysis on Internal Defect of 319S Aluminum Alloy Impeller by Semi-Solid Thioxcasting

In the present investigation a batch of impellers made of 319S aluminum alloy with about 300 pieces and produced by semi-solid thixocasting process were subjected the ultrasonic testing (UT). The experimental results revealed that the equivalent size of the defects in the impellers was not more than φ0.4mm FBH+12dB. And there were two main types of defects in 12 mm depth area from the machined surface, where defect was apt to form. One type was single defect and the other was intensive defect. Moreover, other nondestructive testing (NDT) was used to test some impellers containing typical defect, and the result suggests that UT was more sensitive than x-ray testing. The defects existed in the impellers were investigated by Optical Microscope (OM) and Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectrometer (EDS). The observation showed that the both types of defects existed in eutectic zone. The single defect derived from billet and the intensive defect came from die casting process.

Research on Microstructure and Mechanical Properties Evolution of Rheocast and Thixocast 319s Aluminum Alloy

In semi-solid rheocast and thixocast industry, T6 heat treatment was one key factor to improve the mechanical properties of the castings. The microstructure evolution was closely influenced by heat treatment temperature and time. In this paper, the morphology change of eutectic silicon in semi-solid alloy during different heat treatment time was firstly observed. The changes of both roundness and aspect show that the silicon particles underwent fragmentation, coarsening and growing up processes during solution treatment. Then, the mechanical properties after stand T6 and T6 with higher temperature were compared. It may be concluded that the higher temperature doesn’t have obvious effect to increase the mechanical strength, but severe negative effect on the elongation. Finally, the incipient melting defect appeared in higher temperature T6 was proved and its relationship with elongation was analysed.

Development of Semi-Solid Die Casting Process Technology for Aluminium Alloy Clamp

Compared with traditional liquid and solid processing methods, semi-solid die casting process can apparently overcome shrinkage cavity and porosity defects in castings and high deformation resistance and high residual stress shortcomings in forging parts. Semi-solid die casting process with advantages such as high efficiency and low cost, will become the optimal process for high quality automobile parts. In this study, using the clamp as an example, the author introduced product structure optimization and die design for semi-solid die-casting process of aluminum alloy in a new product development.The Computer Aided Engineering technology was applied to the product structure optimization according to the stress analysis. The optimal mold structure, including cavity layout, gating system, overflow and vent systems, were confirmed based on the die design criteria for traditional die casting, combining with the characteristics of semi-solid forming and the simulation results. The semi-solid aluminum alloy clamp parts with excellent performances were finally developed successfully by means of product structure optimization, die design, parameters optimization of die casting process, and the mechanical properties test of products.The existing parts were optimized to make them more suitable for semi-solid die casting processing. In addition, a reasonable die design specially for semi solid processing was an important guarantee for a successful semi solid product applied in industry. Computer numerical simulation was applied in product structure design for semi-solid die casting, die design, die-casting process optimization and other aspects, to shorten the development cycle of new product, reduce cost and improve efficiency.

Experimental and Simulation Study of Microstructure of the Aluminium Alloy 319s in Induction Reheating Process

Semi-solid processing of metallic alloys has been developing over the last 30 years. Millions of components are now manufactured by semi-solid processing. A semi-solid processing so called thixoforming requires reheating the feedstock to a semi-solid state in relatively short time interval with a uniform temperature distribution as well as an optimum liquid fraction. Microstructure, which makes significant impact on processing parameters and quality of the component, changes during the reheating process. The main objective of this study is to establish a quantitative relationship of the microstructure and the induction heating process parameters of the aluminum alloy 319s. This quantitative relationship is employed in the numerical simulation of calculating solid/liquid fraction changes during induction heating process. The simulation results are then successfully applied in aiding optimization of process parameters to make an automobile engine turbocharger compressor wheel, which has very complex geometry.