Materials Science Forum Vols. 675-677

Abstract: Thin-walled aluminum components are widely used in aerospace industry. The presence of residual stresses in aluminum components usually causes distortion after machining process, making it difficult to guarantee their integrated precision. Therefore, control of machined surface residual stresses is important for this industry. This paper presents an efficient method for modeling the residual stress imparted by machining process. The methodology is based on mathematical model and finite element method to predict the residual stresses during machining process. Through a thermo-mechanically model, machining process output parameters such as cutting forces and cutting temperatures are predicted as part of the overall modeling effort. To verify the modeling simulation, a test on aluminum alloy 7050T7352 is carried out. It is shown that the simulation result is consistent with the experimental one.

Abstract: Four kinds of the welding tools made of PCBN, W, WC and Al2O3 were applied to FSW of 304 stainless steel, and then the wear and reaction between the tool debris and the steel matrix were examined for each tool material. Severe tool damage was found in W, WC and Al2O3 tools after FSW, while the PCBN tool was hardly worn. Formations of Cr-rich borides, ferrite phases containing the high W contents and Cr-rich carbides were found in the stir zones produced using PCBN, W and WC tools, respectively. It was suggested that formation of those phases could be expected by calculation using the thermochemical database.

Abstract: The primary purpose of the present work is to investigate the effects of annealing after equal channel angular pressing (ECAP) on microstructure and microhardness of TA15 titanium alloy. A study was performed by annealing treatment on the microstructure evolution and microhardness variation of ECAPed TA15 alloy. The results state that, static recrystallization occurred distinctly during annealing after ECAP. Since a sample was annealed at a proper temperature and for proper time after ECAP, a larger amount of well globularized and more homogeneous equiaxed α phase has been attained, grains have not grown observably, and the relief of residual stress and work hardening for subsequent processing and using has also been achieved. Accordingly, the optimum annealing parameters of ECAPed TA15 alloy were optimized to be 973 K and 1 hour. The microhardness level of the sample annealed after ECAP was lower than that unannealed, and the microhardness level decreased with the increasing annealing temperature and time.

Abstract: Invar 36 alloy is increasingly used as a structural material for manufacture of liquefied natural gas (LNG) transporters and storage tanks. However, the conventional arc welding of Invar 36 alloy has high susceptibility of hot-cracking. As a high-energy-beam welding process, laser welding could be effective for producing defect-free Invar 36 weld. In the present study, defect-free Invar 36 weld was successfully produced by Nd:YAG laser welding. The microstructure and mechanical properties of weld were also tested.

Abstract: The tensile ductility of the Ti-23Al-17Nb (at.%) alloy laser welded joints is very low at 650°C, so the embrittlement of the joints at high temperature was investigated. The metastable coarse B2 phase of the as-welded metal transformed into coarse O phase in the weld zone when the joints were heated up to 650°C. This type of phase transformation was based on shear transformation. Coarse orthorhombic O phase has lower ductility compared with body-centered cubic B2 phase. Meanwhile, the shear transformation from B2 to O phase could induce stress-strain concentrations in the grain boundary, which seriously weakened the grain boundary of the columnar grains and led to the brittle fracture characteristics in the weld zone during tensile test at 650°C.

Abstract: Water cavitation peening (WCP) with aeration, namely, a new ventilation nozzle with aeration is adopted to improve the process capability of WCP by increasing the impact pressure induced by the bubble collapse on the surface of components. In this study, in order to investigate the process capability of the WCP with aeration a standard N-type almen strips of spring steel SAE 1070 was treated byWCP with various process conditions, and the arc height value and the residual stress in the superficial layers were measured by means of the Almen-scale and X-ray diffraction method, respectively. The optimal fluxes of aeration and the optimal standoff distances were achieved. The maximum of arc height value reach around 150μm. The depth of plastic layer observed from the results of residual stresses is up to 150μm. The results verify the existence of macro-plastic strain in WCP processing. The distributions of residual stress in near-surface under different peening intensity can provide a reference for engineers to decide the optimal process conditions of WCP processing.

Abstract: Nanocrystalline diamond film was deposited on silicon by double bias hot filament chemical vapor deposition (HFCVD) system. The effect of substrate temperature on the microstructure and mechanical properties of the film were investigated systematically. More defects and non-diamond contents were found as the decrease of grain size, which cause the decrease of hardness and elastic modulus. It is shown that the proper substrate temperature is in the range of 720~760 . The excessively high substrate temperature leads to the °C dramatic increase of nondiamond content and the decrease of mechanical properties.

Abstract: The composite die steel was prepared by vacuum diffusion welding with the cold work die steel Cr12MoV and the alloy steel 40Cr. The composition, microstructure and property changes of the welded joints were studied experimentally, and the relationship between the microstructure and the property after the final heat treatment was analyzed. Based on the experiment results, the suitable welding and heat treatment process were approached. The results show that the good metallurgical bonding is obtained via welding at 1100oC for 30 minutes with a pressure of 20 MP and not more than 0.01 Pa in vacuum. A number of voids are found in the tensile fracture surfaces when the welding temperature is below 1100oC. The grains of 40Cr steel grow obviously when the welding temperature is higher than 1100oC. After quenching at 1050oC and tempering at 200oC, the better mechanical properties are obtained in Cr12MoV because fine carbides distribute evenly. At the same time, the mechanical properties of 40Cr do not decrease dramatically because even though high temperature quenching brings coarse grain size, the amount of lamellar martensite, which is hard and brittle, decreases a lot and the amount of lath matensite increases. After quenching and tempering treatment, the bending strength of the composite die steel reaches 73% of that of Cr12MoV when treated by conventional processing. In the course of welding, carbon migrates from 40Cr to Cr12MoV, which results in a decrease in hardness at the interface of 40Cr.

Abstract: External fields (e.g. electromagnetic field, electric field, etc.) are often applied to modify the solidification structure of metallic alloys. The dynamic process of structure modification is invisible because of the opacity of metallic alloys. To understand the modification mechanism, it is necessary to discover the microstructure evolution during solidification. In present paper, we focus on studying the influence of pulsed electric current (PEC) on dendrite growth of Sn-Bi alloy by insitu and real time observation using the third synchrotron X-ray source at Shanghai Synchrotron Radiation Facility (SSRF) in China. The modification mechanism of PEC on solidification structure is analyzed based on a series of real time observations.

Abstract: Employing a cooling slope technique to produce semisolid slurry, in-situ 3wt%TiB2 particulate reinforced Al-12Si alloy composites have been successfully fabricated by flux-assisted synthesis method. It has been shown that the minute TiB2 particles with size 200-500nm uniformly distribute in the spherical α-Al structure and at the boundary of the eutectic Si. Two groups of contrast experiments with and without flowing on the slope have been performed to investigate the influence of the semisolid processing on the microstructure of the composites. Results have shown that α-Al grain is rather spherical and with an average size of 47.4μm, which indicates semisolid processing can be a potential route to produce metal matrix composites with high strength and good ductility.