Zeng Gao

FE modelling of microstructure evolution during friction stir spot welding in AA6082-T6

Friction stir spot welding (FSSW) is a solid-state joining method, which is a variant of friction-stir welding. Microstructure analysis shows that the FSSW joint contains four different zones, namely the stir zone, thermo-mechanical affected zone, heat-affected zone and base metal, respectively. In this paper, the results of a FE analysis of the FSSW process of AA6082-T6 considering geometric dynamic recrystallization are presented. A physically based model taking into account three internal state variables was implemented into the commercial FE package DEFORM-3D to describe the microstructure evolution during FSSW. This model allows predicting the dislocation density, grain size, temperature, effective strain, and strain rate during FSSW. The microstructure in stir zone was analysed by electron backscattered diffraction. Experimental and simulation results have been compared to validate the model.

Analysis of Plastic Flow during Friction Stir Spot Welding Using Finite Element Modelling

Friction stir spot welding (FSSW) as a variant of the linear friction stir welding is implemented in automotive industry as a partial replacement of resistance spot welding for aluminium. FSSW as a solid state joining technology, primarily takes advantage of severe thermoplastic deformation, to achieve the joining between two parts, which can be from the same material or even dissimilar. In this paper, the coupled thermo-mechanical viscoplastic finite element formulation is presented based on the character of FSSW. The model was calibrated by comparing temperature history obtained from the simulation with experimental data and subsequently used to investigate the effective strain distribution in the weld zone as well as the material flow and the shape of the stir zone.

Vacuum brazing of high volume fraction SiC particles reinforced aluminum matrix composites

This experiment chooses A356 aluminum matrix composites containing 55% SiC particle reinforcing phase as the parent metal and Al–Si–Cu–Zn–Ni alloy metal as the filler metal. The brazing process is carried out in vacuum brazing furnace at the temperature of 550°C and 560°C for 3 min, respectively. The interfacial microstructures and fracture surfaces are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy spectrum analysis (EDS). The result shows that adequacy of element diffusion are superior when brazing at 560°C, because of higher activity and liquidity. Dislocations and twins are observed at the interface between filler and composite due to the different expansion coefficient of the aluminum alloy matrix and SiC particles. The fracture analysis shows that the brittle fracture mainly located at interface of filler and composites.

Physical and Numerical Simulations of the Microstructure Evolution in AA6082 during Friction Stir Processing by Means of Hot Torsion and FEM

A reproduction of the conditions occurring during friction stir processing, where a fine grained structure according to the process parameters rpm, transverse speed and pressure develops is the main focus in the present work. To physically simulate such a friction stir process, hot torsion tests at constant temperatures were carried out in a Gleeble ® 3800 machine at different strains and strain rates. The specimens were immediately water quenched after hot deformation to avoid any static recrystallization. The microstructure was investigated to characterize the grain size evolution and misorientation as a function of the local strain, strain rate and temperature. Dynamic recovery was observed followed by continuous dynamic recrystallization at large deformations. By means of DEFORMTM3D the occurring strain, strain rate and temperature distributions, which are decisive for the observed microstructure evolution, were evaluated.

Effect of Ni–P alloy coating on microstructures and properties of vacuum brazed joints of SiCp/Al composites

Compared without electroless Ni–P alloy coating on the SiCp/Al composites, the paper describes the effect of Ni–P deposited layer on the microstructure evolution, shear strength, airtightness and fracture behavior of vacuum brazed joints. Void free and compact reaction layers along the 6063Al/Ni–P deposited layer/filler metal interfaces indicated that the joints exhibit high airtightness with He-leakage less than 2.0 × 10−8 Pa ⋅m3/s. Energy Dispersive X-ray Spectroscopy (EDS) analysis showed that the reaction layers mainly included brittle Al–Ni and Al–Cu–Ni intermetallics, where fracture occurred in priority and the shear strength was less than 90 MPa. However, without Ni–P alloy coating, sound joints with high shear strength of 100.1 MPa but low airtightness with He-leakage higher than 1.45 × 10−7Pa ⋅m3/s were also obtained at 590∘C for soaking time of 30 min. In this case, a few holes that occurred along the filler metal/SiC particle interface significantly decreased the compactness of the joints. Therefore, according to the requirements in practical applications, suitable choice was provided in this research.

Microstructural evolution of AA6082 with small aluminides under hot torsion and friction stir processing

The hot rolled AA6082 aluminium alloy with aluminide dispersoids is deformed up to large strains to obtain a fine grained microstructure. Friction stir spot welding (FSSW) is carried out on rolled plates by means of a device provided by MTS System Corporation. FEM simulations determine that the material can flow up to local strains between 10 and 50 when the material reaches temperatures between 300-500°C. With this information, hot torsion tests at constant temperatures are carried out in a Gleeble ® 3800 machine for different strain rates. In both cases, in situ water quenching is applied to freeze the microstructure and avoid any static recrystallization effect after hot deformation. Light optical microscopy is used to identify the evolution of the grains as a function of the local deformation parameters determined by FEM simulations. The microstructure development by FSSW as well as by torsion is then further characterized by means of EBSD. At small strains the material deforms mainly by dynamic recovery with small low angle grain boundary formation and boundary dragging by fine aluminides and Mg2Si. At large strains grain refinement by continuous dynamic recrystallization takes place heterogeneously as a function of the original crystallographic orientation and precipitation state of each grain.

Welding of Aluminum Matrix Composites

Aluminum metal matrix composites (Al-MMCs) are new promising materials for aviation, aerospace and automotive industries. However, due to the poor weldability they have very limited applications. In this paper, the authors present the welding achievements of Al-MMCs developed by their scientific research team in recent years. Laser welding, liquid phase impact diffusion welding and vacuum brazing were utilized. Based on analysis of microstructure, good joints can be achieved by using these welding methods.