No abstract

An Al-Mg-Si alloy 6061 and an Al-Zn-Mg alloy 7A52 were joined by friction stir welding successfully. Pre- and post- heat treatment were employed to improve the strength of the weld. The results show a best weld joint with the lowest hardness of 100 HV in 6061 matrix, being achieved by post-solid-solution and subsequent two-stage artificial aging for the whole weld joint of the 7A52 and 6061 solid solution. Under this condition, the weld nugget zone (WNZ) is stronger than 6061 matrix but it has lower hardness than 7A52 matrix. The hardness of WNZ is contributed by the combination of η′ and L precipitates, dynamically changes along with the ratios between the number of η′ and L precipitates. The higher the number density of η′ precipitates, the hardness of WNZ is closer to that of the 7A52 matrix. Otherwise, the higher number density of L precipitates, the hardness of WNZ is closer to that of 6061 matrix. The coexistence of η′ and L precipitates is a direct result from the mixture of 7A52 and 6061 alloys achieved by stirring. Precipitates identification and composition analysis reveal a dynamic WNZ with constituent transition in hardness and composition.

A high strength Al-Zn-Mg alloy 7A52 with T6 treatment was successfully friction stir welded. The grain structure, dislocations and precipitates in typical regions of the weld joint, including the weld nugget zone (WNZ), thermos-mechanically affected zones (TMAZ) and heat affected zones (HAZ) were investigated to understand the mechanical properties of each zone and the weld joint. In WNZ, a relatively higher density of dislocations is observed on the advancing side, caused by vacancy collapse induced by severe plastic deformation during stirring. However, in the center and on the retreating side, the dislocation density is very low. The strength of the WNZ is influenced by grain refinement, solution strengthening, or natural ageing hardening. In TMAZs, different mechanical properties on each side are due to different grain structures and precipitates introduced by the asymmetrical thermo-mechanical cycle. In HAZs, the mechanical properties are a strong function of the ratio of η to η phase. Compared to the micro-tensile results, premature failure of the weld joint occurs in HAZs on the advancing side, resulting from stress concentration near the area with the lowest hardness.welding joints [4,[9][10][11][12][13]. These improvements are a result of reduced heat input during FSW compared with conventional fusion weld. The solid phase joining of FSW is achieved by introducing frictional heat, interface deformation and solid-state diffusion, which results in gradual local microstructure changes in the aluminum alloy [14,15]. Understanding the microstructural evolution during the thermomechanical process imposed by FSW is a very important step in understanding the weld's mechanical properties.A number of research papers have been published on the development of microstructures and mechanical properties [4,9,11,[15][16][17]. Concerning Al-Zn-Mg alloys, such studies in alloys 7075 [18][19][20][21], 7050 [10,12,22], 7449 [11], 7039 [4,23-27], 7010 [28] are presented. Results from these studies suggested six different microstructure zones: the nugget in the center of the weld (WNZ), thermo-mechanical affected zones (TMAZ) on each side of the nugget and under the shoulder contact zone, heat affected zones (HAZ) in between the TMAZ and the unaffected parent material. An asymmetrical microstructure produced by FSW has been revealed by hardness testing. This is due to differences in strain levels and thermal cycles on the advancing side and on the retreating side. In addition to the grain structure evolution, different precipitate distributions are induced within each zone by the severe thermo-mechanical condition, which results in higher deformation and temperature gradients to the passing tool.Although microstructural evolution has been reported, the direct link between microstructures and tensile properties has not been established. Previous works have noted more interesting observations on microstructures but not macroscopic tensile properties. The present paper focuses on the direct relationship between microstructures a...

A new method of thermo-mechanical processing has been designed by introducing pre-aging before general cold rolling for an Al-Zn-Mg alloy. This process results in an increase of 200 MPa in yield strength compared to that of the peak-aged samples. The microstructures were examined by transmission electron microscope and X-ray diffraction. It has been found that the enhanced strength is mainly contributed to by ultra-fine lamella structures containing a high density of dislocations pinned by nanoprecipitates. Extra strength is provided by the "interlocking" of precipitates and dislocations. Fractographic features analysis shows that crack propagation along the interface of the lamella structures is the direct reason for resulting in fracture, due to intra-granular strength exceeding grain boundary cohesion.

Electrodeposition as a kind of significant electrochemical technique was used to prepare CoPtMo magnetic materials on copper substrate from an alkaline electrolyte. The influence of hypophosphite addition on the deposition process, alloy content, microstructure and magnetic properties of CoPtMo thin films was studied. The films that were prepared from the electrolytes without hypophosphite exhibited an fcc structure, with higher Pt percentage but lower coercivity. The presence of hypophosphite shifted alloy deposition to more negative potentials, limited the platinum percentage, caused higher Co content and exhibited an hcp structure. It was found that much higher coercivity of CoPtMo(P) than that of CoPtMo was mainly due to the incorporation of P at the grain boundaries.