It was reported that the microstructure in HAZ
It was reported that the microstructure in HAZ, TMAZ and WN vary particularly in gpr40 agonist hardenable aluminium alloys like 7xxx series . The variation in microstructure leads to the variation in hardness and corrosion resistance of AA7075 aluminium weld. The softening of HAZ is one of the reasons why the corrosion resistance of AA7075 aluminium weld is poor. As the FSW process induces a dramatic change in microstructures, there is every need to understand the microstructure and corrosion behavior of friction stir welds .
Several methods are being considered for minimizing the softening and improving the properties of FSW weld joints, such as optimization of process parameters, post weld heat treatment, in process cooling using external coolants, and underwater or submerged FSW. Many authors reported that the post weld heat treatment of AA7075 alloys at overaged (T73) temper is used to improve the corrosion resistance. However, there is a loss of strength about 10%–15% compared to T6 temper. RRA treatment is used to recover the strength of 7xxx series alloys without impairing the corrosion resistance of the material. Post weld treatment to RRA leads to the dissolution of the less stable precipitates (GP zones and η′) inside the grains and the increase of grain boundary precipitates. It also enhances the re-precipitation of η′ (MgZn2) precipitates [9–12].
Results and discussions
Introduction In recent years, with the continuously appearing of near space hypersonic vehicles, such as X-43  and X-51A , the defense against rapid strike weapons has become an urgent problem to be solved. The near space target is different from the traditional aerodynamic target  and the satellite orbital target . It has its unique hypersonic sliding trajectory , and can rapidly attack any target in the world in two hours, but the existing warning defense system is unable to track and intercept the threat target effectively. Therefore, it is necessary to research the tracking of hypersonic sliding target in near space. The domestic & foreign researches have been still in the primary stage for the study of target tracking in near space. The target trajectory, from launch to impact, is divided into three major phases : boost, coast, and reentry. The boost phase of motion, which lasts from launch to thrust burnout, was discussed in Ref.  where the vehicle is accelerated to the designed altitude and velocity for beginning a sliding trajectory. The coast phase, sustaining the motion of skidding to the desired range, was analyzed in Ref. . And the reentry phase, beginning when the sustaining burns are discontinued, was discussed with the state augment tracking methods in Ref. . In view of the tracking of this sliding trajectory target, a modified constant turning (MCT) model was proposed to enhance the performance of tracking high maneuvering target . A variable structure multiple model tracking algorithm based on the directed graph (DG-VSMM) was proposed to further overcome the weakness of maneuvering target tracking with the single model . And in Refs. [12,13], the IMM tracking algorithm is considered one of the most dependable methods in the field, and the robust tracking can be achieved through the interaction among different models. From Ref.  we know that the linear frequency modulated (LFM) signal is one of the most famous radar signals, which is of large time-bandwidth product, and can significantly improve the signal-to-noise (S/N) ratio when the matching filter is performed. But the disadvantage of this signal is that Histones is not sensitive to the Doppler shifts . While the Doppler shifts exist as the target moves at a radial velocity. That is to say, the radar detection and tracking are inevitably affected by the radial velocity. In the case of low radial velocity, this property from target echo can be approximately neglected. But when the target moves at a hypersonic velocity, the effect of Doppler shift on the target tracking has not been discussed for hypersonic movement of the target, and a solving method has not been mentioned yet in the existing references.