| Aluminium alloy AA7050is one of the strongest aluminium alloys in aviation industrial use today. Some several problems is present during fusion welding, such as porosity, lack-of-fusion, and solidification cracking that renders them very useful in aerospace applications and other industries.Friction stir welding(FSW) is a new kind of solid-phase welding process which is used for butt and lap joints, Relative to fusion welds, the FSW joint may avoid thermal crack, lower residual stresses and eliminating brittleness phase. This new technology can join difficult to weld aluminium alloys by traditional fusion techniques, for example, alloys belonging to the2XXX series and the7XXX series with limited weldability. Friction stir welding has received considerable attention since it is invented.Microstructure of transverse cross section of friction stir weld joint of aluminium alloy7050consists of nugget zone, thermo-mechanically affected zone (TMAZ), the heat affected zone (HAZ) and base material. The microstructure of nugget zone is greatly refined due to dynamic recrystallization.crystal grain becomes small. Crystal grains of TMAZ show an elongated shapd due to severely plastic deformation during FSW. The grain structure in the HAZ region, which has not been disturbed mechanically by FSW, is similar to that of the base metal, but it is larger than that of the base metal.All the welding parameters, for example,pin size,pin shape,material of pin, the pin rotation speed, the pin travel speed,axial pressure,which of them influences microstructure when one of these parameters changes..In this paper, FSW test is done under different rotation speed and the different ratio of rotation to travel speed. For defect-free welding joint, The microstructure features of the nugget zone and TMAZ zone and HAZ zone have been characterized by optical microscopy and scanning electron microscopy.grain size have been measured by mean linear intercept. Distribution of precipitation have been analyzed by transmission electron microscopy.Micrograph feature of nugget is equiaxed recrystallized grains, however, macrograph of nugget demonstrate that a segregated, banded, microstructure consisting of alternating hard particle rich and hard particle poor regions is developed.This band structure is called "onion ring".In addition, Dislocation density is different in each zone. Most grains in TMAZ contained a high dislocation density, is similar to that of the base metal, even higher that of the base metal, Grains in nugget zone contained low density of dislocation. The size, morphologies and distribution of precipitation in different regions are also discussed. In nugget zone, the phase was composed of dissolved high temperature stable phase. In TMAZ zone, strengthening precipitates were severely coarsened and partly dissolved while a small amount of very fine η particles were re-precipitated.In HAZ zone, Precipitates of both intragranular and intergranular coarsen. the precipitate-free zone increases in width and more phase forming. Such a microstructure leads to "W type" the hardness distribution characteristics for the joint.Either the HAZ zone or the TMAZ zone will be weakest zone for FSW joints, the TMAZ zone will be weakest zone for FSW joints while hard particle poor regions is developed at the bottom of TMAZ zone. Of course, the HAZ zone will be weakest zone because of grain coarsening when there is not defeat in the TMAZ zone. The strength coefficient of joint is low, at the same time, the strength of joint can not be increased by post-weld solution and aged treatment. When the TMAZ zone will be weakest zone. But if the HAZ zone will be weakest zone, Not only the strength coefficient of joint is high, but also the strength of joint can be increased significantly by post-weld solution and aged treatment.Aluminium alloy7050belongs to age hardening aluminium alloys, In order to develop the high mechanical properties of aluminium alloy7050, reduce the negative impact of welding process.Post-weld heat treatment is particularly important.But aluminium alloy is unstability at high temperature, It is easy to appear the abnomal grain growth when it is heated to a high temperature. Temperature and time are two factors that affects grain heat stability.besides, welding parameters also determine stability of the microstructure. For FSW joint of aluminium alloy7050, the ratio of the square of the pin rotating speed to welding speed is on behalf of instability at high temperature. Stability of the joints is low at high temperature for some joints welded with the small ratio of the square of rotation speed to travel speed, It is prone to abnormal grow for nugget grain. thermal stability of the joints welded with the high ratio of the square of rotation speed to travel speed is improved, abnormal grain growth can be resisted or reduced by high heat rate during solution treatment. It is stable during solution treatment at the heat rate of900℃/h.On the other hand, Aluminum alloys containing a high concentration of alloying elements are considered "quench sensitive"where the cooling rate from thermal exposures has an influence on the mechanical properties by fine precipitation. In general, for ideal as-FSW joint, Increasing cooling rate after solution treatment, the abnormal growth of grain is inhibited, the strength of the joint can increased significantly.For the as-welded joint, facture location depends on welding parameters. facture in tensile test occurs in the HAZ because of the HAZ zone is weakest zone when the joint is friction stir welded with the parameter,which the ratio of rotation speed to travel speed equals to10.The facture location shifted from the HAZ zone to the stir zone after solution and aged treatment. When the ratio of rotation speed to travel speed tended to reach15, the weakest zone of the joint shifted from the HAZ zone to the TMAZ zone. For this kind of joint, facture happened in the TMAZ zone both as-welded and post heat treatment joint. The TMAZ zone is the weakest zone all the time.In summary, this paper has obtained some new ideas and achieved some research results based former studies. microstructure character of defect-free FSW joint and weak zone was analysed in detail, the effect of welding parameter on microstructure and property was investigated, the factor that influence propertier of FSW joints was found during solution and aged treatment.some measures that increase the property of the FSW joints were put forward, which could promote development of FSW process in the connection field of7050aluminium alloy. |
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