| Magnesium alloy have a bright application in aerospace,defense and consumer electronics industries because of their excellent properties(high specific strength,low density,exceptional vibration and outstanding electromagnetism-shielding).However,the application of magnesium is limited due to its hexagonal close-packed(hcp)crystal structure which led to huge difficulty in manufacture of casting and forging.In order to obtain complicated structure of magnesium alloy,the research and creation of welding technology of magnesium are necessary.Tungsten Insert GAS Arc Welding(TIG)is extensive applied in industry because of its excellent economic applicability.In this paper,a new welding technology was created.In this new nanoparticles strengthening activing flux TIG welding(NSA-TIG),the effect of TiC particles on the microstructure and mechanical property of AZ31 magnesium alloy joint were researched.Besides,the distribution of nanoparticles in the joints was investigated by numerical simulation and experiment.The behavior of magnesium alloy joints also was investigated to reveal the mechanism of NSA-TIG welded joints.Contrapose the multiple technological parameter of NSA-TIG,an optimization method which combine response surface method and genetic algorithm was adopted to guarantee quality stability.In this paper,the effects of TiC nanoparticles on the microstructures and mechanical properties of nanoparticles strengthening activating flux tungsten inert gas(NSA-TIG)welded AZ31 magnesium alloy joints were investigated.The scanning electron microscope(SEM),energy dispersive spectroscopy(EDS),tensile strength test and microhardness tests were adopted.In addition,numerical simulation was adopted to predict the distribution of Ti C nanoparticles in the welded joints.The values of ultimate tensile strength(UTS)and microhardness increased which mainly resulted from the grain boundary strengthening of ?-Mg and the dispersion strengthening of Ti C nanoparticles.The theoretical calculation suggested that the improvement of the UTS was the combination of the CTE mismatch between the matrix and the particles,the Orowan and Hall-Petch strengthening mechanisms.Numerical simulation showed that the TiC nanoparticles had a centralized tendency toward middle and bottom part of welded pool and the EDS analysis and the micro-zone microhardness test results of the FZ of the welded joints confirmed this.Some local regions of weld joint sufferrd slight local corrosion attack after 0.5h immerse test in 3.5% Na Cl solution,but the corrosion of base zone BZ is not apparent.It is well known that some small precipitated phases form in the weld joint during the weld process,such as ?-Mg17Al12 phase.After 6h immerse test,for AZ31 magnesium alloy joints,the NSA-TIG welded joint own more corrosion crack than the TIG welded joint.But for AZ91 magnesium alloy joints,the NSA-TIG welded joint suffer less crack.The electrochemical analysis demonstrate that the number of corrosion current of AZ91 NSATIG welded joint was minimum in the case of continuous ?-Mg17Al12 phase in joint.The amount of TiC on the corrosion behavior was also investigated.The polarization potential was-1.564 V when the amount of coating was 15 mg/cm~2.The corrosion rate increase with the increase of coating density.The optimize parameters are as follow: welding speed: 273.6mm/min;welding current: 115.4A;surface activating flux density: 14.5 mg/cm~2;Ar flow: 12.3L/min.After that,the AZ31 magnesium alloy were welded with those optimize parameters.The UTS reached to 189.2MPa(92.3% of base metal),the depth-to-width ratio was 0.63 and the average microhardness of joints was 96.2 HV.In a word,those research solved the shallow penetration of welding of magnesium and could promote the application of NSATIG welding. |
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