Study On Weldability And Weld Joint Mechanical Properties Of The Co-Al-W Based Superalloys

Welding is an indispensable manufacturing technique in the processing of the superalloy compoents.However,the conventional nickel based superalloys are considered to be unweldable due to various welding cracks defectes including liquation cracks,solidification cracks,ductility-dip cracks and strain-aging cracks.The novel Co-Al-W based precipitation strengthened superalloys are promising to be used in the aero-engine compoents.Many merites of the Co-Al-W based superalloys indicate that this kind of materials could have great weldability.However,few studies have been systematic evolved on the weldability of the Co-Al-W based superalloys and the tensile deformation properties of the weldments.To fill the research gap on this field and promote the application of Co-Al-W based superalloys,present thesis is fouced on the elment effects of B and Nb?Ta on the weldability and micro structures of the Co-Al-W based superalloys weldments,the tensile and rupture properties of the weldments and the post weld treatment effects on the microstructure and mechanical properties of the welmdnts.The major research context and conclusions could be drawing as following:Firstly,the effects of the B-doping on the micro structures and weldability of the fusion zones and heat-affected zones in the weldments are analyzed.SEM and TEM results indicate that the moderate boron addition of 0.05at.%could superssed the percipiatation of ? phase in the fusion zone,and the EBSD results indicate that the boron addititon promotes the sub-microstructures recovery in a continuous dynamic way,which effectively eliminate the stress concentration on the high angle grain boundaries and ductility dip cracking in the fusion zones.Further boron addition up to 0.2at.%,would induce the the M3B2 boride,which percipiatate at the terminal stage of solidification.Meanwhile the solidificaition temperature range is significantly widen by boron additions,which induce high suseptibitliy of solidification cracks in the fusion zone.Eliminate the fusion zone cracks could avoid the brittle fracture under high temperature deformation condiditon and promote the high temperature tensile strength.Secondly,the effects of Nb and Ta additions on the microstructures and weldability of the weldments were investgated.The differential thermal analysis method was used to determinate the eutectic phase transformation temperature during cooling,and the finite elment analysis methods were used to analyze the Nb?Ta effects on the stress distribution in the multi-pass welding joints.Nb?Ta additions could significantly affected the HAZ liquation cracking behviors from two aspects:On the one side,the Nb and Ta additions could influence the phase transformation temperatures,on the other hands,the Nb and Ta additions influence the stress in the weldments.Thirdly,the uniaxial tensile tests of the weldments and base metals were performed under the strain rate of 1.2×10-3 range from room temperatures up to 1000?.The fracture mechanisms of the tensile samples were studied by the SEM methods,and deformation mechnisms were studied by the TEM analysis.The experimental results indicates that under the intermediate temperature range of 760? to 850?,the strength and elongation rate of the weldments is much lower than that of the base metals,also the tensile samples are fractured in a brittle manner at intermediate temperature.At room temperature,the fracture occurrs in the BM regions,as the strength of the fusion zone and heat affected zone are higher than that of the base metal.At high temperatures range from 760? to 1000?,fracture occurrs in the fusion zones.At 760?,the cross-slip of the dislocations causes the formation of the dislocation cells,which leads to the strain concentration at interdendritic regions and the dislocasion pile-ups occurrs alongside the interface of the interdendritic phase in the fusion zones,which causes the brittle fracture alsogside the interdendritic regions.At 850? and 1000?,the cracks initiate in the interdendritic x phase or the voids preferred to nucleate around the MC carbides,the fusion zone provide more initial sites of cracks than the heat affected zone and base metal.Finally,to promote the tensile properties and rupture properties of the weldments,differernt post weld treatments(PWHTs)processes were applied on the weldments.The microstructures after PWHTs were charaterizaed by the SEM and TEM methods.The results indicate that solution treatments are necessary for the weldments.The solution treatments could releave the residual stress in the weldments,which prohibit the strain-aging cracks and rafting of the y' phase in the fusion zones and heat affected zones.The high temperature solution treatments at 1200?/2h could eliminate the formation of the needle-like ? phase,which promote the tensile strength of the weldment at 760?.On the other side,the anealling solution treatments at 1100?/2h before aging are more situable for the property enhancement of rupture.After PWHT,both the rupture property at 980?/100MPa and tensile strength at 760? are significantly improved as compared with the as-welded weld joints.