Investigation Of Microstructure Evolution And Property Strengthening Mechanisms Of Narrow-gap Welded Great Thick TC4 Titanium Alloy Joints

Titanium alloys have been widely used in various fields as diverse as aerospace,petrochemical,electronic power,ships,buildings,automobiles,medicine and so on.With the gradual shift of marine development activities to the deep-sea area,the requirements of underwater hyperbaric,temperature and corrosion conditions for equipment are more stringent,so the demand for heavy-thickness titanium alloy structure is increasing.Joining heavy-thickness titanium alloys is a key technology.The 60 mm thick welded joint of TC4 plates investigated in this paper is a high-quality and defect-free joint using narrow-gap multi-pass gas tungsten arc welding(GTAW)welding technology.The weld seam of large thick plates presents a uniform and beautiful I-type morphology as a whole.Joint strength is the important indicator of joint quality and guarantee of security.This paper aims at revealing rule of microstructure and orientation evolution of joins under GTAW thermal action to investigate joint strengthening mechanism from them,combined with dynamic mechanical property characterization.The main conclusions are listed as follows:The macrostructure of weld seam is large columnar grains.Some columnar grains with preferred orientation can grow across multi-layers.The fusion zone(FZ)and heat affected zone(HAZ)are typical ?+? dual-phase structures while their morphologies are distinct.During the cooling stage,the high temperature ? phase of each layer will transform to the basket weave structure of lath martensite through non-diffusion martensite transformation and ? phase are formed by steady-state transformation.The dispersed granular ? phase will be uniformly precipitated during the subsequent cooling process.The martensite in HAZ is mainly acicular shape and a portion of parent ? phase can be retained due to the absence of phase transformation during the thermal cycle.From the base metal(BM)to FZ,the content of Al is almost unchanged,while element V decreases gradually.Due to diffusion mechanism,the content of Al is much higher than the nominal content of filler metal in each layer of FZ.The grain orientation of different layers in FZ with different welding thermal cycle characteristics was studied.It is found that,intensive texture will be formed in FZ after welding while it becomes much stronger after thermal cycle of cover layer(TCCL)and thermal cycle of filled layer(TCFL)than thermal cycle of back layer(TCBL).According to the Schmid Factor distribution of cylindrical slip system,the plastic deformation ability of BM is the best,and the strengthening effect of the strong and complex texture in FZ will hinder the deformation to increase strength while reduce plasticity.In addition,for TCCL and TCFL weld seam,the proportion of low angle grain boundaries decreases and that of high angle grain boundaries increases.High angle grain boundaries can effectively hinder the movement of dislocations to improve weld strength,but also reduce plasticity.Mechanical properties of welded joints are also analyzed in this paper.For tensile test,it is found that the tensile strength of each layer is much higher than the nominal strength of wire,which is almost equal matching with BM.Therefore,the dynamic in-situ tensile test was designed to characterize and analyze the strengthening mechanism of weld microstructures on joint properties.During the tensile process of BM,uniform deformation of ? and ? grains occur in each region,and the cracks propagate along the slip lines after initiation along the softer ? phase.Therefore,strength and plasticity of BM are better.For TCBL weld seam,the size of harder lath martensite is smaller and its distribution is dispersed.Cracks can continue to propagate along the ? phase after initiation in the softer stable ? phase and strength and ductility of layers are better as well.For TCFL weld seam,due to the stronger texture and grain boundary strengthening,the grains are not easy to deform.The small number of micro-cracks will be hindered by basket weave martensite after initiation and cannot expand until a large number of micro-cracks are combined and instantaneous instability fracture.Although this mechanism can improve the strength of weld,it reduces the plasticity and increases the tendency of brittleness.As basal research of joint microstructure and performance,this paper can display significant guiding role in design of joint matching and process optimization.It is favorable to joint quality improvement and ocean equipment development.