| Wire and arc additive manufacturing(WAAM)uses the method of material accumulation layer by layer to manufacture solid parts,which with the characteristics of low cost,easy control and high efficiency.It is the most widely used technology in industrial production.This paper aimed at the different performance requirements of titanium alloy materials in complex application environments,a method of arc augmentation was proposed to manufacture gradient titanium alloy materials.By using the technical features of slice stratification in additive manufacturing,different proportions of nitrogen are mixed into the shielding gas during the manufacturing process,combining nitriding strengthening of titanium alloy materials with additive manufacturing technology,and in the process of layered manufacturing,gradient titanium alloy materials with performance differences can be obtained effectively and controllably.On the basis of wire and arc additive manufacturing,combining with the nitriding strengthen characteristics of titanium alloy,a gradient titanium alloy arc augmentation manufacturing test system was established.A single-pass single-layer stacking test was carried out on a gradient titanium alloy wire and arc additive manufacturing platform,explored the influence of technological parameters on the stacking process.Finally,the final process of gradient titanium alloy arc additive was carried out with a current of 100 A,a stacking speed of 150mm/min,a wire feeding speed of 180cm/min,an arc length of 5mm and a wire height of 0mm.The results show that the formation of titanium alloy stacking has a great relationship with the matching of the main control parameters,while the nitrogen flow has little effect on the stacking formation.Adding different flow rates of nitrogen in the stacking process has obvious strengthening effect on the stacking layer,and the whole stacking layer basically has performance strengthening,which proves that the stacking layer has good strengthening depth.Observing the arc and the shape of the molten pool by high-speed photography,the introduction of nitrogen causes the arc to shrink significantly,thereby increasing the center temperature of the arc column and increasing the thermal efficiency of the stacking process,and is positively correlated with the increase of nitrogen flow rate.Using Optical Microscope(OM)Scanning Electronic Microscopy(SEM)to observe the structure of single-channel single-layer stacking samples,it can be seen that under the same technological parameters,theamount of nitrogen flow determines the structure of TiN strengthening phase in the stacking layer,which is mainly characterized by the formation of typical lamellar ?phase under non-nitrogen conditions,the formation of TiN dendrites under low nitrogen conditions,and TiN particle linking microstructure under high nitrogen conditions.The three-part transport mechanism of ionization,dissolution and transportation of nitrogen in the stacking process is also analyzed.The transmission behavior of nitrogen in the additive manufacturing process is summarized.It is mainly divided into three parts: ionization of nitrogen molecules in the arc,dissolution and adhesion of nitrogen ions by liquid titanium alloy,and the transport of nitrogen ion with the flow of molten pool.At the same time,the single-channel and multi-layer stacking tests of gradient titanium alloy materials were carried out,and the forming characteristics of single-channel and multi-layer stacking samples were analyzed,and the effects of interlayer cooling time and fixed nitrogen flow rate on the properties and structure of gradient titanium alloy materials during multi-layer stacking were mainly studied.The results show that the magnitude of nitrogen flow has a great influence on the formation of single-pass multi-layer stacking.On the one hand,the arc shrinks,which affects the coupling form of molten pool and welding wire;on the other hand,the excessive flow reduces the plasticity of materials and easily causes the stacking layer to crack or even fall off.Compression experiments were carried out on stacked specimens with different nitrogen flow rates and interlayer cooling time.Taking the parameters of nitrogen flow rate of 1.5L/min as an example,the interlayer cooling time is shorter and the thermal accumulation of the stacking layer is larger.The main structure of TiN is dendrite,and its compressive strength is about 1.47 GPa.The longer the interlayer cooling time is,the less thermal accumulation of the stacking layer.At this time,the TiN structure is a mixture of dendrite and granular crystal,and the compressive strength of the stacking layer is about 1.3GPa.With the increase of nitrogen flow rate,the plasticity of the material decreases obviously,the compressive strength increases from 0.9 GPa to 1.5 GPa,and the morphology of the strengthened structure changes from TiN dendrite to TiN particle bonding.Finally,the numerical simulation analysis of gradient titanium alloy wire and arc additive manufacturing was carried out,and the stress and deformation of titanium alloy wire and arc additive manufacturing process were analyzed.The influence of interlayer cooling time on titanium alloy wire and arc additive manufacturingbehavior was studied.The stress and deformation state of gradient titanium alloy wire and arc additive manufacturing process under extreme conditions were analyzed emphatically.The results show that the interlayer cooling time has little effect on the stress of the stacking layer produced by titanium alloy wire and arc additive manufacturing,but it has a great influence on the stress and deformation of the substrate.When the interlayer cooling time is 20 s,the final maximum deformation of the stacking layer is 0.3mm.When it is 100 s,the maximum deformation is 0.8mm,and the average residual stress of the stacking layer and the joint is about 400 MPa,which is caused by the different heat distribution during the stacking process.At the same time,the arc additive manufacturing process of gradient titanium alloy under extreme conditions is analyzed.The stacking of TiN material has a great influence on both the base material and the stacking layer.Specifically,the base material has greater deformation and residual stress,and the stacking layer has greater interlaminar residual stress.Under the same parameters of the interlayer cooling time of 20 s,the maximum deformation of the TiN material is 0.84 mm,and the average residual stress at the joint exceeds 700 MPa.It is considered that the different thermal expansion coefficients between the stacking layer and the material on both sides of the interface lead to the stress concentration at the interface,and the effect of the stacking layer on the substrate mainly comes from the stress difference on the interface between the stacking layer and the base material. |
PDF Full Text Request