| Metal additive manufacturing technology has undergone rapid development in recent years,but its application and development have been limited due to the easy formation of coarse columnar grain structures during the process.To solve this problem,a technique is needed that can improve the structure and properties of the material synchronously or in situ during the additive manufacturing process.Therefore,this paper proposes a dual ultrasound-assisted metal additive manufacturing method,in which the upper ultrasound acts at close range on the solidified deposition layer to transmit the ultrasound energy field through the deposition layer to the liquid melt,thus realizing the control of the solidification process,while the lower ultrasound transmits the ultrasound energy field to the liquid melt through the substrate to realize the control of the solidification process,and the combined effect of the two achieves in situ improvement of the structure and properties during the additive manufacturing process.This paper introduces ultrasound technology into the laser and wire additive manufacturing(LWAM)process and studies its relevant scientific issues in regulating the structure and properties of metal additive manufacturing.The influence rule and action mechanism of ultrasound-assisted solidification process were studied by numerical simulation,and the solidification process of LWAM-Ti-6Al-4V alloy was analyzed.Combined with the study of structure and properties,the effectiveness of ultrasound-assisted metal additive manufacturing in improving structure and properties was demonstrated.This passage discusses the effects of adjusting different process parameters on the morphology,grain structure,and microhardness of a single-layer deposition.Specifically,increasing laser power flattens the single-layer deposition and results in a rapid decrease followed by a slow decrease in the contact angle.The influence of deposition speed on the morphology of the deposition is more complicated,as it first slowly decreases,then quickly increases,and finally decreases again,with the contact angle exhibiting a similar trend.Increasing the wire feed speed makes the deposition overall fuller,with the aspect ratio showing a linear decrease and the contact angle gradually increasing.Under the assistance of ultrasound,the grain structure of the single-layer deposition changes from columnar grains to equiaxed grains,with the most regular and smallest grain size observed under the coupling of dual ultrasound waves.In terms of microhardness,increasing the amplitude of the upper ultrasound wave results in an increase in the microhardness of the deposition within the top 0mm-0.8mm,while under the coupling of dual ultrasound waves,the microhardness increases significantly within the top 0mm-1mm distance from the deposition surface.When applying ultrasonic waves from the top,the deposit layer can be divided into three regions,with smaller grain sizes at the top and bottom and larger grain sizes in the middle.When ultrasonic waves are applied from the bottom,the grain size of the deposit layer becomes more uniform overall,and as the ultrasonic power increases,the grain size decreases.With the assistance of dual ultrasonic waves,the grain size of the deposit layer becomes the smallest.Ultrasonic assistance can significantly reduce the texture orientation of the grains,increase the content of small-angle grain boundaries inside the deposit layer,and reduce the KAM value.Introducing a large number of small-angle grain boundaries and in the deposit layer can refine the grain size of the interlayer region and impede the epitaxial growth of columnar crystals.Ultrasonic assistance can reduce the microhardness of single-layer and multi-layer deposit layers,but can promote the refinement of grain size,thereby significantly improving plasticity while increasing strength.In this study,after using ultrasonic assistance,the tensile strength of the deposit layer increased from 930.70±9.07 MPa in the original sample to 999.30±15.90 MPa when applying 18μm ultrasonic waves from the top,to 968.30±17.20 MPa when applying1000 W ultrasonic waves from the bottom,and to 1178.80±23.83 MPa,respectively.The elongation at break increased from 3.5% in the original sample to 4.6%,6.5%,and 7.0% when using ultrasonic assistance from the top,bottom,and dual ultrasonic waves,respectively.Overall,ultrasonic assistance can significantly affect the grain structure and mechanical properties of the deposit layer,improving the plasticity and strength of the deposit layer by refining grain size,increasing the number of grain boundaries,and improving the structure of grain boundaries.The amplitude of ultrasound decays with increasing propagation distance,but the energy of ultrasound propagating into the melt can still reach the required threshold for Ti-6Al-4V alloy cavitation,and the average pressure inside the melt exceeds the threshold more as the amplitude of ultrasound increases.The effect of ultrasonic assistance on cavitation mainly concentrates at the bottom of the melt,and the cavitation area inside the melt becomes larger with the increase of ultrasound amplitude.Ultrasonic assistance can accelerate the solidification of the melt,and the complete solidification time of the melt under ultrasound amplitudes of0μm,6μm,12μm,18μm,and dual 18μm+18μm are 0.325 s,0.235 s,0.12 s,0.075 s,and 0.058 s,respectively.By extracting the temperature gradient(G)and solidification rate(R)from the bottom to the top of the liquid-solid interface of the melt and using a solidification map to determine and predict the solidification conditions and grain morphology inside the melt,it was found that the solidification process of the original sample mainly occurred in the columnar crystal and mixed crystal regions.When an 18μm amplitude ultrasound sample was applied from above,there were almost no columnar crystals,while the solidification process of the sample with 1000 W power ultrasound and dual ultrasound was all in the equiaxed crystal region of the solidification map. |
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