Microstructures And Properties Of Laser Welded Dissimilar Metal Joints By Using High-entropy Filler Metal Powders

Dissimilar metal welding can make full use of the advantages of two materials and has a wide range of applications in many fields.However,due to the different physical and chemical properties of two materials,Dilution of the weld seam and intermetallic compound are easy to occur during the dissimilar metal welding process,which may reduce the safety and reliability of the dissimilar joints.High-entropy alloys have high mixed entropy effects,high lattice distortion effects,and performance cocktail effects.High strength and toughness are easily achieved in those alloys.Therefore,it has important research significance for high-entropy alloys introducing into the field of dissimilar metal welding to improve the mechanical properties of dissimilar joints.In this thesis,multi-component mixed powders based on a high-entropy design were used as the filler metals,to achieve the dissimilar welding between different materials by laser deposition welding.And the effects of high-entropy filler metal powders on the microstructures,microhardness,tensile properties and fracture behaviors of those dissimilar joints were studied.Firstly,three types of filler metal powders,including Fe Co Cr Ni Mn,Cr Fe Ni_2.4Al_0.6 and316L,were used to achieve dissimilar metal welding between SMA490BW steel and 304stainless steel.The feasibility of using high-entropy filler metals to achieve the weld seams with high-entropy effects was explored.Moreover,the high-entropy effects in the weld seams were tried to be modified by changing the welding process parameters and the composition ratio of the filler metals.It is found that the high-entropy structures can be obtained in the weld seam by using a suitable filler metal(such as Cr Fe Ni_2.4Al_0.6 powder).However,owing to the melting of base metal the dilution of weld seam during the welding process can significantly change the set elemental composition in the weld seam,which prevents the high-entropy structure obtained in the weld seam by using the Fe Co Cr Ni Mn powders.In addition,the effects of dilution of weld seam on the high-entropy structures could be modified by improving the elemental distribution ratio in the filler metals,as well as opening the V-groove in the plates.It is found that high hardness and tensile strength can be obtained in the dissimilar metal joints of SMA490BW steel to 304 stainless steel by using the high-entropy filler metals,which are significantly higher than that of the base metal of SMA490BW steel.Secondly,five types of?Cr Mn Fe?_x?Co Ni?_yfiller metal powders,with different ratios of BCC and FCC components in those filler metals,were designed and prepared.And the effects of the ratio of BCC and FCC components in those filler metals on the mechanical properties of joints were investigated.It is found that the ratio of BCC to FCC components in filler metals has an important effect on the mechanical properties of the joints.The microhardness in the weld seam increases significantly as the ratio of BCC to FCC components increasing?especially the ratio?1?.However,the best comprehensive mechanical properties could not be obtained as the ratio value of BCC to FCC is too large or too small.And the best comprehensive mechanical properties of the joints are achieved by using the?Cr Mn Fe?₅?Co Ni?₅ filled metal powders.Finally,three types of filler metal powders of Cu_xCo Cr Mn Al_0.8Si_0.2 were designed and prepared to achieve dissimilar metal welding between TC4 titanium alloy and N4 pure nickel plate.The effects of high-entropy filler metals on the weld forming,microstructure and mechanical properties of the TC4/N4 dissimilar joints were explored.It was found that Cu₂Co Cr Mn Al_0.8Si_0.2 and Cu₃Co Cr Mn Al_0.8Si_0.2 powders can achieve the dissimilar metal joining of TC4/N4.Snowflake-like and high-entropy structures are found in the weld seam center.However,a significant transition zone was observed in the weld seam near the Ti plate?about 43?m?.The content of Ti element is higher than 50 at.%.And high entropy effects had not been achieved in the transition zone.The content of Cu element in filler metals has an important effect on the hardness and tensile strength of the dissimilar metal joints.And the highest tensile strength was obtained by using the Cu₂Co Cr Mn Al_0.8Si_0.2 powders.