Metallurgical Study On Interface Chemical Reaction Of Laser Melting Brazing Of Magnesium Alloy With Steel

Magnesium alloy is a lightweight material.The Mg alloy has high specific strength,specific stiffness,and good shock absorption,and it can be widely used in non-bearing parts instead of traditional structural steel.It is very significant to study the connections of magnesium/steel dissimilar metal because of steel is always used as the main bearing material.There is a large differences in the physical and chemical properties of magnesium and steel,such as melting point,thermal conductivity,coefficient of thermal expansion,crystal structure,atomic radius,and so on.Therefore,it is difficult to achieve effective connection between magnesium and steel using conventional welding methods,and cracks,porosity,collapse,and other defects are prone to occur during welding.Currently,brazing,TIG brazing,MIG and cold metal transitions are mainly used to connected magnesium/steel dissimilar metals.However,these welding methods are defective.Due to the laser has small size spot,concentrated energy,precise distribute energy,and high power density,this welding method has unique advantages in the field of dissimilar metal connections.In this paper,laser welding with MIG filler wire is used to weld 2mm AZ31B magnesium alloy and Q235 steel plate.The welding wire adopts 1.2mm AZ31B magnesium alloy welding wire which is consistent with the magnesium alloy base material.AZ31B magnesium alloy base metal and different coated steel plates were subjected to laser welding and brazing tests.Different process parameters such as laser power,offset,and welding speed were changed.The macroscopic appearance of the welding-brazing joint was observed,and the mechanical properties were analyzed using an extensograph and a microhardness tester.The metallographic microscope,SEM,EDS,and XRD are used to discuss and analyze the joint formation and microstructure of the brazed joint.Firstly,this paper will discuss the influence of different laser power,offset and welding speed on the welding-brazing joint of magnesium alloy and galvanized steel,and the experiment can get a beautiful surface weld with no obvious defects.Thus,the best laser welding parameters are obtained.In the welding-brazing joint,the zinc-rich zone of the weld toe mainly consists of the fragile phase of MgZn and MgZn₂,which is easy to form cracks;The homogeneous reaction zone is mainly composed of??-Mg+MgZn?eutectic structure;FeAl strengthening phase is found in the stretching process and the direct laser action zone and tensile fracture,but due to strengthening phase is few,the strengthening effect is small;defects such as stoma,un-welded,and cracks exist at the welding-brazing joints,among which the cracks and un-welded have a significant effect on the mechanical properties.Different laser powers are used to weld magnesium alloys and copper-plated and nickel-plated steels.The microstructure of welding-brazing joints changes,and the mechanical properties are greatly improved.When the copper-plated steel is welded,Cu element can improve the wetting-spreading ability of the molten alloy on the surface of the solid steel,and the surface forming of welding-brazing joint is become better.The liquid-solid transition temperature can be reduced,the interfacial reaction time can be prolonged,and the non-welding defects can be reduced effectively by adding Cu element,thereby the strength of the welded joint is increased.When the laser power is 1600W,the welding-brazing joint joints are formed.The weld is mainly composed of?-Mg and Mg2Cu,and the?Fe,Cu?Al phase appears in the interface layer,which improves the joint strength.When the magnesium alloy is welded to the nickel-plated steel,the Ni element can effectively improve the wetting and spreading performance,and the molten brazing joint can be formed better.The weld toe at the welding-brazing joints mainly consists of?-Mg and Mg₂Ni phases.It can be known by the detection of welds and tensile fractures that the weld and interfacial layer are mainly composed of?-Mg,Mg₂Ni and NiAl phases.The NiAl phase has excellent properties and can improve the mechanical properties of the fusion brazing joint.When the laser power is 1800W,the mechanical properties of the brazed joint are the best.Through thermodynamic calculations,it can be seen that the intermetallic compounds in addition to Fe₃Al consist of Fe and Al elements can be spontaneously formed.In theory,the higher the Al content,the easier the formation of intermetallic compounds.In the interfacial layer,Fe and Al are the easiest ratios achieved at 1:1,so only the FeAl phase exists in the fusion brazing joint.According to the kinetics,the growth process of the interfacial layer consists of four stages:wetting and spreading,dissolution-diffusion stage,generation-growth stage and diffusion stage.Since the Zn element has a small atomic radius,it can react with the Fe-Al phase in the form of interstitial solid solution to form a Fe-Zn-Al phase.The Cu element is dissolved in the FeAl phase by substitution,some of the Cu atoms replace the Fe atoms,and a small amount of Al-Cu phase is formed,finally forming the?Fe,Cu?Al compound.Due to the infinite solid solution of Ni atoms and Fe atoms,in the interface layer of near-steel side,Fe atoms react with Ni atoms to form Fe-Ni solid solution phases,and Ni atoms are formed between Al atoms and Ni atoms in the near-bead interface layer.