Microstructure and Properties of Mg/Ni/Ti Laser Welding-Brazing Joint
With the rapid development of aerospace industry,magnesium/titanium dissimilar metal connectors have been widely used in aerospace industry.However,the joint between Mg alloys and Ti alloys is restricted by their huge differences in physical and metal-lurgical properties.The melting point of magnesium and titanium are 649 and 1668 ℃ respectively and the boiling point of magnesium is 1090 ℃,hence both of Mg and Ti alloys are difficult to remain in molten state at the same time.The severe evaporation of magne-sium would occur when using the conventional fusion welding techniques.In addition,no mutual diffusion or reaction occurs during so-lidification process due to the metallurgical immiscibility of Mg/Ti,which make it difficult to obtain high-quality fusion welded joint when connecting magnesium and titanium.In order to solve the problem that Mg/Ti dissimilar metals do not dissolve or react with each other and realize the metallurgical combination of Mg/Ti dissimilar metals,the laser welding experiment of Mg/Ni/Ti plate fixed on the worktable was carried out with IPG ylr-4000 fiber laser.The base metals adopted in the present work were 1.0 mm thick AZ31B magne-sium alloys and 1.0 mm thick TC4 titanium alloys with dimension of 120 mm×50 mm.Ni foil with thickness of 0.1 mm and purity of 99.9％was added as intermediate layer.Laser welding joints with different microstructures and properties were obtained by adjusting the laser power and welding speed.Properties of Mg/Ni/Ti laser welding-brazing joints with different welding parameters were tested by WDW-100 microcomputer controlled electronic universal testing machine.Samples with different welding parameters were cut perpen-dicular to the weld seam by electrical discharge machining(EDM).Metallographic specimens were prepared by mounting in denture base resins and then grinded by different SiC grades abrasive paper and polished a mirror-like surface by 3.5 μm diamond suspension.The weld appearances and microstructures were examined by optical microscopy(OM)and scanning electron microscopy(SEM).The distribution of chemical elements across the interface of joint were analyzed through energy dispersive X-ray spectrometer(EDS)analy-sis.The results showed that the laser welding-brazing joint of Mg/Ni/Ti was in good shape and scale lines on the surface of weld seam.The weld zone of joint could be composed of magnesium side weld zone,titanium side weld zone and interface intermetallic compound layer.The width of magnesium side weld zone was larger than that of titanium side.The liquid metal phase in the weld zone formed a re-gion similar to the brazing reaction between the incomplete molten Ni and Ti interlayer.The molten Ni,Mg and Ti formed a region simi-lar to fusion welding reaction in the weld zone.The magnesium side weld zone was mainly composed of α-Mg phase and the titanium side weld zone was mainly composed of α-Ti phase.The interface between the two sides of the weld zone was close,in which part of the Mg/Ti liquid phase was mixed with each other.The interface intermetallic compound layer of magnesium side weld zone was mainly composed of Mg2Ni+AlNi phase and the interface intermetallic compound layer of titanium side weld zone was mainly composed of Ti2Ni phase.Therefore,the metallurgical bonding of Mg/Ti laser welding joint was realized by adding Ni layer,the problem that magne-sium and titanium did not dissolve or react with each other was solved effectively.Meanwhile,with the increase of laser power and welding speed,the bonding area of Mg/Ti interface increased,but the shear strength of the joints increased first and then decreased.When the laser welding power P=1250～1350 W and welding speed v=17～19 mm·s-1,the shear strength of joint could be greater than 80 MPa.When the laser power was too low or the welding speed was too high,there was an obvious gap between the weld zone of mag-nesium side and titanium side.When the laser power was too high or the welding speed was too low,more cracks appeared in the titani-um side weld zone.This was also the reason for the decline of mechanical properties of the joint.Mechanical properties of Mg/Ni/Ti la-ser welding-brazing joint were improved because the molten Ni had good metallurgical bonding ability in the liquid metal phase of Mg and Ti.Under the effect of laser stirring on the molten pool,part of the liquid metal phase in the weld zone on both sides of magnesium and titanium was stirred into the Ni-Ti interlayer.The bonding area of both sides of the weld zone increased with the increase of laser stirring.With the increase of welding heat input,the amount of molten base metal on the titanium side and the effect of laser stirring on the molten pool increased,the interface area of Mg/Ti increased further.Then the metallurgical reaction bonding area between Ni/Mg/Ti was increased further and the mechanical properties of the joint were greatly improved.As the welding heat input continued to in-crease,a large amount of brittle phase Ti2Ni was formed in the titanium side weld zone,which made the joint brittleness and hardness.Under the influence of welding residual stress,a large number of cracks appeared in the titanium side weld zone,which was unfavor-able to the mechanical properties of the joint.Therefore,the interfacial bonding area and the intermetallic compound layer formed by the reaction between elements were the keys to improve the mechanical properties of Mg/Ni/Ti laser welding joint.
pre set Ni sheetlaser welding-brazinginterfacial bondingintermetallic compoundelement diffusion
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