Research On Non-penetrating Laser Lap Welding Technology Of Austenitic Stainless Steel

Plate heat exchanger,as an efficient heat exchange equipment,has been widely used in the fields of thermoelectricity,food,medicine,waste heat recovery and petrochemical industry.Austenitic stainless steel is widely used in the production of plate heat exchangers because of its good heat resistance,corrosion resistance and hot and cold workabilities.Due to the spatter problem caused by resistance spot welding of heat exchanger plates,a non-penetrating laser lap welding method of austenitic stainless steel was proposed by using the advantages of laser welding technology,such as small welding deformation,fast welding speed and high mechanical properties of joints.In this work,the welding technology of 2mm 304 austenitic stainless steel for plate heat exchanger was studied by using 8kW disc laser.The influence of laser welding parameters on the weld morphology and mechanical properties was investigated.The results showed that low welding speed and high laser power led to surface irregularity and spatter of weld.The tensile and shear resistance of the joint increased with the increasement of the welding width.With the increase of defocusing amount,the oxidation discoloration degree of weld surface decreased gradually,while the penetration decreased significantly.The low shielding gas flow rate couldn't protect the welding surface,while excessive flow rate resulted in narrowing of the joint width and reducing the mechanical properties of the weld.The optimum test combination as follows: welding speed 1.5m/min,laser power 2.5kW,defocusing rate +4mm and gas flow rate of protection 30L/min.In order to further studied the method of blowhole suppression in non-penetrating laser welding,the measures of blowhole suppression were explored by adjusting special welding process.The mechanism of porosities in laser deep penetration welding was discussed and investigated.The results showed that the increase of laser beam tilt angle contribute to the trend of porosity increased then decreased.With the increase of clearance,the weld residual height decreased or even collapsed.The molten metal overflowed from the clearance results in the width of the joint surface increases significantly,and the tensile properties of the joint improved significantly.When the clearance was 0.2mm,the splash was serious in the clearance between the two plates.The splash between the two plates disappeared basically when the clearance continues to increase,but excessive clearance would lead to higher porosity.Under the same heat input,the weld with higher mechanical properties could be obtained by pulsed laser welding.When the frequency was moderate,the weld had well shape,the irregularities and spatters on the front of the weld were improved.When the pulse frequency was controlled between 40 Hz and 45 Hz,the weld pore disappeared completely.During the welding process,the dendrites on both sides of the weld became smaller and the growth trend changed significantly assisted by ultrasonic impact.The weld structure was refined.It could significantly reduce the porosity of the weld,but excessive power ultrasound led to cracking along the joint and reduce the mechanical properties of the joint.