The Effects of Process Variables on Pulsed Nd: YAG Laser Spot Welds: Part II.AA 1100 Aluminum and Comparison to AISI 409 Stainless Steel

摘要

In this two-part article ,the weldabilities of AA1100 aluminum and AISI 409 stainless steel by the pulsed Nd :YAG laser welding process have been examined experimentallyand compared .The effects of laser pulse time and power density onlaser spot weld characteristics ,such as welddiameter ,penetration ,melt area ,melting ratio ,porosity ,andsurface catering ,have been studied and explained qualitatively inrelation to material-dependent variables such as absorptivity andthermophysical properties .The weldability of AISI 409 stainlesssteel was reported in Part I of this article .In the present article ,theweldability of AA 1100 aluminum is reported and compared tothat of AISI 409 stainless steel .Weld pool shapes in aluminumwere found to be influenced by the mean power density of the laserbeam and the laser pulse time .Both conduction-mode andkeyhole-mode welding were observed in aluminum .Unlikestainless steel ,however ,drilling was not observed .Conduction-mode welds were produced in aluminum at power densitiesranging from 3.2 to 10GW/m2 .The power density required formelting aluminum was approximately 4.5 times greater thanstainless steel .The initial transient in weld pool development inaluminum occurred within 2 ms ,and the aspect ratios (depth/width)of the steady-state conduction-mode pools were approximately0.2.These values are about half those observed in stainlesssteel .The transition from conduction- to keyhole-mode weldingoccurred in aluminum at a power density of about10GW/m2 ,compared to about 4GW/m2 for stainless steel .Welddefects such as porosity and cratering were observed in bothaluminum and stainless steel spot welds .In both materials ,therewas an increased propensity for large occluded vapor pores nearthe root of keyhole-mode welds with increasing power density .Inaluminum ,pores were observed close to the fusionboundary .These could be eliminated by surface milling andvacuum annealing the specimens ,suggesting that such pored weredue to hydrogen .Finally ,excellent agreement was obtainedbetween experimental data from both alloys and an existinganalytical model for conduction-mode laser spot welding .Twonondimensional parameters ,the Fourier number and anondimensional incident heat flux parameter ,were derived andshown to completely characterize weld pool development inconduction-mode welds made in both materials.