In Situ Observation of Solidification Conditions in Pulsed Laser Welding of AL6082 Aluminum Alloys to Evaluate Their Imp
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A. Pulsed Laser Welding of Aluminum
PULSED lasers are mainly used for welding when precise and low heat input is required. Heat dissipates between each laser pulse so that the area near the fusion zone receives low thermal stress. This leads to very low heat distortion and enables welding applications where electronics or other sensitive components are close to the fusion zone.[1] Encapsulation of electronics is an example of where pulsed laser welding processes are utilized. Aluminum alloys are often chosen in this context as housing material, since they offer good machinability, electromagnetic shielding capabilities, and corrosion resistance while possessing a low density.[2] Hot cracking must be avoided to ensure hermetic tightness of the weld. In comparison to continuous wave laser welding processes, pulsed laser welding has a higher tendency to generate hot cracks because the pulsed mode leads to higher cooling rates and thus short solidification times and higher strain rates.[3,4] PHILIPP VON WITZENDORFF, Head of Glass Group, STEFAN KAIERLE, Head of Materials and Processes Department, OLIVER SUTTMANN, Head of Production, and Systems Department, and LUDGER OVERMEYER, Scientific-Technical Director, Board of Directors, are with the Laser Zentrum Hannover e.V., Hollerithallee 8, 30419 Hannover, Germany. Contact e-mail: p.witzendorff@lzh.de Manuscript submitted September 19, 2014. Article published online 21 January 2015 1678—VOLUME 46A, APRIL 2015
B. Background Information Hot cracking was viewed from several perspectives in which the hot cracking mechanism was explained by metallurgical factors, stresses, strains, and strain rates.[5] Recent hot cracking models are based on the assumption of the Rappaz–Drezet–Germaud model, where the initiation of hot cracking is caused by the inability of the molten material to compensate for the pressure drop at the dendritic roots.[6] The inner-dendritic pressure drop is induced by the strain rate, e_ , transverse to the columnar dendrites. Hot cracking is initiated when this pressure falls below a specific material value for cavitation. Coniglio et al. showed that dissolved hydrogen promotes pore formation, which can act as crack initiation points.[7] The pressure compensation to avoid hot cracking is determined by the permeability of the molten material to flow to the dendritic roots. The permeability term K can be approximated by K¼
k2 ð1 fs Þ3 ; 180 f2s
½1
where k is the secondary dendrite arm spacing and fs is the fraction of solid.[8,9] Hot cracking sensitivity coefficients were introduced, which are based on an innerdendritic feeding balance increase for higher strain rates at a given fraction of solid.[10,11] Studies on slow-bending trans-varestraint testing in arc welding of aluminum alloys have shown that the critical strain to induce hot cracking decreases for higher strain rates.[12–14] In addition to external applied loads, solidification shrinkage METALLURGICAL AND MATERIALS TRANSACTIONS A
and thermal contraction cause a strain originating from the welding p
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