Welding Metallurgy of

Solidification cracking can occur in the fusion zone during the solidification of the weld metal Liquation cracking, on the other hand, can occur in the partially melted zone during the solidification of the liquated material These two types of cracking are reviewed in this article, including the factors that affect cracking and the remedies

Solidification and liquation cracking issues in welding

It is a known fact that 2024 aluminium alloy is susceptible to solidification cracking in the weld metal and liquation cracking in the base metal when welded with fusion processes. The main purpose of this study is investigating whether these two types of cracks act independently or are related with each other in terms of initiation and propagation as this can lead to enhancing the understanding of the hot cracking phenomena in these alloys. Laser welding whether continuous or pulsed has promising outlooks for welding heat treatable aluminium alloys. But the fast heating and cooling rates involved in pulsed laser welding give rise to unique successively repeating microstructural features which provides an interesting base for studying the cracks. Thus, the experimentation involved Nd:YAG pulsed laser welding of 2024 aluminium alloy. The observations indicate that liquation cracks in the partially melted zone of wrought base metal have strong association with solidification cracks in the weld metal and accordingly it is proposed that the liquation cracks act as a strong initiation sites for solidification cracks. It is also shown that healing of liquated grain boundaries through backfilling can have a significant role on resistance to liquation cracking in the partially melted zone and that in turn can affect tendency for solidification cracking in the weld metal.

The relation between liquation and solidification cracks in pulsed laser welding of 2024 aluminium alloy

The fusion zone and heat affected zone (HAZ) microstructures of electron beam welded superalloy 718PLUS™ (718 Plus) that has been newly developed by ATI ALLVAC were examined. The microsegregation pattern during solidification of the fusion zone indicated that while Fe, Co, W, and Cr segregated to the core of the gamma dendrites, Nb, Ti, and Al were extensively rejected into the interdendritic liquid. Electron diffraction and X-ray microanalysis using transmission electron microscopy (TEM) of the fusion zone showed that the major secondary phases that formed from the interdendritic liquid were gamma/MC type carbide eutectic and gamma/Laves eutectic constituents. HAZ microstructure showed partially melted zone immediately adjacent to the fusion zone and intergranular microfissuring associated with resolidified products which suggested that HAZ cracking in this alloy occurred by liquation cracking. Microstructural examination of the HAZ using analytical scanning electron microscope showed resolidified gamma/Laves eutectic on the cracked and backfilled grain boundaries. Fine resolidified MC type carbide particles were also observed in the HAZ. Causes of grain boundary liquation were identified and the solidification of intergranular liquid in the HAZ was discussed.

Microstructural analysis of fusion and heat affected zones in electron beam welded ALLVAC® 718PLUS™ superalloy

Liquation cracking was investigated in the partially melted zone (PMZ) in welds of Al-Si alloys, which are widely used as casting alloys and often rich in Si to ensure good castability Alloy A357 (Al-7Si) was selected as an example for studying liquation cracking in castings repaired or joined by welding The crack susceptibility was evaluated by the circular-patch test, and full penetration, gas metal arc welds made with filler metals 1100 (Al), 4043 (Al-5Si), 4047 (Al-12Si), and 5356 (Al-5Mg) The PMZ consisted of a dendrites and interdendritic eutectic similar to the base metal, and the interdendritic eutectic coarsened and became a and Si particles in the heat-affected zone(HAZ) Liquation cracking was significant with filler metals 1100 and 5356 but slight with filler metals 4043 and 4047 Based on the multicomponent Scheil model and including as many as 10 different elements, curves of temperature (T) vs fraction solid (f S ) were calculated for the weld metal and the PMZ, which arc competing with each other in increasing f S , and hence crack resistance It is proposed that liquation cracking can occur if the weld-metal f S > the PMZ f S during the last 40°C of PMZ solidification before f S reaches 099 (beyond which there is too little grain-boundary liquid to cause liquation cracking) It was found that in welds with significant liquation cracking (filler metals 1100 and 5356), the weld-metal f S was significantly higher in the temperature range, while in welds with slight liquation cracking (filler metals 4043 and 4047), it was only slightly higher In all welds, liquation cracks were completely backfilled and healed, instead of open as in full penetration welds of wrought aluminum alloys, such as Alloys 2219 and 6061 The T-f S curves showed that, as compared to Alloys 2219 and 6061, Alloy A357 had a much higher fraction liquid (1 - f S ) for backfilling before PMZ solidification was essentially over (f S = 099) The T-f S curves were also calculated for two other high-Si, Al-Si casting alloys, A356 and 359, and the fraction liquid was high before PMZ solidification was essentially over, thus suggesting a strong tendency for backfilling and healing

Liquation cracking in full-penetration Al-Mg-Si welds

Aluminium alloys are susceptible to liquation cracking in the partially melted zone (PMZ), where grain boundary liquation occurs during welding. Alloys 2024, 6061 and 7075 were gas-metal arc welded with fillers 1100 and 4043, and liquation cracking near the weld root was examined. Curves of temperature (T) versus solid fraction (f S), based on the Scheil equation for Al‐ Cu‐ Mg‐ Mn‐ Si‐ Zn alloys, were calculated for the solidifying PMZ and weld metal at the fusion boundary. Judging from the freezing temperature range and the liquid fraction, these curves suggested that liquation decreases in the order of 7075, 2024 and 6061, consistent with experiments. They also suggested that 1100 increases the weld metal f S, thus promoting liquation cracking (by strengthening the solidifying and contracting weld metal that pulls the PMZ) and discouraging backfilling (by reducing the interdendritic liquid), while 4043 does the opposite except during PMZ terminal solidification in 7075 and 2024. These are consistent with experiments. STWJ/396

G. Cao

Papers

Liquation cracking in partial penetration aluminium welds: assessing tendencies to liquate, crack and backfill