| Abstract Scope |
INTRODUCTION
Significant progress has been made recently in understanding the fundamental aspects of welding metallurgy. A summary of the progress has been provided in Welding Metallurgy (3rd edition, Sindo Kou, published in 2020 by John Wiley). Examples are shown as follows.
DISCUSSION
I. Transport Phenomena:
a) In computer modeling of GMAW of Al alloys, Murphy et al. (2017) showed the size of an Al weld can be overestimated because the Al vapor in the arc reduces the arc plasma temperature.
b) Wei et al. (2015) showed rotational asymmetry of the weld pool along the butt joint between two stainless steels of different sulfur contents.
II. Fusion-Zone Phenomena:
a) Microsegregation in the fusion zone is typically measured across several secondary dendrite arms. Liu and Kou (2017) demonstrated a statistically significant microsegregation measurement, which has been used in casting.
b) Mendez and co-workers (2014, 2015) studied the effect of welding conditions on overlays of Cr-carbide and Ni-WC on steel substrates, including the effect of AC balance on volume fraction of primary carbide.
c) Kou (2015, 2016) proposed |dT/d(fS)1/2| near fS = 1 (T: temperature; fS: fraction solid) as a simple index for the susceptibility to solidification cracking. The validity of the index was verified against Al alloys, Mg alloys and carbon steels.
d) Soysal and Kou (2017, 2018) developed a simple new test for solidification cracking, i.e., the Transverse Motion Weldability (TMW) test. Unlike the widely used Varestraint test, a filler metal can be used to evaluate its effect on cracking, and the tensile strain is applied to the mushy zone alone instead of the entire workpiece (the global strain is not responsible for cracking in the mushy zone).
e) By quenching during welding, Liu et al. (2017) showed the high equilibrium partition ratio k of Mg in Al promoted much Mg back diffusion in the mushy zone of 5085 Al (~Al-4Mg) and hence early bonding between dendrites to resist solidification cracking.
f) Liu et al. (2020) showed lacy/skeletal δ cannot resist solidification cracking in austenitic stainless steels (e.g., 304) as widely believed for decades. Quenching showed it does not even exist in the mushy zone, where solidification cracking occurs during welding.
g) Liu et al. (2020) proposed a new mechanism for the resistance of austenitic stainless steels to solidification cracking. With primary-δ solidification, fast back diffusion in bcc (δ dendrites) and significant L+δ+γ reaction (which consumes L while forming γ) deplete the intergranular liquid L to allow early bonding between δ to resist solidification cracking.
h) Yu et al. (2020) proposed a new mechanism for the resistance of austenitic stainless steels to ductility-dip cracking (DDC), i.e., γ forms between δ dendrites by post-solidification δ-to-γ reaction and grows deep into δ to bond the dendrites firmly to resist DDC.
III. Partially-Melted-Zone Phenomena:
a) Chai et al. (2016) demonstrated in welding Mg alloys the susceptibility to liquation cracking exists if (weld metal fS) > (PMZ fS). This criterion, developed earlier for Al alloys, can predict the effect of the filler metal on the liquation cracking susceptibility.
b) Firouzdor and Kou (2010) showed liquid droplets squeezed out of the lap joint during friction-stir lap welding between Al (top) and Mg (bottom). Higher peak temperatures (more liquation and cracking and lower joint strength) were found when Al was placed at the top in lap welding or placed on the advancing side in butt welding.
IV. Heat-Affected-Zone Phenomena
a) Chamanfar et al. (2012) showed HAZ weakening in linear friction welding of Waspaloy due to dissolution of γ' precipitates.
b) Farabi et al. (2012) showed significant HAZ weakening in laser welding of dual-phase steel DP 980.
c) Kant and DuPont (2019) ranked the stress-relief cracking susceptibility among high-temperature ferritic and austenitic alloys. Ranking was based on Risk Priority Number (a prioritization tool in Six Sigma)
d) Wang et al. (2017, 2018) showed cracking along the outer HAZ edge of Grade 91 steel associated with large fluctuations in hardness in the intercritical HAZ.
V. Next Frontiers/Future Challenges
a) To explain spatter in GMAW with Al filler wires containing volatile elements Mg, Zn or Li, Soysal et al. (2017) proposed Marangoni flow in the pendant drop at the wire tip carries bubbles nucleated inside the drop to the arc, causing them to expand suddenly and burst. One next challenge is to confirm the flow pattern and bubble bursting by computer modeling.
b) Bahrami et al. (2016) showed the rotation of outward surface flow in a GTA weld pool between 1018 steel and 304 stainless steel. One next challenge is to calculate the the formation of macrosegregation features in dissimilar-metal welding, such as the formation of a steel peninsula and a Cu beach in Cu-to-steel butt welding.
c) In Al-to-Mg butt and lap FSW Firouzdor and Kou (2009) showed the position of Al relative to Mg affects the peak temperatures, which in turn affect the extent of liquation and hence strength of the resultant welds. One next challenge in modeling FSW is to demonstrate the position effect.
d) In GTAW of Al-7Si containing 1 wt% Al2O3 nanoparticles, Wang et al. (2011) discovered the fusion zone became bigger than that without nanoparticles. It is desirable to explain the difference by computer modeling of weld pool convection.
e) Solidification cracking and liquation cracking can occur in additive manufacturing. They are affected by microsegregation during solidification. It is desirable to better understand solute redistribution considering the dendrite-tip undercooling and nonequilibrium solute partitioning.
f) Various types of high-entropy alloys have been investigated. It is desirable to understand their welding metallurgy and evaluate their weldability.
CONCLUSIONS
Significant advances in fundamental understanding of welding metallurgy have been made recently as described in Kou's new book Welding Metallurgy, 3rd edition, 2020. In light of these advances, examples of next frontiers/future challenges have been shown in the this presentation. |