John C. Lippold

Hot Cracking Phenomena in Welds III

Hot tearing remains a major problem of casting technology despite decades-long efforts to develop a working hot tearing criterion and to implement it into casting process computer simulation. Existing models allow one to calculate the stress–strain situation in a casting (ingot, billet) and to compare it with the chosen hot tearing criterion. Two kinds of hot tearing criteria are available in literature: mechanical and non-mechanical one. The mechanical criteria of hot tearing are derived based on mechanical behaviour semi-solid, and the non-mechanical one is based on other properties of semi-solid. In most successful cases, the simulation shows a relative probability of hot tearing and the sensitivity of this probability to such process parameters as casting speed, casting dimensions, and casting practice. None of the existing criteria, however, can give the quantitative answer on whether the hot crack will appear or not and what will be the extent of hot cracking (position, length, shape). This chapter outlines the requirements for a modern hot tearing criterion as well as the future development of hot tearing research in terms of mechanisms of hot crack nucleation and propagation. Introduction – Mechanisms of Hot Tearing Various defects of as-cast product are still frequently encountered in casting practice. One of the main defects is hot tearing or hot cracking, or hot shortness. Irrespective of the name, this phenomenon represents the formation of an irreversible failure (crack) in the still semi-solid casting. in Aluminium Alloys Delft University of Technology, Netherlands Institute for Metals Research, Delft, The Netherlands 4 L. Katgerman, D.G. Eskin From many studies [1, 2, 3, 4, 5, 6, 7, 8] started already in the 1950’s, and reviewed by Novikov [9] and Sigworth [10], it appears that hot tears initiate above the solidus temperature and propagate in the interdendritic liquid film. In the course of solidification, the liquid flow through the mushy zone decreases until it becomes insufficient to fill initiated cavities so that they can grow further. The fracture has a bumpy surface covered with a smooth layer and sometimes with solid bridges that connect or have connected both sides of the crack [7, 8, 11, 12, 13, 14, 15, 16]. Research studies show that hot tearing occurs in the late stages of solidification when the volume fraction of solid is above 85–95% and the solid phase is organized in a continuous network of grains. It is also known that fine grain structures and controlled casting (without large temperature and stress gradients) help to avoid hot cracking. During direct-chill (DC) casting of aluminium alloys, primary and secondary cooling cause strong thermal gradients in the billet/ingot, resulting in uneven thermal contraction in different sections of the billet/ingot. As a result, macroscopic stresses cause distortion of the billet/ingot shape (e.g. butt curl and swell, rolling face pull-in) and/or may trigger hot tearing and cold cracking in the weak sections. The terms “hot” or “cold” refer to the temperature range where the cracking occurs – in the semi-solid mushy zone or below the solidus, respectively. In DC casting, the name “mushy zone” is frequently applied to the entire transition region between liquidus and solidus, which is misleading, as the semi-solid mixture in the top part of the transition region is actually a slurry. Only after the temperature has dropped below the coherency temperature, a real mush is formed. On the microscopic level solidification shrinkage and thermal contraction impose strains and stresses on the solid network in the mushy zone. The deformation behaviour of the mush is very critical for the formation of hot tears. The link between the appearance of hot tears and the mechanical properties in the semi-solid state is obvious and has been explored for decades; see for example reviews [9, 17]. Another important correlation – between the hot cracking susceptibility and the composition of an alloy – has been established on many occasions. A large freezing range of an alloy promotes hot tearing since such an alloy spends a longer time in the vulnerable state in which thin liquid films exist. A lot of efforts have been devoted to the understanding of the hot tearing phenomenon. Compilations of research in this field have been done by Novikov [9], Sigworth [10], and Eskin et al. [17]. Several mechanisms of hot tearing are already suggested in literature. Some of those are outlined in Table 1. In Search of the Prediction of Hot Cracking in Aluminium Alloys 5 Table 1. Summary of hot tearing mechanisms Mechanism Suggested and developed by Ref. Cause of hot tearing Thermal contraction Heine (1935); Pellini (1952); Dobatkin (1948) [18, 2, 19] Liquid film distribution Vero (1936) [20] Liquid pressure drop Prokhorov (1962); Niyama (1977) [37, 39] Vacancy supersaturation Fredriksson et al. (2005) [21]

The effect of annealing twin-generated special grain boundaries on HAZ liquation cracking of nickel-base superalloys

The susceptibility to weld heat-affected-zone (HAZ) liquation cracking of wrought Waspaloy and Alloy 718 was quantified by using hot ductility testing. The intergranular (IG) cracking behavior of these alloys was influenced by long term isothermal heat treatments. Such long holds at the solution temperature resulted in continuous grain growth in Waspaloy. However, the IG liquation cracking was not solely controlled by grain size. Annealing twin-generated special grain boundaries increased in volume fraction as grain size increased and reduced the tendency for IG cracking. Intense δ phase precipitation occurred in Alloy 718 following the long isothermal holds. δ phase pinning of grain boundaries restricted grain growth and hence the fractional increase of special grain boundaries. However, special grain boundaries did provide resistance to IG liquation cracking once the δ-phase was dissolved using a “rejuvenation” heat treatment.

Degradation and failure mechanisms in thermally exposed Au-Al ball bonds

During the manufacturing and the service life of Au-Al wire bonded electronic packages, the ball bonds experience elevated temperatures and hence accelerated interdiffusion reactions that promote the transformation of theAu-Al phases and the growth of creep cavities. In the current study, these service conditions were simulated by thermally exposing Au-Al ball bonds at 175 and 250 °C for up to 1000 h. The Au-Al phase transformations and the growth of cavities were characterized by scanning electron microscopy. The volume changes associated with the transformation of the intermetallic phases were theoretically calculated, and the effect of the phase transformations on the growth of cavities was studied. The as-bonded microstructure of a Au-Al ball bond typically consisted of an alloyed zone and a line of discontinuous voids (void line) between the Au bump and the bonded Al metallization. Thermal exposure resulted in the nucleation, growth, and the transformation of the Au-Al phases and the growth of cavities along the void line. Theoretical analysis showed that the phase transformations across and lateral to the ball bond result in significant volumetric shrinkage. The volumetric shrinkage results in tensile stresses and promotes the growth of creep cavities at the void line. Cavity growth is higher at the crack front due to stress concentration, which was initially at the edge of the void line. The crack propagation occurs laterally by the coalescence of sufficiently grown cavities at the void line resulting in the failure of the Au-Al ball bonds.

Single Sensor Differential Thermal Analysis of Phase Transformations and Structural Changes During Welding and Postweld Heat Treatment

The technique of Single Sensor Differential Thermal Analysis (SS DTA) has been subjected to series of verification experiments. Its accuracy and sensitivity to various phase transformations and structural changes has been evaluated by comparison to the classic differential thermal analysis (DTA) and to dilatometric analysis (DA). The reliability of thermocouple and SS DTA measurements in the typical ranges of heat treatment and weld heating rates has been estimated utilizing the endothermic effect of the ferromagnetic to paramagnetic transition and the Curie temperature as a reference point. The sensitivity of SS DTA to various phase transformations and structural changes has been demonstrated by in-situ applications during fusion and solid-state welding, casting, heat treatment and post weld heat treatment (PWHT). Its application range includes construction of continuous cooling transformation (CCT) diagrams, development and testing of processing procedures, weldability studies, development of welding consumables and new alloys.

Development of the Strain-to-Fracture Test for Evaluating Ductility-Dip Cracking in Austenitic Stainless Steels and Ni-Base Alloys

The strain-to-fracture test has been developed as a reproducible and robust test technique for evaluating susceptibility to ductility-dip cracking (DDC) and other elevated temperature cracking phenomena. Samples are tested over a range of temperature and strain, producing a temperature-strain “envelope” in which cracking occurs. Threshold strain for fracture (Emin) and the ductility-dip temperature range (DTR) can then be determined from these envelopes and used to compare susceptibility among materials. The test is very flexible and allows for changes in the testing parameters and material conditions that permit the factors that affect cracking to be identified. In this investigation, the weld metals of three austenitic stainless steels, two Ni-base alloys, and two Ni-base filler metals have been tested using the strain-to-fracture test. The stainless steels include type 304, type 310, and the superaustenitic grade AL-6XN. The Ni-base alloys tested were alloy 690 and C-22. Filler metal samples produced using the gas-tungsten arc welding (GTAW) process included alloys 52 and 82. Alloy 690 and filler metal 52 were found to be the most susceptible to DDC with a low threshold strain and wide DTR. Type 310 stainless steel exhibited a similar DTR to alloy 690 but had a higher threshold strain. Type 304 stainless steel was found to be the most resistant. Metallographic evaluation of strain-to-fracture samples revealed that cracking occurred preferentially along migrated grain boundaries (MGBs) in the weld metal. Cracking was most severe in the weld metals that were free of second phases or precipitates and exhibited very straight MGBs. The high resistance of type 304 was related to the presence of ferrite in the weld deposit. Recrystallization was observed at temperatures near the upper end of the DTR and was accompanied by a recovery of ductility.

Weldability Studies of High-Cr, Ni-Base filler metals for power generation applications

The solidification behaviour and weld solidification cracking susceptibility of high-Cr, Ni-base filler metals that are widely used, or proposed for use, in the nuclear power industry have been investigated. Two heats of ERNiCrFe-13 (filler metal 52MSS), one heat of ERNiCrFe-7A (filler metal 52M), and one heat of a modified ERNiCr-3 (filler metal 82 with higher Cr content, designated here as filler metal 52i) have been tested using both the Transvarestraint test and the Cast Pin Tear test (CPTT). The solidification behaviour in these alloys has been studied by a newly developed procedure that accurately replicates the solidification process in fusion welds of Ni-base alloys and is based on the patented technique for Single Sensor Differential Thermal Analysis (SS DTA™). Results of the solidification studies showed that filler metal 52i has the widest solidification range, followed by the two heats of filler metal 52MSS, and filler metal 52M. The filler metal 52i also has the widest eutectic temperature range. The interdendritic eutectic constituent formed in weld metal of this filler metal and filler metal 52MSS is enriched in Nb and results from the eutectic reaction of γ + L → γ + NbC at the end of solidification. Both the CPTT and the Transvarestraint test provided the same ranking of solidification cracking susceptibility among these filler metals. Both heats of 52MSS and the heat of 52i were found to be more susceptible to solidification cracking than filler metal 52M. The slightly higher resistance to solidification cracking of filler metal 52i relative to the 52MSS filler metals is attributed to crack “healing” during the final stages of solidification. This is the result of the higher fraction of eutectic liquid of filler metal 52i, as confirmed by metallographic studies. The results of this study confirm the higher solidification cracking susceptibility of high-Cr, Ni-base filler metals that contain higher Nb levels to counteract ductility-dip cracking, relative to filler metals that are Nb-free. This study has also shown that the CPTT can be used as an alternative, and reliable, tool for ranking the solidification cracking susceptibility of high-Cr, Ni-base filler metals proposed for use in nuclear power plants and other applications.

Weld Solidification Cracking in Solid-Solution Strengthened Ni-Base Filler Metals

The weld solidification cracking susceptibility of several solid-solution strengthened Ni-base filler metals was evaluated using the transverse Varestraint test. The alloys tested included Inconel 617, Inconel 625, Hastelloy X, Hastelloy W, and Haynes 230W.* Susceptibility was quantified by determining the solidification cracking temperature range (SCTR) which is a direct measurement of the range over which cracking occurs. This temperature range was then compared to the equilibrium solidification temperature range derived from Calphad-based ThermoCalc™ calculations, Scheil-Gulliver solidification simulations, and in-situ measurements using the single sensor differential thermal analysis (SS-DTA) technique.

Transient thermal response in ultrasonic additive manufacturing of aluminum 3003

Purpose – Ultrasonic additive manufacturing (UAM) is a rapid prototyping process through which multiple thin layers of material are sequentially ultrasonically welded together to form a finished part. While previous research into the peak temperatures experienced during UAM have been documented, a thorough examination of the heating and cooling curves has not been conducted to date.Design/methodology/approach – For this study, UAM weldments made from aluminum 3003‐H18 tapes with embedded Type‐K thermocouples were examined. Finite element modeling was used to compare the theoretical thermal diffusion rates during heating to the observed heating patterns. A model was used to calculate the effective thermal diffusivity of the UAM build on cooling based on the observed cooling curves and curve fitting analysis.Findings – Embedded thermocouple data revealed simultaneous temperature increases throughout all interfaces of the UAM build directly beneath the sonotrode. Modeling of the heating curves revealed a del...

Further Investigations of Ductility-Dip Cracking in High Chromium, Ni-Base Filler Metals

The ductility-dip cracking (DDC) susceptibility of high-chromium, nickel-base filler metals was evaluated using the strain-to-fracture (STF) test technique. These filler metals were of the Ni-30Cr type and included INCONEL® Filler Metals 52 and 52M supplied by Special Metals Welding Products Company, and Sanicro 68HP® and Sanicro 69HP® supplied by Sandvik AB. In addition, two experimental Ni-30Cr filler metals were evaluated which contained variations in other alloy additions including Nb above 1 wt % and Mo up to 4 wt %. A wide range in DDC susceptibility was observed with these filler metals, including large heat-to-heat variations in filler metals with similar compositions. The experimental filler metals were found to be remarkably resistant to DDC. Cracking susceptibility is primarily associated with the type and form of precipitate that forms along the weld metal migrated grain boundaries. The formation of Nb-rich, M(C, N) at the end of solidification has the most profound effect on DDC, since these precipitates are most effective in pinning the boundaries. The formation of M23C6 carbides during weld cooling or subsequent reheating can also affect DDC susceptibility in these filler metals.

Development of a chromium-free consumable for austenitic stainless steels-Part 1: Monel (alloy 400) filler metal

Abstract During fusion welding of stainless steels and other high-Cr alloys, evaporation and oxidation of Cr from the molten weld pool result in the generation of carcinogenic hexavalent chromium (Cr[VI]) in the welding fume. Stringent new exposure limits from the Occupational Safety and Health Administration (OSHA) might limit the environment in which the welding of stainless steel will be possible in the future. Therefore, a Cr-free filler metal is needed to reduce the release of Cr(VI) during the welding of stainless steel. From the galvanic series, the corrosion potential of the Ni-Cu alloy K400 (UNS N04400) is located in the same potential range as Type 304 (UNS S30400) and Type 316 (UNS S31600) stainless steels. In this study, the applicability of the Monel-type filler metals for the welding of austenitic stainless steel is examined. Type 304L (UNS S30403) stainless steel plate was successfully welded with Monel filler wire, resulting in high-quality welds with no cracks. The welds survived long-ter...