Papers by Keyword: Hot Cracking

Abstract: Nucleation and growth conditions of single-crystallinity control are convincingly elaborated by multi-scale mathematical modeling of heat and mass transport to totally abate undesirable weld defects, e.g. disoriented crystal and hot cracking inside molten pool of nonequilibrium crystallization, in order to illustrate the usefulness of predictive capability through theory and experiment procedures. Crystal growth is complicated by crystallinity-dependent thermal and chemical driving forces in front of dendrite tip during viable laser surface modification of Ni-based single-crystal superalloy. These two thermal metallurgical determinants play crucial role in crack-insusceptible columnar crystal growth, which is favorably oriented throughout weld depth. There is particular challenge in complete elimination of disoriented crystal, i.e. stray grain formation, for acceptable surface quality. Conservative (001)/[100] crystalline orientation is desired to diminish Al concentration and supersaturation, and morphologically satisfy epitaxial growth kinetics to successfully lessen central cracking with satisfactory variability of laser power and welding speed. Comparatively, (001)/[110] crystalline orientation is disadvantageous to asymmetrically augment Al concentration and supersaturation and aggressively increase interface instability, microstructure heterogeneity and hot cracking vulnerability along disoriented crystal boundaries. Disoriented crystal is increasingly withstood if the Al concentration and supersaturation in front of dendrite tip are low enough and crack-unsusceptible part is relatively large enough in case of attractive (001)/[100] crystalline orientation with optimal range of heat input to ameliorate microstructure homogeneity. Crystalline orientation region varies with diverse welding configurations, and epitaxy across solid/liquid interface is also sensitive to heat input of laser processing, which necessitate high efficient welding conditions optimization. Considerable effort is made to distinguish diffusion-driven crystal growth between a series of combinations of multiple welding conditions, such as critical welding configuration and heat input. Metallographically, the morphologies of crystal growth and hot cracking are experimentally observed to consistently support kinetics calculation result and well explain correlation between solidification behavior and crystal growth.

Abstract: The hot cracking (solidification cracking) susceptibility in the weld metals of duplex stainless steels were quantitatively evaluated by Transverse-Varestraint test with gas tungsten arc welding (GTAW) and laser beam welding (LBW). Three kinds of duplex stainless steels (lean, standard and super duplex stainless steels) were used for evaluation. The solidification brittle temperature ranges (BTR) of duplex stainless steels were 58K, 60K and 76K for standard, lean and super duplex stainless steels, respectively, and were comparable to those of austenitic stainless steels with FA solidification mode. The BTRs in LBW were 10-15K lower than those in GTAW for any steels. In order to clarify the governing factors of solidification cracking in duplex stainless steels, the solidification segregation behaviours of alloying and impurity elements were numerically analysed during GTAW and LBW. Although the harmful elements to solidification cracking such as P, S and C were segregated in the residual liquid phase in any joints, the solidification segregation of P, S and C in LBW was inhibited compared with GTAW due to the rapid cooling rate in LBW. It followed that the decreased solidification cracking susceptibility of duplex stainless steels in LBW would be mainly attributed to the suppression of solidification segregation of P, S and C.

Abstract: The knowledge about the formation of hot cracking in magnesium alloys, such as in twin-roll cast magnesium sheets and strips, is fundamental for a good quality of the strips during the further processing by rolling or welding and minimize the reject. Hot cracking often occurs in the so-called mushy zone, when solid phases and melt coexist, at temperatures where the material no longer exhibits ductility. For the evaluation of the hot cracking tendency of an alloy, the width of the HTBR (High-temperature brittleness range) can be used. On the basis of a test on a Gleeble HDS-V40, the HTBR was determined for a twin-roll cast AZ31 magnesium alloy. The transition between ductile forming behaviour and complete brittle reaction of the AZ31 alloy is confirmed by the observation of the fracture surfaces (determination of the fracture type) in the scanning electron microscope (SEM) and is located at 555 °C. The HTBR shows a range 35 K.

Abstract: The Inconel 718 alloy owes high strength and ductility at high temperature due to precipitation strengthening. In order to upgrade productility of Inconel 718 alloy, the Inconel 718 alloy solve hot crackings through Zr additions. The result shows that, the Inconel 718 alloy with Zr addition achieves grain size refinement and homogenization effect. It is suggested that, homogenization process, such as temperature point and time control, realizes low content of Nb segregation which is the key to prevent hot crackings. At the same time, through dendrite space measurement, the grain refinement realize high productivity of forged Inconel 718 alloy, as a another method of soft effect. In conclusion, adding Zr element is one of dominant methods for producing high quality of Inconel 718 alloy.

Abstract: Generally, nickel superalloy Inconel 625 is good weldability. However, it should be kept in mind his tendency to hot cracking during welding. Gamma matrix is strengthening by the alloying elements chromium, titanium, niobium. These elements favour the formation of carbides and intermetallic compounds that may cause embrittlement and hot cracking in a welded joint. Based on research the system Gleeble 3800 and transvarestraint test appointed criterion for susceptibility to hot cracking and high embrittlement temperature range (HTBR). Analysis of results allowed for evaluation of hot cracking structural mechanism of welded joints Inconel 625.

Abstract: The article presents the course and the results of research on welding conditions and tendency of hot cracking occurrence connected with MIG (CMT or Pulse) welding method choice. EN AW 6082 alloy is considered hard-to-weld by using conventional arc welding processes (MIG-Pulse) due to the heat sensitivity and the tendency to hot cracking in the thin walled joints (weld, HAZ). MIG low energy method (CMT) was developed inter alia to solve this problem.

Abstract: Defining the susceptibility to hot cracking of Inconel 617 alloy welds is essential for assessment welding and pad welding technology. Because of that technological transvarestraint test was performed in the study. Test simulates strains that form in the material during welding. Transvarestraint test enables the assessment of susceptibility to hot cracking and resistance to hot cracking characterized by cracking threshold (ε_p) and critical strain speed (CSS). Performed investigations enabled to characterize the phenomena occurring in Inconel 617 during welds crystallization, which are important for engineers selecting the joining technology of Inconel 617.

Abstract: In-situ tensile properties during the solidification of three commercial aluminium die casting alloys, ADC5, ADC6 and ADC12, were investigated by utilizing a high temperature tensile testing machine. Mid-length portions of the tensile specimens were melted by electromagnetic induction heating. The in-situ data was recorded in the temperature range in which the melt zone was cooling down to the solidified state and immediately after solidification. The technique allows measurement and evaluation of tensile strength and elongation of the alloys at the temperature ranges critical to many manufacturing processes. The relationship between fraction of solid and temperature during the solidification of each alloy was predicted using the Gulliver-Scheil model applied with the thermodynamic database, i.e., Thermo-Calc. Zero Strength Temperature (ZST) and Zero Ductility Temperature (ZDT) of each alloy were determined from the experiments and were correlated to the calculated fractions of solid. The morphology of the specimens’ fractured surfaces was also investigated. The tensile behavior during solidification in this study was used for characterization of hot tearing behavior in each alloy.

Abstract: Development of hot cracks during welding of austenitic materials is a challenge which must be coped with when a suitable welding method is to be chosen. Boundary conditions about hot crack formation are not sufficiently known. One factor is the state of strain during welding. Therefore strain is determined via neutron diffractometry next to the fusion line during welding. This evaluation shall allow to draw conclusions about the influence of the state of thermal strain on the hot crack formation.

Abstract: The article presents the course and the results of research on material and technological welding conditions of 7xxx aluminium alloy using low energy welding method (CMT) as well as discusses the properties of welded joints and the application fields of modern low energy welding devices for joining thin aluminium sheets.