Hot Crack Initiation Research Articles

Hot Cracking Behaviors of Mg-Zn-Er Alloys with Different Er Contents.

The hot cracking behaviors of Mg-5Zn-xEr (x = 0.83, 1.25, 2.5, 5 wt.%) alloys are investigated by optimized hot cracking experimental apparatus, optical microscope, and scanning electron microscope, such as contraction behaviors, feeding behaviors, and permeability characteristics. It is found that the solid phase fraction at hot crack initiation and within the freezing range both increased with increasing Er contents up to 2.5 wt.% and then decreased at 5 wt.% Er content. The Mg-5Zn-5Er alloy exhibits the lowest solid phase fraction (87.4%) and a reduced freezing range (74.2 °C), which leads to more effective liquid feeding in the latter stages of solidification. Combined with the grain size, the permeability of the mushy zone, and fracture morphology, the overall permeability is optimal in the Mg-5Zn-5Er alloy, which is beneficial for feeding the cavities and micro-pores. Meanwhile, a large amount of W phase precipitated by the eutectic reaction (L→α-Mg + W phase), which facilitates healing of the incurred cracking. Conversely, the Mg-5Zn-2.5Er alloy shows inferior feeding ability due to the lowest solid phase fraction (98.3%), wide freezing range (199.5 °C), and lowest permeability. Therefore, the Mg-5Zn-2.5Er alloy exhibits maximal hot cracking susceptibility, and the Mg-5Zn-5Er alloy exhibits minimal hot cracking susceptibility. This work provides guidance for improving the hot cracking resistance of cast Mg-Zn-Er alloy and enables an understanding of the hot cracking behaviors of Mg-Zn-RE alloys.

Hot cracking in autogenous welding of 6061-T6 aluminum alloy by rectangular pulsed Nd:YAG laser beam

Due to high hot crack susceptibility of Al-Mg-Si alloys, autogenous welding by rectangular pulsed laser beam has not been generally successful in the removal of cracks. In this research, the effect of pulsed Nd:YAG laser parameters and preheating on the creation of hot cracks in the 6061-T6 aluminum alloy was investigated. The sample that was fabricated by the laser parameters including 1 Hz, 0.12 mm/s, 10 ms, and without preheating exhibited the highest cooling rate and the smallest dendrite arm spacing but no hot crack was observed. The tensile test specimens of this sample fractured at a point far from the weld metal and a decrease in the precipitation of silicon and magnesium in the inter-dendritic space and the reduction of thermal stresses resulted in the elimination of hot cracks. However, according to macro-scale models for the creation of hot cracks, preheating decreased the tensile stresses in the BTR (brittle temperature range), but increasing the preheating temperature led to increasing rather than decreasing the hot crack length. In this case, the formation of hydrogen porosity, the segregation of silicon and magnesium, and the creation of low melting point compounds were the important parameters affecting the hot crack initiation and growth.

Comparison of cracking sensitivity between high-permeability 1.5 wt% Si steel and 6.5 wt% Si steel in mushy zone

Twin-roll thin strip casting process (SC) has been utilized in laboratory to fabricate typical high-permeability electrical steel (HPES) such as 1.5 wt% Si and 6.5 wt% Si steel with ideal crystallographic orientation and excellent permeability after a series of heat treatments. Whereas, there exists a problem associated to the cracking sensitivity in high temperature mushy zone, which further restricts the industrial application of SC to produce HPES. Mechanical behavior of 1.5 wt% Si and 6.5 wt% Si steels in high temperature mushy zone were comparatively studied by Gleeble-3800 physical simulation system under strain rate 3 s−1, systematically. The high temperature brittle region composed of zero strength temperature (ZST) and zero ductility temperature (ZDT) in the mushy zone were quantitatively determined, and the critical strain threshold associated to hot crack initiation in the brittle region were further evaluated following empirical model proposed by Y Won. Results show that the tensile strength and fracture ductility in the mushy zone are both closely associated with the varying volume fraction of network-shaped structure during solidification, and are decreased with decreasing solid fraction concerning studied 1.5 wt% Si and 6.5 wt% Si steels. The high temperature brittle region corresponding to the studied 1.5 wt% Si and 6.5 wt% Si steels in mushy zone are separately confirmed to be 1459 ~ 1465℃ and 1390 ~ 1427℃, and the thresholds of critical strain for hot cracks initiation in high temperature brittle region are calculated to be ~ 1.18% and ~ 0.24%, respectively. Considering from perspective of width of high temperature brittle region and critical strain threshold for hot crack initiation, the cracking sensitivity of 6.5 wt% Si steel in the mushy zone is more serious compared to 1.5 wt% Si steel.

Thermomechanical numerical analysis of hot cracking during laser welding of 6XXX aluminum alloys

The finite element method is applied to study the mechanisms inducing hot cracking during laser welding of 6xxx aluminum alloys. The welding process of a single-sheet plate is modeled by adopting a three dimensional fully coupled thermomechanical finite element formulation, where the energy distribution due to a fiber laser heat source and the motion trajectory of the moving laser beam are specified. The temperature dependent material behavior of aluminum alloys is described by a thermo-elastic-plastic constitutive law which assumes isotropic hardening, and the plastic strain history is reinitialized during the melting and solidification process. With this numerical model, the critical mechanical and thermal responses in the welded metal sheet are used to investigate potential hot cracking mechanisms. It is found that the heat flux vector field obtained through simulation is consistent with the grain orientation in the weld bead observed experimentally. The large tensile stress in the semisolid mushy zone near the fusion line is used as an indicator for identification of hot crack initiation, which is shown to be in the direction normal to the welding line. As the crack grows from the fusion line into the weld centerline, due to the high transverse tensile thermal stress developed inside the weld central region, the crack propagation then shifts to the weld line direction. Furthermore, the effects of several manufacturing process parameters on hot cracking susceptibility are investigated. The results reveal that a smaller weld line-to-edge distance would result in a higher tensile stress state near the fusion line in the mushy zone, and thus increase the possibility of crack formation which is consistent with the experimental observations. Preliminary results also suggest that higher laser power or slower welding speed could increase the hot cracking susceptibility.

Residual Stress Tensor Distributions in Cracked Austenitic Stainless Steel Measured by Two-Dimensional X-Ray Diffraction Method

The residual stress tensor for cracked austenitic stainless steel was measured by a two-dimensional X-ray diffraction method. Higher von Mises equivalent stress concentrations, attributed to hot crack initiation, were obtained at both crack ends. The stress of 400 MPa at the crack end in the columnar grain region was about two-fold larger than that of 180 MPa in the equiaxed grain region. This difference was caused by a depression in the cast slab.

Hot tearing susceptibility of binary Mg–Y alloy castings

The influence of Y content on the hot tearing susceptibility (HTS) of binary Mg–Y alloys has been predicted using thermodynamic calculations based on Clyne and Davies model. The calculated results are compared with experimental results determined using a constrained rod casting (CRC) apparatus with a load cell and data acquisition system. Both thermodynamic calculations and experimental measurements indicate that the hot tearing susceptibility as a function of Y content follows the “λ” shape. The experimental results show that HTS first increases with increase in Y content, reaches the maximum at about 0.9wt.%Y and then decreases with further increase the Y content. The maximum susceptibility observed in Mg–0.9wt.%Y alloy is attributed to its coarsened columnar microstructure, large solidification range and small amount of eutectic at the time of hot tearing. The initiation of hot cracks is monitored during CRC experiments. It corresponds to a drop in load increment on the force curves. The critical solid fractions at which the hot cracks are initiated are in the range from 0.9 to 0.99. It is also found that it decreases with increasing the content of Y. The hot cracks propagate along the dendritic or grain boundaries through the interdendritic separation or tearing of interconnected dendrites. Some of the formed cracks are possible to be healed by the subsequent refilling of the remained liquids.

Welding simulation at the development of exhaust systems

In the production of exhaust systems, welding is one of the most used joining processes. The distinctive hot crack inclination of the usually used nickel alloyed materials poses during this a special challenge. The article of J. Eberspacher describes a numerical method for the welding simulation, with which, amongst others, the hot crack inclination of welded components can be tested. With the aid of this method that is based on the finite element method, the relation between component geometry and hot crack initiation could be demonstrated, whereby component designs can be optimised in this aspect.

Investigation of Structure and Crack Formation in Welded Joints of Single Crystal Ni-Base Alloys

Conditions of initiation of hot cracks in welding of single crystal heat-resistant alloys with a 56% content of the γ’-phase were studied. Conditions for providing a polycrystalline structure in HAZ and part of the weld metal were artificially created in one region of a welded joint with the single crystal structure. Welding of the single crystal alloy was performed by the electron beam method in vacuum. Cracks in a low-temperature ductility dip range (ductility dip cracking) were found to form in HAZ and weld of a welded joint with the polycrystalline structure. The nature of cracking of the single crystal weld is related to formation of low-melting point eutectoid interlayers, as well as to exhausting of the safety factor for ductility. Cracks in the single crystal weld along the strain grain boundaries are formed by a similar mechanism.

Effect of Rare Earths on Hot Cracking Resistant Property of Mg-Al Alloys

The effect of rare earths (RE) ranging from 0.1% to 1.2% (mass fraction) on hot cracking resistant property of Mg-Al alloys was investigated. The results show that hot cracking resistant property of Mg-Al alloys remarkably declines with an increase of RE addition. The causes of the decline are as follows: First, grain coarsening of Mg-Al alloys caused by RE addition reduces the fracture strain required for hot crack initiation. Second, RE reduces the eutectic microstructure of Mg-Al alloys, and as a result, shortens the time that the feeding channel remains open, making it difficult to feed the alloy. Furthermore, RE elevates the eutectic reaction temperature, which leads to the decrease in the strength of the interdendritic liquid film at the late stage of solidification. Third, when α-Mg dendrites form continuous skeletons, the interdendritic Al 11 RE 3 phase tends to block the feeding channels and increases the difficulty of feeding. Last, the shrinkage ratio discrepancy between Al 11 RE 3 phases and α-Mg matrix is prone to cause shrinkage stress and promote hot crack initiation.

Different Optimization Models for Crack‐Free Laser Welding

AbstractConventional laser beam welding of aluminum alloys often leads to hot cracking. This is caused by a complex process where thermo‐mechanical and metallurgical aspects are involved; cf. [3], [2]. A possibility to prevent hot crack initiation yields the multi‐beam welding technique (cf. [2]), where additional laser beams are led parallelly besides the main laser beam. There by optimal positions, sizes, and powers of the additional laser beams play an important role otherwise hot cracking can even be enhanced.In [1], [4], resp., a mechanical 1D and thermal 2D model of hot cracking was derived. It provides the basis for different formulations of constrained nonlinear programming problems to identify the optimal parameters of the additional laser beams. In the present paper a comparison between these formulations and between two different optimizers for the so far best formulation are presented. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Elimination of Hot Cracking in Laser Beam Welding

AbstractHot cracking is one of the big problems in laser beam welding. By multi‐beam welding, first suggested in the 70‐s (cf. [1], [4]), hot cracking can be avoided. Hereby two additional laser beams are employed. These beams generate a compression which compensate for the critical tensile strain in the solid‐liquid region of the weld induced by the main laser beam. However, hot cracking can only be prevented if the positions, sizes, and powers of the additional laser beams are suitably chosen, i.e. are optimized. Non‐optimal values can even enhance hot cracking. Until now these quantities have been found either by trial and error or by prescribing them intuitively. In the present paper a constrained nonlinear programming problem is formulated to solve the problem of hot crack initiation by minimizing the accumulated transverse strain, i.e. the opening displacement, in the solid‐liquid region. This approach is based on the so‐called strip expansion technique, cf. [2]. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)