Welding Configuration Research Articles

Numerical Analysis of Microstructure Anomalies during Laser Welding Nickel-Based Single-Crystal Superalloy Part III: Amelioration of Solidification Behavior

The contribution of crystallography-dependent metallurgical factors, such as supersaturation of liquid aluminum and minimum dendrite tip undercooling, to solidification behavior and microstructure development is numerically analyzed during Ni-Cr-Al ternary single-crystal superalloy molten pool solidification to better understand thermodynamic and kinetic driving forces behind solidification cracking resistance. The variation of supersaturation of liquid aluminum and minimum dendrite tip undercooling with location of solid/liquid interface is symmetrically consistent in (001)/[100] welding configuration. By comparison, the variation is asymmetrically consistent in (001)/[110] welding configuration. The different distribution is attributed to growth crystallography and dendrite selection. Significant increase of supersaturation of liquid aluminum and dendrite tip undercooling from [010] dendrite growth region to [100] dendrite growth region preferentially aggravates microstructure development as result of nucleation and growth of stray grain formation with the same heat input on each half of the weld pool in (001)/[110] welding configuration. High heat input (both increasing laser power and decreasing welding speed) exacerbates supersaturation of liquid aluminum and dendrite tip undercooling by faster diffusion to incur stray grain formation with severity of contributing thermometallurgical factors for susceptibility to solidification cracking, while low heat input (both decreasing laser power and increasing welding speed) ameliorates microstructure development and increases resistance to solidification cracking. Weld microstructure of optimum welding conditions, such as combination of low heat input and (001)/[100] welding configuration, is less susceptible to solidification cracking to suppress asymmetrical microstructure development and improve weld integrity potential rather than insidious welding conditions, such as combination of high heat input and (001)/[110] welding configuration. Severer supersaturation of liquid aluminum and wider dendrite tip undercooling occur in the [100] dendrite region as consequence of alloying enrichment, while smaller supersaturation of liquid aluminum and narrower dendrite tip undercooling occur in the [001] dendrite region as consequence of alloying depletion to spontaneously facilitate epitaxial growth of single-crystal essential. Symmetrical (001)/[100] welding configuration decreases growth kinetics of dendrite tip with smaller overall supersaturation of liquid aluminum and dendrite tip undercooling than that of asymmetrical (001)/[110] welding configuration regardless of combination of laser power and welding speed. Mitigation of supersaturation of liquid aluminum and dendrite tip undercooling simultaneously alleviate crack-susceptible microstructure development and solidification cracking. Additionally, the appropriate mechanism of solidification cracking resistance improvement through modification of crystallography-dependent supersaturation and undercooling of dendrite tip is proposed. Calculation analyses are sufficiently explained by experiment results in a reasonable way. The additional purpose of this theoretical analysis is to evaluate solidification cracking susceptibility of similar nickel-based or iron-based single-crystal superalloys.

Numerical Analysis of Solidification Behavior during Laser Welding Nickel-based Single-crystal Superalloy Part III: Auspicious Control of Dendrite Tip Undercooling

Location-dependent dendrite tip undercooling is numerically elucidated to predict crystallography-assisted resistance to centerline grain boundary formation and morphology transition of stray grain formation ahead of dendrite tip in the ternary Nickel-Chromium-Aluminum molten pool during course of nonequilibrium solidification for explanation arduous solidification behavior control of microstructure melioration. Heat input is not so salient as welding configuration for auspicious solidification behavior and beneficial microstructure development. Advantageous symmetry of welding configuration efficiently lessens dendrite tip undercooling for prevalent dendrite morphology stability of planar interface with alleviation of columnar/equiaxed transition (CET) phenomenon. The bimodal distribution of undercooling ahead of dendrite tip is symmetrically dominant for (001)/[100] growth crystallography with capability of increasing morphology of interface kinetics for epitaxial growth and guarantees single-crystal potential. Alternatively, the distribution of undercooling ahead of dendrite tip is asymmetrically prevalent for (001)/[110] growth crystallography with inefficiency of nonhomologous solidification behavior for discontinuous intersection of solidification interface. Undercooling ahead of dendrite tip inside [010] growth region is not so wide as inside [100] growth region, where thermometallurgically initiates unstable solidification interface and inferior solidification behavior, with unfavorable crystallography in the case of asymmetrical (001)/[110] welding configuration. The smaller heat input is applied, the narrower undercooling ahead of dendrite tip is acquired to significantly mitigate microstructure anomalies with favorable solidification conditions, meliorate metallurgical properties and potentially improve weldability with viability of epitaxial columnar morphology and vice versa. Optimum heat input, especially low laser power and high welding speed together, is a viable and robust way to limit plethora of undercooling and easily decrease solidification behavior anomalies. When low laser power or rapid welding speed is chosen, low heat input not only lessens [100] dendrite growth region, where is spontaneously vulnerable to columnar/equiaxed transition, as ramification of prominent dendrite tip undercooling, but also metallurgically ameliorates [001] dendrite growth region, where morphologically aids epitaxial growth and activates stable planar interface, with achievable diminution of dendrite tip undercooling. Symmetrical (001)/[100] welding configuration, in which undercooling ahead of dendrite tip is preferably narrower than asymmetrical (001)/[110] welding configuration, is one of the most important ingredient for auspicious control of dendrite tip undercooling, once other welding conditions are similar. The main reason, why welding conditions (both low heat input and (001)/[100] welding configuration) is quite superior to welding conditions (both high heat input and (001)/[110] welding configuration), is attributable to favorable crystallography-dependent thermometallurgical factors to suppress inhomogeneous microstructure as long as solidification conditions within marginal stability range. Satisfying crack-free microstructure development is strongly interdependent on kinetics-related solidification behavior through scrupulous control of dendrite tip undercooling to balance between microstructure amelioration and weld depth requirement. The mechanism of columnar/equiaxed transition elimination, by which kinetic driving forces of abnormal microstructure development within high-undercooling region on either left or right side of weld pool is diminished through challenging method of crystallography-dependent dendrite tip undercooling control, is therefore proposed. Finally, there is reasonable consensus between numerical analysis results and experiment results. The numerical analysis provides credible insight into where is liable to microstructure anomalies and why dendrite tip undercooling suppresses stray grain formation for successful laser surface modification of Ni-based single-crystal superalloy.

Numerical Analysis of Solidification Behavior during Laser Welding Nickel-based Single-crystal Superalloy Part IV: Optimization of Thermo-metallurgical Factors

Important metallurgical factors, such as alloying aluminum redistribution, supersaturation and undercooling of dendrite tip around solid/liquid interface, are separately optimized to alleviate stray grain formation and columnar/equiaxed transition (CET) with series of welding conditions and provide a very efficient method for microstructure control through modification of growth kinetics of dendrite tip under nonequilibrium solidification conditions of ternary Ni-Cr-Al molten pool. Asymmetrical (001)/[110] welding configuration is inferior to symmetrical (001)/[100] welding configuration, because overall area-weighted alloying redistribution, supersaturation and undercooling of dendrite tip throughout the solid/liquid interface of weld pool are consistently severer to exacerbate solidification behavior and microstructure development and incur morphology instability of columnar/equiaxed transition. High heat input, such as combination of higher laser power and slower welding speed, monotonically increases aluminum enrichment, supersaturation and undercooling of dendrite tip near solidification interface to simultaneously deteriorate nucleation and growth of stray grain formation and weaken columnar dendrite morphology, while low heat input, such as combination of lower laser power and faster welding speed, decreases solute buildup, relieves supersaturation and beneficially suppresses dendrite tip undercooling to minimize equiaxed dendrite morphology in the crack-susceptible region, and thereby facilitate single-crystal epitaxial growth with decrease of thermo-metallurgical factors for columnar/equiaxed transition in order to provide prerequisite for optimization of welding conditions. Favorable solidification conditions are obtainable with preferential crystallographic orientation to eliminate columnar/equiaxed transition under which the epitaxy of single-crystal metallurgical properties across fusion boundary of substrate is predominantly promoted to essentially reduce stray grain formation in (001)/[100] welding configuration, and is kinetically capable of significant reduction of microstructure anomalies and nonuniform solidification behavior. The useful relationship among welding conditions, alloying aluminum redistribution, supersaturation and undercooling of dendrite tip is properly established within dendrite stability range through thorough analysis. In addition, the validation of theoretical predictions is fairly reasonable by the experiment results. It is worth that the contributions of kinetics-related solidification phenomena with advancement of solid/liquid interface are imposed altogether to understand why stray grain formation occurs on the basis of controlling mechanism of minimum undercooling or minimum velocity by the reproducible methodology procedure.

Numerical Analysis of Stray Grain Formation during Laser Welding Nickel-Based Single-Crystal Superalloy Part II: Multicomponent Dendrite Growth

Multicomponent dendrite growth is theoretically predicted to optimize solidification cracking susceptibility during ternary Ni-Cr-Al nickel-based single-crystal superalloy weld pool solidification. The distribution of dendrite trunk spacing along the weld pool solidification interface is clearly symmetrical about the weld pool centerline in beneficial (001)/[100] welding configuration. The distribution of dendrite trunk spacing along the weld pool solidification interface is crystallography-dependent asymmetrical from bottom to top surface of the weld pool in detrimental (001)/[110] welding configuration. The smaller heat input is used, the finer dendrite trunk spacing is kinetically promoted by less solute enrichment and narrower constitutional undercooling ahead of solid/liquid interface with mitigation of metallurgical contributing factors for solidification cracking and vice versa. Vulnerable [100] dendrite growth region is predominantly suppressed and epitaxial [001] dendrite growth region is favored to spontaneously facilitate single-crystal columnar dendrite growth and reduce microstructure anomalies with further reduction of heat input. Optimum low heat input (both lower laser power and higher welding speed) with (001)/[100] welding configuration is the most favorable one to avoid nucleation and growth of stray grain formation, minimize both dendrite trunk spacing and solidification cracking susceptibility potential, improve resistance to solidification cracking, and ameliorate weldability and weld integrity through microstructure modification instead of inappropriate high heat input (both higher laser power and slower welding speed) with (001)/[110] welding configuration. The dendrite trunk spacing in the [100] dendrite growth region on the right side of the weld pool is considerably coarser and grows faster than that within the [010] dendrite growth region of the left side in the (001)/[110] welding configuration to deteriorate weldability, although the welding conditions are the same on the either side. Furthermore, the alternative mechanism of crystallography-dependent solidification cracking as consequence of asymmetrical microstructure development and diffusion-controlled dendrite growth of γ phase is therefore proposed. The theoretical predictions are comparable with experiment results. The reliable model is also useful for welding conditions optimization for crack-free laser processing.

Numerical Analysis of Stray Grain Formation during Laser Welding Nickel-Based Single-Crystal Superalloy Part III: Crystallography-Dependent Solidification Behavior

The solidification temperature range was numerically analyzed to optimize nonequilibrium solidification behavior during ternary Ni-Cr-Al nickel-based single-crystal superalloy weld pool solidification with variation of laser welding conditions (either heat input or welding configuration). The distribution of solidification temperature range along the fusion boundary is beneficially symmetrical about the weld pool centerline in the (001)/[100] welding configuration. The distribution of solidification temperature range along the fusion boundary is detrimentally asymmetrical about the weld pool centerline in the (001)/[110] welding configuration. The stray grain formation and solidification cracking are preferentially confined to [100] dendrite growth region. [001] epitaxial growth region with columnar dendrite morphology is favored at the expense of undesirable [100] growth region with equiaxed dendrite morphology to facilitate essential single-crystal solidification with considerable reduction of heat input. The smaller heat input is used, the narrower solidification temperature range is thermodynamically promoted to reduce nucleation and growth of stray grain formation with decrease of constitutional undercooling ahead of dendrite tip and mitigate thermo-metallurgical factors for morphology instability and microstructure anomalies. Potential low heat input(both decreasing laser power and increasing welding speed) with (001)/[100] welding configuration decreases solidification temperature range to significantly minimize columnar/equiaxed transition (CET) and stray grain formation, and improve resistance to solidification cracking through microstructure control. On both sides of weld pool are imposed by the same heat input, while the solidification temperature range along the fusion boundary inside of [100] dendrite growth region on the right part of the weld pool is spontaneously wider than that of [010] dendrite growth region on the left part to increase solidification cracking susceptibility in the (001)/[110] welding configuration. Furthermore, another mechanism of solidification cracking as consequence of severe solidification behavior and anomalous microstructure with asymmetrical crystallographic orientation is therefore proposed. The theoretical predictions are well verified by experiment results. The useful and satisfactory numerical modeling is also available for other single-crystal superalloys during successful laser repair process without stray grain formation.

Investigations on the weld metal composition and associated weld metal cracking in laser beam welded steel copper dissimilar joints

Cracking as one of the most common weld defects for joining dissimilar metals can lead to challenges as lack of mechanical properties, lack of joint tightness, lowered electrical conductivity or can be a point of attack for corrosion. Joints between stainless steel and copper also suffer from weld metal cracking which is caused by the effect of liquid copper on the microstructure of solidifying steel matrices. The crack formation is dependent on the dilution of copper in the weld metal, the overall metal intermixing and thermal stresses while cooling down. To investigate the crack susceptibility of laser beam welds with that material combination, lap joints with stainless steel on the top side were welded with a wide range of parameter settings to create different weld metal compositions. The joints were evaluated regarding metal intermixing, cracking, micro hardness and shear tensile strength by metallographic cross sections, x-ray spectroscopy line scans, x-ray spectroscopy element mapping and shear tensile tests to create sufficient datasets for analyses. The results show a correlation between the mixing ratio of copper and the formation of hot cracks with a critical amount of approximately 10 w co .% copper in the fusion zone. Based on that, a hypothesis in order to reduce cracks by decreasing the amount of copper in Fe-rich weld metals or to create copper dominated weld metals was put forward. The crack formation has no influence on the tensile shear strength, which is dependent on the connection width reached between the sheets with a maximum shear strength of 9.6 kN at full penetration of both sheets and shows ductile fraction behavior. Consequently, the crack formation in this welding configuration at copper contents above 10 w co .% has to be weighed against the better mechanical properties at higher connection width and mixing ratios for joining stainless steel and copper. Hardness measurements of the weld metal show results mostly independent of the overall mixing ratio with median values of 170 HV0.1.

Experimental investigation and optimization of thermal gradients by infrared welding

Infrared welding technology is currently used in the automotive industry to assemble very complex composite shapes made of short glass fiber reinforced polymers. Such applications are targeted more towards short joining times than towards the optimization of adhesive properties. Through this work, the authors joined their efforts on the experimental investigation of driving mechanisms and their optimization, enabling the welding of high-performance materials typically selected for aeronautics applications. The thermal field through the thickness and on the surface was investigated. The best configuration of the LM-PAEK/C laminate presented a single lap shear (SLS) strength of 43.5 MPa with a standard deviation of 0.7 MPa compared to a strength of 24.9 MPa with a standard deviation of 2.3 MPa obtained with the welding configuration without insulation. These results highlight the major effect of a thermal gradient during infrared welding. It is specific to infrared welding to observe that the major resulting defects are located not at the interface area but inside the composite substrate, where voids, generated during the heating step, are unable to be reconsolidated during pressure application. The impact of the decompaction behavior on the thermal gradient was studied through a MATLAB© implemented 1D numerical model, developed internally and called “LysIR”.

Numerical Analysis of Stray Grain Formation during Laser Welding Nickel-based Single-crystal Superalloy Part I: Columnar/Equiaxed Morphology Transition

The thermo-metallurgical modeling of stray grain formation was further developed by couple of heat transfer model, dendrite selection model, multicomponent dendrite growth model, nonequilibrium solidification model and minimum undercooling model during three-dimensional nickel-based single-crystal superalloy weld pool nonequilibrium solidification over a wide range of welding conditions (laser power, welding speed and welding configuration). Welding configuration simultaneously influences distributions of stray grain formation and columnar/equiaxed transition (CET). The stray grain formation and dendrite morphology ahead of solid/liquid interface are symmetrically distributed about the weld pool centerline in the (001)/[100] welding configuration. The stray grain formation and dendrite morphology ahead of solid/liquid interface is asymmetrically distributed in the (001)/[110] welding configuration. Vulnerable [100] dendrite growth region is suppressed in favor of epitaxial [001] dendrite growth region to predominantly facilitate single-crystal dendrite growth with further reduction of heat input. Stray grain formation and solidification cracking are preferentially confined to [100] dendrite growth region. The smaller heat input is used, the less nucleation and growth of stray grain formation with decreasing constitutional undercooling ahead of solid/liquid interface is incurred with mitigation of metallurgical driving forces for solidification cracking and columnar dendrite morphology is increased and vice versa. Symmetrical crystallographic orientation of dendrite growth spontaneously ameliorates microstructure development, and improves resistance to solidification cracking. The mechanism of asymmetrical solidification cracking because of crystallography-dependent stray grain formation and morphology instability is therefore proposed. Optimum low heat input (low laser power and high welding speed) with (001)/[100] welding configuration essentially minimizes both stray grain formation and columnar/equiaxed morphology transition and is beneficial to weldability and weld integrity through morphology control, while undesirable high heat input (high laser power and slow welding speed) with (001)/[110] welding configuration leads to microstructure anomalies and worsens solidification cracking susceptibility. The stray grain formation and morphology transition in the [100] dendrite growth region on the right side of the weld pool are more severe than that in the [010] dendrite growth region on the left side, although the same heat input imposes on both sides of the weld pool in the (001)/[110] welding configuration. The theoretical predictions agree well with the experiment results. Moreover, the promising and reliable model is also applicable to other single-crystal superalloys with similar metallurgical properties for successful crack-free laser welding or laser cladding.

Numerical Analysis of Solidification Behavior during Laser Welding Nickel-Based Single-Crystal Superalloy Part I: Crystallography-Dependent Solid Aluminum Distribution

The thermal metallurgical modeling of alloying aluminum redistribution was further developed through couple of heat transfer model, dendrite selection model, multicomponent dendrtie grwoth model and nonequilibrium solidification model during three-dimensional nickel-based single-crystal superalloy weld pool solidification over a wide range of welding conditions (laser power, welding speed and welding configuration) to facilitate understanding of solidification cracking phenomena. It is indicated that the welding configuration plays more important role than heat input in aluminum redistribution. The bimodal distribution of solid aluminum concentration along the solid/liquid interface is crystallographically symmetrical about the weld pool centerline for (001) and [100] welding configuration, while the distribution of solid aluminum concentration along the solid/liquid interface is crystallographically asymmetrical throughout the weld pool for (001) and [110] welding configuration. The size of vulnerable [100] dendrite growth region is beneficially suppressed in favor of epitaxial [001] dendrite growth region through optimum low heat input (low laser power and high welding speed) to facilitate single-crystal dendrite growth for successful crack-free weld at the expense of shallow weld pool geometry. The overall aluminum concentration in (001) and [100] welding configuration is significantly smaller than that of (001) and [110] welding configuration regardless of heat input. Severe aluminum enrichment is confined to [100] dendrite growth region where is more susceptible to solidification cracking. Heat input and welding configuration are optimized in order to minimize the solidification cracking susceptibility and improve microstructure stability. The relationship between welding conditions and alloying aluminum redistribution are established for solidification cracking susceptibility evaluation. The higher heat input is used, the more aluminum enrichment is monotonically incurred by diffusion with considerable increase of metallurgical driving forces for morphology instability and microstructure anomalies to deteriorate weldability and vice versa. The mechanism of asymmetrical solidification cracking because of crystallography-dependent alloying redistribution is proposed. The theoretical predictions agree well with the experiment results. Moreover, the useful modeling is also applicable to other single-crystal superalloys with similar metallurgical properties during laser welding or laser cladding.

Numerical Analysis of Microstructure Development during Laser Welding Nickel-Based Single-Crystal Superalloy Part II: Multicomponent Dendrite Growth

A thermal metallurgical coupling model was further developed for multicomponent dendrite growth of primary γ gamma phase during laser welding nickel-based single-crystal superalloys. It is indicated that welding configuration has a predominant role on the overall dendrite trunk spacing than heat input throughout the weld pool, and modifies the dendrite growth kinetics. The dendrite trunk spacing in (001) and [100] welding configuration is finer than that of in (001) and [110] welding configuration. In (001) and [100] welding configuration, the bimodal distribution of dendrite trunk spacing is symmetrical about weld pool centerline, the dendrite trunk spacing in [100] growth region near the weld pool center is coarser than [010], [0ī0] and [001] dendrite growth regions. In (001) and [110] welding configuration, the distribution of dendrite trunk spacing is crystallographically asymmetrical, and the dendrite trunk spacing in [100] growth region is severely coarser than that of [010] and [001] dendrite growth regions. (001) and [110] welding configuration is of particular interest, because dendrite trunk spacing decreases in [100] dendrite growth region and dendrite trunk spacing increases in [010] dendrite growth region from the maximum weld pool width to the end due to crystallography-dependent growth kinetics. Moreover, strict control of low heat input (low laser power and high welding speed) beneficially promotes fine dendrite trunk spacing and reduces the size of dendrite growth regions. High heat input (high laser power and low welding speed) monotonically coarsens dendrite trunk spacing. The dendrite trunk spacing is refined and [100] dendrite growth is suppressed by the optimum low heat input and (001) and [100] welding configuration to improve weldability. An alternative mechanism of solidification cracking because of asymmetrical dendrite trunk growth is proposed. The useful results facilitate understanding of single-crystal superalloys microstructure development and solidification cracking phenomena. The theoretical predictions agree well with the experiment results. Moreover, the model is also applicable to other single-crystal superalloys with similar metallurgical properties by feasible laser welding or laser cladding.

Numerical Analysis of Microstructure Development during Laser Welding Nickel-Based Single-Crystal Superalloy Part I: Stray Grain Formation

The mathematical modeling of microstructure development is further extended through coupling of heat transfer model and columnar/equiaxed transition (CET) model during nickel-based single-crystal superalloy weld pool solidification with different welding conditions (laser power, welding speed and welding configuration). It is indicated that crystallographic orientation plays an important role in stray grain formation ahead of the solid/liquid interface on the basis of constitutional undercooling mechanism. (001) and [100] welding configuration promotes symmetrical distribution of microstructure morphology about the weld pool centerline that is favored for reduction of stray grain formation, while detrimental (001) and [110] welding configuration induces asymmetrical distribution of microstructure morphology with more stray grain formation and deteriorates the weldability. The mechanism of increasing stray grain formation due to misorientation of dendrite growth crystallography is proposed. Appropriate low heat inputs (low laser power or high welding speed) of solidification conditions prevents stray grain formation and vice versa, and suppress the size of vulnerable [100] dendrite growth region. Weld pool geometry, θ-φ of solid/liquid interface, morphology transition and stray grain formation on either side of weld are closely correlated. In order to eliminate stray grain formation through microstructure control, it is imperative to optimize the welding configurations for defect-free weld through useful welding configuration-microstructure map. The theoretical predictions are verified by the experiment results in a consistent way. In addition, the model is also applicable to other single-crystal superalloys with similar metallurgical properties by feasible laser welding or laser cladding.

Numerical Analysis of Solidification Behavior during Laser Welding Nickel-Based Single-Crystal Superalloy Part: II Crystallography-Dependent Supersaturation of Liquid Aluminum

The thermal metallurgical modeling of liquid aluminum supersaturation was further developed through couple of heat transfer model, dendrite selection model, multicomponent dendrite growth model and nonequilibrium solidification model during three-dimensional nickel-based single-crystal superalloy weld pool solidification. The welding configuration plays more important role in supersaturation of liquid aluminum, morphology instability and nonequilibrium partition behavior. The bimodal distribution of liquid aluminum supersaturation along the solid/liquid interface is crystallographically symmetrical about the weld pool centerline in (001) and [100] welding configuration. The distribution of liquid aluminum supersaturation along the solid/liquid interface is crystallographically asymmetrical throughout the weld pool in (001) and [110] welding configuration. Optimum low heat input (low laser power and high welding speed) with (001) and [100] welding configuration is more favored to predominantly promote epitaxial [001] dendrite growth to reduce the metallurgical factors for solidification cracking than that of high heat input (high laser power and slow welding speed) with (001) and [110] welding configuration. The lower the heat input is used, the lower supersaturation of liquid aluminum is imposed, and the smaller size of vulnerable [100] dendrite growth region is incurred to ameliorate solidification cracking susceptibility and vice versa. The overall supersaturation of liquid aluminum in (001) and [100] welding configuration is beneficially smaller than that of (001) and [110] welding configuration regardless of heat input, and is not thermodynamically relieved by gamma prime γˊ phase. (001) and [110] welding configuration is detrimental to weldability and deteriorates the solidification cracking susceptibility because of unfavorable crystallographic orientations and alloying aluminum enrichment. The mechanism of asymmetrical solidification cracking because of crystallography-dependent supersaturation of liquid aluminum is proposed. The eligible solidification cracking location is particularly confined in [100] dendrite growth region. Moreover, the theoretical predictions agree well with the experiment results. The useful modeling is also applicable to other single-crystal superalloys with similar metallurgical properties for laser welding or laser cladding. The thorough numerical analyses facilitate the understanding of weld pool solidification behavior, microstructure development and solidification cracking phenomena in the primary γ phase, and thereby optimize the welding conditions (laser power, welding speed and welding configuration) for successful crack-free laser welding.

Numerical Analysis of Microstructure Development during Laser Welding Nickel-based Single-crystal Superalloy Part III: Nonequilibrium Solidification Behavior

The thermal-metallurgical modeling of primary γ gamma phase nonequilibrium solidification behavior during laser welding nickel-based single-crystal superalloy is further developed through coupling of heat transfer model, multicomponent dendrite growth model and nonequilibrium solidification model to evaluate solidification cracking susceptibility and advance the understanding of the material weldability. The useful relationship among welding conditions (laser power, welding speed and welding configuration), weld pool geometry, dendrite selection, dendrite growth, solidification temperature range, solidification cracking susceptibility and weldability are established for welding conditions optimization and microstructure control. It is indicated that for (001) and [100] welding configuration, solidification cracking susceptibility along the solid/liquid interface is symmetrically distributed about the weld pool centerline. It is crystallographically favorable for mitigation of solidification cracking with bimodal distribution of solidification temperature range. By contrast, for (001) and [110] welding configuration, solidification cracking susceptibility is asymmetrically distributed. The solidification cracking susceptibility of the unfavorable [100] dendrite growth region on the right side of the weld pool is higher than that of [010] dendrite growth region on the left side due to the enlargement of solidification temperature range. The mechanism of asymmetrical crystallography-dependent solidification cracking is proposed. Asymmetrical dendrite growth region induces asymmetrical solidification behavior and solidification cracking susceptibility along the solid/liquid interface, although heat transfer is symmetrical. The overall solidification temperature range of (001) and [100] welding configuration is beneficially narrower than that of (001) and [110] welding configuration throughout the weld pool regardless of heat input. The theoretical predictions agree well with the experiment results. In addition, the promising model is also applicable to other single-crystal superalloys with similar metallurgical properties during laser welding or laser cladding.

Numerical Analysis of Aerospace Nickel-Based Single-Crystal Superalloy Weldability Part III: Solidification Cracking Susceptibility

The thermal-metallurgical model of primary γ gamma phase through couple of heat transfer model, dendrite selection model, columnar/equiaxed transition (CET) model, multicomponent dendrite growth model and nonequilibrium solidification model is further developed on the basis of criteria of minimum growth velocity, constitutional undercooling and marginal stability of planar front during nickel-based single-crystal weld pool solidification. It is clearly indicated that crystallographic orientation plays more important role than heat input in microstructure development and solidification behavior. The dendrite trunk spacing and solidification temperature range along the solid/liquid interface are symmetrically distributed about the weld pool centerline in (001) and [100] welding configuration, while they are asymmetrically distributed in (001) and [110] welding configuration. The dendrite size and solidification temperature range are beneficially smaller in (001) and [100] welding configuration than that of (001) and [110] welding configuration regardless of heat input. The mechanism of asymmetrical solidification cracking because of crystallography-dependent growth kinetics and solidification behavior is proposed. Optimum low heat input (low laser power and high welding speed) refines dendrite size and suppresses the solidification temperature range to minimize the solidification cracking susceptibility and ameliorate the weldability through microstructure control, while high heat input (high laser power and low welding speed) deteriorates the weldability and weld integrity. It is therefore imperative to optimize the welding conditions for successful defect-free laser welding. Moreover, the promising theoretical predictions agree well with the experiment results. The useful model is also applicable to other single-crystal superalloys with similar metallurgical properties by laser welding or laser cladding, and provide a more accurate and reliable way of solidification cracking susceptibility evaluation.

Numerical Analysis of Microstructure Development during Laser Welding Nickel-based Single-crystal Superalloy Part IV: Welding Conditions Optimization

The thermal-metallurgical modeling of columnar/equiaxed transition(CET), multicomponent dendrite growth and nonequilibrium solidification behavior was further developed during single-crystal superalloy weld pool solidification over a wide range of laser welding conditions (laser power, welding speed and welding configuration). It facilitates the unprecedent understanding of the effect of welding conditions on overall stray grain formation, dendrite trunk spacing and solidification temperature range throughout the weld pool. In order to eliminate the solidification cracking through microstructure control, it is imperative to optimize the welding conditions for successful defect-free laser welding. Crystallographic orientation of dendrite growth plays an important role than heat input in microstructure development and solidification cracking susceptibility. Optimum low heat inputs (low laser power and high welding speed) of solidification condition simultaneously prevent stray grain formation, refine dendrite size and mitigate the solidification temperature range to improve the weldability and weld integrity and vice versa. Nonequilibrium solidification behavior of the weld pool suppresses the solidification temperature range and dendrite size, and improves the solidification cracking resistance. The overall stray grain formation, dendrite trunk spacing and solidification temperature range in (001) and [100] welding configuration are beneficially reduced than that of in (001) and [110] welding configuration regardless of heat input. The theoretical predictions agree well with the experiment results. Moreover, this promising model is also available to other single-crystal superalloys with similar metallurgical properties during laser welding or laser cladding.

Numerical Analysis of Aerospace Nickel-Based Single-Crystal Superalloy Weldability Part I: Crystallography-Dependent Dendrite Growth

The thermal-metallurgical modeling of microstructure development was further advanced during single-crystal superalloy weld pool solidification by coupling of heat transfer model, columnar/equiaxed transition (CET) model and multicomponent dendrite growth model on the basis criteria of minimum dendrite velocity, constitutional undercooling and marginal stability of planar front. It is clearly indicated that heat input (laser power and welding speed) and welding configuration simultaneously influence the stray grain formation, columnar/equiaxed transition and dendrite growth. For beneficial (001) and [100] welding configuration, the microstructure development along the solid/liquid interface is symmetrically distributed about the weld pool centerline throughout the weld pool. Finer columnar in [001] epitaxial dendrite growth region is kinetically favored at the bottom of the weld pool. For detrimental (001) and [110] welding configuration, the microstructure development along the solid/liquid interface is asymmetrically distributed. The dendrite trunk spacing along the solid/liquid interface from the beginning to end of solidification morphologically increases on the left side of the weld pool, while it spontaneously decreases on the right side. The vulnerable location of solidification cracking is confined in the [100] dendrite growth region on the right side of the weld pool because of increasing metallurgical contributing factors of severe stray grain formation, centerline grain boundary formation and coarse dendrite size. The mechanism of crystallography-dependent asymmetrical solidification cracking due to microstructure anomalies is proposed. It is crystallographically favorable for predominant morphology instability to deteriorate weldability. Active [100] dendrite growth region is diminished in the shallow elliptical weld pool by optimum low heat input (low laser power and high welding speed) with (001) and [100] welding configuration to essentially facilitate single-crystal solidification conditions and provide enough resistant to solidification cracking. Moreover, the theoretical predictions agree well with the experiment results. The reliable weldability maps are therefore established to determine the prerequisite for successful crack-free laser welding or cladding. The useful model is also applicable for other single-crystal superalloys with similar metallurgical properties.

Numerical Analysis of Aerospace Nickel-Based Single-Crystal Superalloy Weldability Part II: Nonequilibrium Solidification Behavior

The thermal metallurgical modeling by coupling of heat transfer model, dendrite selection model, columnar/equiaxed transition (CET) model and nonequilibrium solidification model was further developed to numerically analyze stray grain formation and solidification temperature range on the basis of three criteria of constitutional undercooling, marginal stability of planar front and minimum growth velocity during multicomponent nickel-based single-crystal superalloy weld pool solidification. It is indicated that the primary γ gamma phase microstructure development and solidification cracking susceptibility along the solid/liquid interface are symmetrically distributed throughout the weld pool in (001) and [100] welding configuration. The microstructure development and solidification cracking susceptibility along the solid/liquid interface are asymmetrically distributed in (001) and [110] welding configuration. Appropriate low heat input (low laser power and high welding speed) simultaneously minimizes stray grain formation, grain boundary misorientation and solidification temperature range in the vulnerable [100] dendrite growth region and beneficially maintains single-crystal nature of the material in the [001] epitaxial dendrite growth region to improve the cracking resistance, while high heat input (high laser power and low welding speed) increases the solidification cracking susceptibility to deteriorate weldability and weld integrity. The solidification temperature range in (001) and [110] welding configuration is detrimentally wider than that of (001) and [100] welding configuration due to crystallographic orientation of dendrite growth regardless of heat input. The mechanism of asymmetrical crystallography-dependant solidification cracking because of nonequilibrium solidification behavior is proposed. The elliptical and shallow weld pool shape is less susceptible to solidification cracking for successful crack-free laser welding. Moreover, the promising theoretical predictions agree well with the experiment results. The useful modeling is also applicable to other single-crystal superalloys with similar metallurgical properties during laser welding or laser cladding.

Numerical Analysis of Microstructure Development during Laser Welding Nickel-based Single-crystal Superalloy Part V: Crystallography-dependent Aluminum Redistribution

The thermal-metallurgical modeling of alloying aluminum redistribution was further developed through couple of heat transfer model, dendrite selection model, multicomponent dendrite growth model and nonequilibrium solidification model during three-dimensional nickel-based single-crystal superalloy weld pool solidification over a wide range of welding conditions (laser power, welding speed and welding configuration). It is clearly indicated that welding configuration plays more important role than heat input in aluminum redistribution. The bimodal distribution of aluminum concentration along the solid/liquid interface is crystallographically symmetrical about the weld pool centerline for (001) and [100] welding configuration, while the distribution is crystallographically asymmetrical about the weld pool centerline for (001) and [110] welding configuration. Optimum low heat input (low laser power and high welding speed) with (001) and [100] welding configuration beneficially suppresses the aluminum concentration, reduces the vulnerable [100] dendrite growth region to minimize the solidification cracking susceptibility and improve weldability and vice versa. The overall aluminum concentration on the left side of the weld pool is beneficially smaller than that of right side in (001) and [110] welding configuration throughout the weld pool due to crystallographic orientation of dendrite growth regardless of the heat input. The mechanism of asymmetrical solidification cracking because of crystallography-dependent alloying solute inhomogeneity was proposed. The welding conditions, weld pool geometry, solidification conditions, dendrite selection, alloying redistribution and solidification cracking susceptibility are closely correlated. The theoretical predictions agree well with the experiment results. Moreover, the promising results facilitate the understanding of solidification cracking phenomena, and the useful model is also applicable to other single-crystal superalloys with similar metallurgical properties for successful defect-free laser welding or laser cladding.

Experimental investigation of plasma arc welded Ti–6Al–4V sheets

In the present work, experimental trials were conducted with Ti–6Al–4V sheet. The influence of process parameters on the weld bead geometry of bead on joint welding and butt joint configuration was studied. It was concluded that at high current and low travel speed, the heat input was found to be maximum. This led to a higher linear heat input over the base metal that subsequently yielded a full depth of penetration. The strength and integrity of the welded butt joint configuration were ascertained by tensile and bend tests. The microhardness values of the fusion and heat-affected zones were concluded to be higher compared to the base metal. In addition, an Erichsen cupping test ensured that the formability of the welded specimen was comparable to that of the base metal.

Mechanical Properties and Microstructure of Pulsatile Current Gas Tungsten Arc Welding (PC-GTAW) of AISI 304L Butt Joint Weld

In this research, investigation of mechanical properties and microstructure on Pulsatile Current Gas Tungsten Arc Welding (PC-GTAW) was carried out. AISI 304L was chosen as a base material with autogenous welding and butt joint configuration. The dimension of the base material was 120 x 50 mm and the thickness of 3 mm. The welding with pulsatile current and constant current were used in the welding process in this study. In the pulsatile current, the mean current was made all the same with a constant current of 100 amperes. The peak current, base current and time of peak and base current were varied to make it the same. The welding speed used is constant at 2.0 mm/s. The result shows that the HAZ area will get shrinkage if welded using pulsatile current welding. The welding with a pulsatile current can produce more fine cellular dendritic structure than the welding with a constant current. The UTS in the welding with a pulsatile current was lower than the welding with constant current, while the impact toughness in the welding with a pulsatile current was higher than the welding with a constant current. The highest of the UTS and the impact toughness of 577.62 MPa on parameter 65-A and 27 J on parameter 65-C, respectively.