Papers by Keyword: Laser Welding

Abstract: This paper focuses on the laser weldability of additively manufactured (AM) Inconel 718. The experiments of this research were conducted on different series of AM Inconel 718 alloy, i.e. as­-built, heat­ treated (HT), and HT after welding, and comprehensively characterized using optical microscope and electron back scattering diffraction (EBSD). The weld morphology and microstruc­tural evolution of the fusion zone were recorded. The mechanical properties of the welded AM Inconel 718 were evaluated by tensile tests and hardness measurements. It was found that solidification crack and micropore defects are induced in the as­built AM Inconel 718. However, defect­free weld was promoted in the HT alloy. The changes in hardness profiles and tensile strength under the experimen­tal parameters were further reported. Homogenous hardness of 500 HV across the weld was obtained when HT was applied after the LW.

Abstract: The aim of the study was to determine the static strength of laser welded lap joint in laser powder bed fusion (LPBF) printed stainless steel material and also a joint formed of printed and commercial sheet metal. Printed 316L test pieces with a thickness of 2 mm and a similar commercial 2 mm thin plate were used as test material. A laser welded lap joint made of a commercial sheet metal was used as a reference. Yb:YAG disk laser with wavelength 1030 nm and maximum output power 4 kW was used in welding tests. All test sets were welded with the same welding parameters and argon shielding gas. One fully penetrated keyhole weld was made to the lap joint. The static strength of the lap joints was determined by tensile tests. The measured shear strength was highest in the reference joint. In other cases, shear strength was only 8-11% lower compared to the reference joint. The cross-sections of the welds were analyzed on the basis of images taken with an optical microscope. Based on the results, the printed 316L is highly laser weldable material.

Abstract: Dissimilar laser welding of ferritic, type EN 1.4509, and austenitic, type EN 1.4307, stainless steel sheets was conducted at different energy inputs 30 and 80 J/mm and under different shield gases Ar and N and without shielding gas to evaluate the microstructure and hardness of the welded zone. The formability tests, using Erichsen principle, were carried out to determine the deformation behaviour of the dissimilar welded joints under biaxial straining. The fusion zone microstructure analysis revealed that the predominant phase structure is columnar coarse ferritic grains with slightly small content of austenite in the ferrite grain boundaries. The formability of the welded joints under Ar and N shielding gases is significantly improved, i.e., higher plasticity, compared with welded joints without shielding gas at both energy inputs.

Abstract: The article describes a method for determination of the welded parts temperature pattern under laser welding conditions. An algorithm is engineered to solve the non-stationary heat conduction problem by finite element method. The boundary conditions are determined by the molten pool parameters and depend on the welding regime characteristics. The dependencies for determining the molten pool geometric dimensions for laser welding conditions are proposed. Calculations of the temperature pattern change during the steel plates joint by laser welding are carried out. It is shown that the proposed model adequately describes the heat transfer process in the welding region.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.