Showing papers on "Shielding gas published in 2018"

Laser powder bed fusion in high-pressure atmospheres

Abstract: High-speed imaging and schlieren imaging were used to investigate the interaction of the laser beam with the powder bed at pressures up to 5 bar, in argon and helium atmospheres. The entrainment of powder particles in the flow of shielding gas generated by the laser plume, and hence denudation, was reduced at high pressure for both gases. However, for argon, high pressure increased the temperature of both the melt pool and the laser plume, which significantly increased the generation of spatter and ionisation of the metal vapour with degraded surface smoothness and continuity. For helium, the formation of spatter and plasma did not increase with the increase in pressure above that observed at atmospheric pressure: its higher thermal conductivity and thermal diffusivity limited the laser plume temperature. Layers built at 5 bar in helium had a surface smoothness and continuity comparable to those built in argon at atmospheric pressure, but achieved at a higher laser scan speed, suggesting that a high-pressure helium atmosphere may be used to enhance the build rate.

Using Genetic Algorithms with Multi-Objective Optimization to Adjust Finite Element Models of Welded Joints

Abstract: To ensure realistic results when modeling welded joints using the finite element method (FEM), it is essential to appropriately characterize the thermo-mechanical behavior of the elastic-plastic Finite Element (FE) models. This task is complex. Any small differences between the actual welded joints and the welded joints based on FEM can be amplified enormously in the presence of nonlinearities. Due to the intense concentration of heat on a small area to create such joints, the regions near the weld line undergo severe thermal cycles. These generate significant angular distortion due mainly to the residual stresses. This paper proposes a method to determine the parameters that are most appropriate for modeling the Butt joint single V-groove welded joint FE models’ thermo-mechanical behavior that were created by the one-pass Gas Metal Arc Welding (GMAW). The method is based on experimental data, as well as genetic algorithms (GA) with multi-objective functions. As a practical example, the proposed methodology is validated with three different welded joints specimens that are manufactured by different voltages and currents (26 volts and 140 amps, 28 volts and 210 amps, and 35 volts and 260 amps). The electrode orientation, shielding gas flow rate, distance between nozzle and plate, and welding speed were considered to be constant for all of the specimens that were studied, and their values were 80°, 20.0 L/min, 4.0 mm, and 6 mm/s, respectively. The base material was EN 235JR low carbon steel, whereas the weld bead was ER70S-6 for the three specimens that were welded. An agreement between the temperature field and the angular distortion that was obtained by the adjusted FE models and those that were obtained experimentally demonstrates that the proposed methodology may be valid for automatically determining the most appropriate parameters.

A numerical and experimental investigation of convective heat transfer during laser-powder bed fusion

Abstract: Parts fabricated using additive manufacturing (AM) methods, such as laser-powder bed fusion (L-PBF), receive highly localized heat fluxes from a laser within a purged, inert environment during manufacture. These heat fluxes are used for melting metal powder feedstock, while remaining energy is transferred to the solidified part and adjoining gas environment. Using computational fluid dynamics (CFD), the local heat transfer between the adjoining shielding gas, laser-induced melt pool and surrounding heat affected zone is estimated. Simulations are performed for the L-PBF of a single layer of Ti-6Al-4 V. Local temperature, temperature gradients, temperature time-rates-of-change (including cooling rates), as well as dimensionless numbers descriptive of important thermophysics, are provided in order to quantify local convective heat transfer for various laser/gas motion directions. Results demonstrate that L-PBF track heat transfer is highly dependent on relative gas/laser direction which can impact the prior β grain sizes in Ti-6Al-4 V material by up to 10%. It is found that when the laser and gas are moving in the same direction, convection heat transfer is the highest and a ‘leading thermal boundary layer’ exists in front of the laser which is capable of preheating downstream powder for a possible reduction in residual stress formation along the track. Presented results can aid ongoing L-PBF modeling efforts and assist manufacturing design decisions (e.g. scan strategy, laser power, scanning speed, etc.) – especially for cases where homogeneous or controlled material traits are desired.

Comparison of bead shape, microstructure and mechanical properties of fiber laser beam welding of 2 mm thick plates of Ti-6Al-4V alloy

Abstract: Fiber laser beam welding experiments are carried out on 2 mm thick plates of Ti-6Al-4V alloy at various welding conditions. A specially designed workpiece fixture with shielding gas delivery system is fabricated and effectiveness of shielding gas is confirmed by energy dispersive X-ray spectroscopy analysis. Two different types of bead cross-sections i.e. T-shaped and nearly X-shaped beads are observed at different line energies. After experiments, width and area of fusion zone and size of heat affected zone are measured and further analyzed. The microstructural studies at different locations in heat affected zone and also in fusion zone are carried out with optical microscope and their morphologies are compared at different welding conditions using field emission scanning electron microscope. The porosities are observed in both T and X-shaped weld beads at different line energies. However, their sizes are in acceptable range as per BS EN:4678 standard. The maximum microhardness is found in fusion zone due to the presence of large amount of α′ martensite. The line energy utilization factor is higher for T-shaped bead than X-shaped bead. The tensile strength of T-shaped bead is lower than X-shaped bead due to higher non-uniformity in the distribution of plastic strain. The experimental results show that T-shaped welding conditions are favorable while considering full penetration in keyhole mode, narrower weld width and lesser heat affected zone.

Optical diagnostics and optimization of the gas-powder flow in the nozzles for laser cladding

Abstract: The results of investigation of gas and gas-powder flows in various cladding nozzles are presented and discussed Schlieren-imaging, high-speed imaging and laser Doppler anemometry were used for optical diagnostics of the flows It is demonstrated that diameter of the outlet in removable cyclone cap has significant impact on the powder flow pattern in an off-axis nozzle It provides a convenient way to optimize focusing of the powder flow It is found that stable vortices that appear near the surface of the substrate significantly reduce efficiency of powder material use when off-axis nozzle is applied It is demonstrated that excessive shielding gas flow rate in four-stream and coaxial nozzles may lead to defocusing of gas-powder flows A possibility to increase the efficiency of powder use in various laser cladding nozzles through modification of their design as well as shielding and carrier gases flow rates is discussed

Experimental and Numerical Assessment of Temperature Field and Analysis of Microstructure and Mechanical Properties of Low Power Laser Annealed Welded Joints.

Abstract: In this present work, laser welding experiments were carried out on 1 mm thin Ti6Al4V sheets using a low power Nd-YAG laser machine without using any filler wire and without edge preparation of welding specimens. The influence of different major process control parameters such as welding speed and power on the yield parameters like temperature field, weld bead geometry, microstructure, and mechanical properties are critically investigated. Experimental results are compared in detail with the simulated results obtained using a commercial 3D finite element model. In the simulation model, temperature-dependent thermal and mechanical properties of plates were considered. The temperature readings were recorded with the aid of K type thermocouples. Forced convection has been assumed near weld zone region because of the movement of the shielding gas. Appreciable agreement is found between the experimental and the simulated temperature fields in most of the cases with few exceptions. These deviations on few occasions may be due to the presence of uncertainties inherently present in the experimental domain and uncertainties in the subsequent temperature sensing techniques by the thermocouples. In addition, annealing has been done at 950 °C, 980 °C, and 1010 °C for one selected parameter (192 W, 6 mm/s). The tensile strength of the samples annealed at 980 °C has been found to be 1048 MPa and it is 3% to 4% higher than that of the usual welded samples.

Parameter optimization of laser welding of steel to Al with pre-placed metal powders using the Taguchi-response surface method

Abstract: Welding of steel to aluminum alloy is a difficult work due to the formation of brittle intermetallic compounds. In order to improve the quality of the dissimilar materials weld joints, metal powders are pre-placed on the surface of aluminum alloy prior to welding. In this study, laser welding technique is employed to join DC04 steel to 6016 aluminum alloy with pre-placed metal powders in a lap joint configuration. The morphology, micro-structure and elemental composition of the joints are characterized using SEM equipped with an EDS and optical micro-scope while the micro-hardness and tensile strength are measured. It is indicated that although metal powders do not form compounds directly with Fe or Al, they prevent the contact of Fe and Al. Tensile experiments show that the pre-placed metal powders can significantly improve the joints’ tensile properties. The combined Taguchi-response surface method approach is employed to optimize the laser welding parameters and the optimal condition to have a quality weld is found at laser power 1344.73 W, welding speed 30 mm/s, Defocus distance −2 mm, Shield gas flow 30 L/min. Confirmation experiment proves that the Taguchi-response surface method is an effective way to optimize welding parameters to gain higher mechanical resistance reach to 165.24 N/mm and the bead appearance of optimization joint with fish scales and without cracks.

Study of the Influence of Shielding Gases on Laser Metal Deposition of Inconel 718 Superalloy

Abstract: The use of the Laser Metal Deposition (LMD) technology as a manufacturing and repairing technique in industrial sectors like the die and mold and aerospace is increasing within the last decades. Research carried out in the field of LMD process situates argon as the most usual inert gas, followed by nitrogen. Some leading companies have started to use helium and argon as carrier and shielding gas, respectively. There is therefore a pressing need to know how the use of different gases may affect the LMD process due there being a lack of knowledge with regard to gas mixtures. The aim of the present work is to evaluate the influence of a mixture of argon and helium on the LMD process by analyzing single tracks of deposited material. For this purpose, special attention is paid to the melt pool temperature, as well as to the characterization of the deposited clads. The increment of helium concentration in the gases of the LMD processes based on argon will have three effects. The first one is a slight reduction of the height of the clads. Second, an increase of the temperature of the melt pool. Last, smaller wet angles are obtained for higher helium concentrations.

Hot cracking behaviour of an autogenously laser welded Al-Cu-Li alloy

Abstract: AA2198 is a relatively new light-weight and high-performance Al-Cu-Li alloy considered for aviation and space applications. However, Al-Cu-Li alloys generally exhibit severe weldability problems for all fusion-welding techniques, such as laser-beam welding. In particular, porosity formation and hot cracking are observed for the laser-beam welding of these alloys. A common remedy for hot cracking is the use of an appropriate filler wire with a high Si content. In the present study, three different approaches for improving the hot cracking susceptibility of AA2198 laser beam welded without any filler material are presented. For this purpose, pre-heating of the weld samples to elevated temperatures, pre-loading of the weld samples perpendicular to the welding direction, or an optimization of the laser-beam welding parameters were conducted. The autogenously welded samples were assessed with regard to the resulting total crack length and their mechanical properties. It was demonstrated that all of the presented approaches led to a reduction of hot cracking. However, the largest effect was observed for the use of low levels of laser power and welding velocity. The mechanical properties of the optimised autogenously welded samples are only marginally inferior as for the samples laser welded with the Al-Si filler wire AA4047.

Review of GTAW Welding Parameters

Abstract: Welding as a fabrication process is one of the vital production routes for most manufacturing industries. Several factors are involved in the choice of welding process for specific applications; notable among these are compositional range of the material to be welded, the thickness of the base materials and type of current. Most metals oxidize rapidly in their molten state, and therefore, the weld area needs to be protected from atmospheric contamination; this is achieved in gas tungsten arc welding GTAW by a shielding gas (argon, helium, nitrogen). GTAW technique is one of the major processes for joining austenitic stainless steels (ASS) and ferritic stainless steel (FSS) fabrication. However, the microstructural change that occurs during welding and at weld joint is still a major challenge today as it affects both the corrosion resistance and the mechanical properties. Therefore, this present paper reviews past research findings on GTA welding of ASS and FSS. Results of the findings have confirmed that, depending on the amount of heat input, which can be controlled by welding parameters (welding speed, voltage and current), welded joints particularly, heat affected zones (HAZs) of both grades of steels can undergo mechanical failure and can be susceptible to corrosion attack if the joints are produced with a less ideal combination of welding parameters.

Effect of Shielding Gas on the Properties of AW 5083 Aluminum Alloy Laser Weld Joints

Abstract: The paper deals with the evaluation of the shielding gas influence on the properties of AW 5083 aluminum alloy weld joints produced with disk laser. Butt weld joints were produced under different shielding gas types, namely Ar, He, Ar + 5 vol.% He, Ar + 30 vol.% He and without shielding weld pool. Light and electron microscopy, computed tomography, microhardness measurements and tensile testing were used for evaluation of weld joint properties. He-shielded weld joints were the narrowest ones. On the other hand, Ar-shielded weld joints exhibited largest weld width. The choice of shielding gas had significant influence on the porosity level of welds. The lowest porosity was observed in weld joint produced in Ar with the addition of 5 vol.% He shielding atmosphere (only 0.03%), while the highest level of porosity was detected in weld joint produced in pure He (0.24%). Except unshielded aluminum alloy weld joint, the lowest tensile strength was recorded in He-shielded weld joints. On the contrary, the highest average microhardness was measured in He-shielded weld joints.

Visualization of the shielding gas flow in SLM machines by space-resolved thermal anemometry

Abstract: Purpose The purpose of this paper is to introduce an approach in measuring the shielding gas flow within laser powder bed fusion (L-PBF) machines under near-process conditions (regarding oxygen content and shielding gas flow). Design/methodology/approach The measurements are made sequentially using a hot-wire anemometer. After a short introduction into the measurement technique, the system which places the measurement probe within the machine is described. Finally, the measured shielding gas flow of a commercial L-PBF machine is presented. Findings An approach to measure the shielding gas flow within SLM machines has been developed and successfully tested. The use of a thermal anemometer along with an automated probe-placement system enables the space-resolved measurement of the flow speed and its turbulence. Research limitations/implications The used single-normal (SN) hot-wire anemometer does not provide the flow vectors’ orientation. Using a probe with two or three hot-films and an improved placement system will provide more information about the flow and less disturbance to it. Originality/value A measurement system which allows the measurement of the shielding gas flow within commercial L-PBF machines is presented. This enables the correlation of the shielding gas flow with the resulting parts’ quality.

Microstructural characterization of Ni-201 weld cladding onto 304 stainless steel

Abstract: Ni-201(ERNi-1), a cladding layer which is resistant to the corrosion of molten salts, was deposited on 304SS substrate by Gas Tungsten Arc Welding (GTAW). Microstructure characterization showed that the cladding interface was obviously divided into three zones from microstructures along depth, weld metal (WM), unmixed zone (UZ) and heat affected zone (HAZ). Element distribution presented that elements Ti, N, Si were segregated into interdendritic region of WM and formed the TiN precipitates during the solidification. The element distribution of UZ was similar to that of HAZ. A large quantity of vermicular δ-ferrite phases were precipitated in laminar UZ. Grain coarsening in HAZ was evident compared with the grains in base metal, and no precipitates were found in HAZ. The hardness of cladding layers was decreased from interface to surface. The cladding layer exhibited excellent corrosion resistance to molten FLiNaK salts.

Influence of welding parameters on diffusible hydrogen content in high-strength steel welds using modified spray arc process

Abstract: In order to satisfy the growing requirements towards lightweight design and resource efficiency in modern steel constructions, e.g., mobile cranes and bridges, high-strength steels with typical yield strength ≥ 690 MPa are coming into use to an increasing extent. However, these steels require special treatment in welding. The susceptibility for degradation of the mechanical properties in the presence of hydrogen increases significantly with increasing yield strength. In case of missing knowledge about how and the amount of hydrogen that is uptaken during welding, hydrogen-assisted cracking (HAC) can be a negative consequence. Moreover, modern weld technology like the modified spray arc process enables welding of narrower weld seams. In this context, a reduced number of weld beads, volume, and total heat input are technical and economical benefits. This work presents the influence of welding parameters on the diffusible hydrogen content in both (1) single-pass and (2) multi-layer welds. Different hydrogen concentrations were detected by varied contact tube distance, wire feed speed, arc length, and varied arc type (transitional arc and modified spray arc). The results show that all welding parameters have significant influence on the diffusible hydrogen concentration in the single-pass welds. By increasing the number of weld beads in case of multi-layer welding, the hydrogen concentration has been reduced. Whereby, differences in hydrogen concentrations between both arc types are present.

Numerical simulation of fume formation process in GMA welding

Abstract: In order to clarify fume formation process in GMA welding, quantitative investigation based on understanding of interaction between the electrode, the arc, and the weld pool is indispensable. This study aims to clarify the fume formation process theoretically through numerical analysis. A fume formation model consisting of a homogeneous nucleation model, a heterogeneous condensation model, and a coagulation model was coupled with a GMA welding model. A series of processes such as evaporation of metal vapor from molten metal, transport of the metal vapor in the arc, fume formation from the metal vapor, and transport of the fume to the surrounding was investigated by employing this coupled simulation model. In this paper, the influence on the fume formation process of changing the shielding gas from Ar to CO2 and the associated change in arc characteristics, leading to different droplet formation and metal transfer phenomena, is discussed. As a result, it was clarified that the fume particles produced around the droplet were transported into the arc to be evaporated again and a large number of fine fume particles were produced in the downstream region of the plasma flow near the base metal in Ar GMA welding. In contrast, approximately 30% of all fume particles produced around the droplet were transported directly to the surrounding and the other particles were transported into the arc to be evaporated to produce fume particles in the downstream region in CO2 arc welding.