Mechanical Properties Of Laser Welded Joints Research Articles

Evolution of NbC during laser welding and its impacts on the performance of molybdenum alloy joint

Influences of addition of niobium carbide (NbC) in the fusion zone (FZ) on room-temperature and high-temperature mechanical properties of laser-welded joints of molybdenum (Mo) alloys were studied. Results show that after adding NbC powder, the average Vickers microhardness in the upper part of the FZ increases from 183.0 HV to 712.3 HV; the room-temperature tensile strength grows from 33.9 MPa to 323.1 MPa, reaching 45.4 % that of base metal (BM). In addition, the fracture mode of joints turns from intergranular fractures into transgranular fracture; at 1100 °C, the high-temperature tensile strength of joints added with NbC is 141.6 MPa, which is 65.9 % that of BM. Energy dispersive spectrometer (EDS) and electron backscattered diffraction (EBSD) results show that after adding NbC powder, the average grain size in the FZ diminishes from 40.3 μm to 32.9 μm, where the number of low-angle grain boundaries (LAGBs) increases; the FZ not only contains NbC phase but also a large quantity of Nb2O5 and Mo2C phases dispersed on grain boundaries (GBs), and the number of MoO2 phase on GBs decreases apparently. Therefore, physical mechanisms underlying significant improvement of room-temperature and high-temperature tensile strengths of laser-welded joints of Mo alloys added with NbC in the FZ mainly include fine-grain strengthening, GB purification, and GB strengthening.

Study on laser welding technology of ultra-deep drawing steel DC06 for automobile

Abstract The forming, microstructure, and mechanical properties of laser-welded joints of DC06 steel sheets with 0.7 mm thickness were studied. The results show that the microstructure of the weld is ferrite and a little pearlite under different welding conditions. The weld surface formation is better with the increase in welding speed and laser output frequency. With the increase in welding speed, the ferrite grain size of the weld decreases, and the elongation and work hardening index increase.

Microstructure evolution and mechanical properties of laser-welded joints of 1.2 GPa-class quenching and partitioning steel

Laser welding of 1.2 GPa-class cold-rolled quenching and partitioning steel with martensite + retained austenite microstructure was performed at different heat inputs ranging from 91–240 J·mm−1 using a CO2 laser. Scanning electron microscopy, electron backscatter diffraction, and uniaxial tensile testing equipped with digital image correlation were employed to characterize the microstructural and mechanical evolution of the joints. The results revealed that the fusion zone and upper-critical HAZ comprised full martensite with high microhardness. The HAZ contained a narrow softened zone with a minimum width of ∼0.6 µm and at least 48 HV microhardness drop because of the martensite tempering. Martensite decomposition and precipitation of carbides were noticeable at higher heat inputs, so that the hardness of the softened zone was declined. At the welding heat input of 91 J·mm−1, the yield and tensile strengths were 841 and 1270 MPa, respectively, which represented reductions of 5.5 and 0.78% compared to the base material properties. The joint efficiency of 99.2% was achieved at the lowest heat input. The tensile deformation was primarily concentrated in the softened zone and was small in the fusion zone. However, the participation of the base material to tensile deformation was enhanced at lower heat inputs, leading to higher elongation values up to ∼11.9% during tensile testing.

Microstructure and Mechanical Properties of Laser-Welded Joints between DP590 Dual-Phase Steel and 304 Stainless Steel with Preset Nickel Coating

Dissimilarities in metal laser welding lead to brittleness in welded joints due to differences in the thermophysical and chemical properties between dissimilar base materials. To overcome such brittleness, the addition of a preset coating onto the base materials as an interlayer is a method for attaining reliable welded joints. Nd:YAG laser butt welding of DP590 dual-phase steel and 304 stainless, both with a thickness of 1 mm, was performed with a preset nickel coating as an interlayer using an electroplating process. The relationship between the microstructure and the mechanical properties of the welded joints was researched, the microstructure and composition of the weldment were analyzed, and the microhardness, tensile strength and corrosion resistance were tested. The results showed that the preset nickel coating increased the content of Ni element in the welded joints, which is beneficial to the formation of lath martensite. The average hardness of the welded joints increased by 12%, and the tensile strength was higher than 370 MPa. The corrosion rate of the welded joints can be slowed down, and the corrosion resistance can be improved by increasing the nickel coating.

Investigation of laser beam welding on lap joints of Al/steel dissimilar materials subjected to alternating magnetic field

In this study, magnetically assisted laser beam welding was experimentally carried out on Al/steel dissimilar materials in a lap configuration. Weld morphology, interfacial microstructure, element distribution, precipitated phases and mechanical properties of laser-welded joints were carefully investigated. The strengthening mechanism of Al/steel joints was also explored. The results revealed that good weldments were successfully acquired with the assistance of an alternating magnetic field. The microstructures at interface were mainly constituted by nubbly structures, island-shaped structures and banded structures. Fe-Al compounds at interface were expectedly dispersed due to the magnetic field-driven effect on liquid metal. During laser beam welding, the susceptibility to weld cracking was obviously decreased on the premise of inhibition of the composition segregation. The Fe/Al interface zone was mainly composed of η-Fe2Al5 phase, β2-FeAl phase, ζ-FeAl2 phase and θ-FeAl3 phase. The maximum tensile load and elongation of the tested joints subjected to a magnetic field reached 122.22 % and 145.76 % of joints without magnetic field, respectively. Performance enhancement of welded joints was mainly attributive to the effective regulation of weld microstructure and composition during magnetically assisted laser beam welding.

Effect of post-weld heat treatment on the microstructure and mechanical properties of laser-welded joints of SLM-316 L/rolled-316 L

Abstract Connecting small pieces of parts manufactured by Selective Laser Melting (SLM) and traditional formed parts into large structural component by using welding technology provides a high-efficiency and low-cost way for expanding additive manufacturing technology. SLMed parts usually exhibit similar or superior tensile strength but lower ductility compared to that of cast or rolled ones due to the rapid cooling and cyclic heating deposition characteristics. What about the weldability of the dissimilar joints of SLMed parts and rolled ones? Whether the properties of the welded joints can be improved by heat treatment under the premise of ensuring the reliability of the joint is unclear. The weldability and the effect of heat treatment temperature on the microstructure and properties of laser-welded joints of SLM-316 L/rolled-316 L are studied in this article. The results show that the joints exhibit good weldability without obvious defects. The low temperature annealing treatment had no significant effect on the multilayered structure and columnar austenite grains, but only changed the morphology and content of ferrite within the grains leading to the slight increase of elongation but decrease of material strength. The corrosion performance was almost unaffected. After solution annealing above 1,000°C, the ferrite was nearly dissolved in the austenite matrix. The austenite recrystallized and the multilayered structure was destroyed, which resulted in decrease of material strength, a slightly improvement in corrosion resistance, and the elongation was greatly enhanced. After the detailed study, it was concluded that the post-weld heat treatment provided an effective way for improving the overall performance of the SLM-316 L/rolled-316 L dissimilar joint.

Microstructure and Mechanical Properties of Laser-Welded Joint of Tantalum and Stainless Steel

Bimetallic components welded by Ta and stainless steel have great promise for engineering applications, but there is relatively little relevant research. In this study, 0.6 mm-thick Ta and 304L stainless steel plates were laser welded, and the forming characteristics, microstructure, and mechanical properties of the welded joints were analyzed. First-principles calculations were also performed to explore the structural stabilization mechanisms of the two intermetallic compounds, TaFe2 and TaCr2, in terms of mechanical properties as well as electronic structure. The results showed that the weld surface was smooth and free from any defects. Fe-based solid solutions formed grains, while TaFe2, TaCr2, and some Fe-based solid solutions formed intergranular and eutectic structures. In addition, due to the presence of the brittle phases of TaFe2 and TaFe, the microhardness of the weld area can reach 650HV, with an average hardness of 530HV, which is much higher than that of the base material. The tensile shear of the joint at room temperature was 154.77 N/mm, and the fracture occurred in the weld zone on the steel side, showing brittle fracture. TaFe2 is a brittle intermetallic compound, while TaCr2 is ductile. Both TaFe2 and TaCr2 systems have dual properties of metallic and covalent bonding within them, and metallic bonding dominates.

Effect of Dynamic Preheating on the Thermal Behavior and Mechanical Properties of Laser-Welded Joints.

The high cooling rate and temperature gradient caused by the rapid heating and cooling characteristics of laser welding (LW) leads to excessive thermal stress and even cracks in welded joints. In order to solve these problems, a dynamic preheating method that uses hybrid laser arc welding to add an auxiliary heat source (arc) to LW was proposed. The finite element model was deployed to investigate the effect of dynamic preheating on the thermal behavior of LW. The accuracy of the heat transfer model was verified experimentally. Hardness and tensile testing of the welded joint were conducted. The results show that using the appropriate current leads to a significantly reduced cooling rate and temperature gradient, which are conducive to improving the hardness and mechanical properties of welded joints. The yield strength of welded joints with a 20 A current for dynamic preheating is increased from 477.0 to 564.3 MPa compared with that of LW. Therefore, the use of dynamic preheating to reduce the temperature gradient is helpful in reducing thermal stress and improving the tensile properties of the joint. These results can provide new ideas for welding processes.

Effect of pre-heating and post-weld heat treatment on structure and mechanical properties of laser beam-welded Ti2AlNb-based joints

The effect of pre-heating and post-weld heat treatment on microstructure and mechanical properties of laser-welded joints in a Ti–23Al–23Nb-1.4V-0.8Zr-0.4Mo-0.4Si (at.%) alloy was studied. Laser beam-welding was carried out at room temperature as well as after pre-heating up to 800°С. The post-weld heat treatment comprised either air quenching from 920°С followed by aging at 800°С or only aging at 800°С. The microstructure of the fusion zone consisted of columnar β-grains after welding at room temperature and 400 °C or both the columnar and large equiaxed crystals at 600 and 800 °C. An increase in the pre-heating temperature caused the columnar β-crystals growth as well as an increase in the fusion zone and heat-affected zone widths. Meanwhile, a decrease in the Al and Ti content, as well as an increase in both the porosity and gaseous elements content (O and N) after welding at 600–800 °C were found. The microhardness of each joint obtained after welding with pre-heating temperatures up to 600 °C was lower than that of the base material. All the welded joints showed the yield strength and ultimate tensile strength levels between 1070 and 1110 MPa, which correspond to approximately 80% of the base metal level. Reasonable total elongation of the joint was achieved after welding at 400 °C (4.3%). The post-weld heat treatment involving air quenching from 920 °C with subsequent aging at 800 °C for 6 h demonstrated the best results. The heat treatment resulted in the precipitation of the O- and α2-phases and an increase in total elongation to 6.5%.

Microstructure and Mechanical Properties of Laser-Welded DP Steels Used in the Automotive Industry

The aim of this work was to investigate the microstructure and the mechanical properties of laser-welded joints combined of Dual Phase DP800 and DP1000 high strength thin steel sheets. Microstructural and hardness measurements as well as tensile and fatigue tests have been carried out. The welded joints (WJ) comprised of similar/dissimilar steels with similar/dissimilar thickness were consisted of different zones and exhibited similar microstructural characteristics. The trend of microhardness for all WJs was consistent, characterized by the highest value at hardening zone (HZ) and lowest at softening zone (SZ). The degree of softening was 20 and 8% for the DP1000 and DP800 WJ, respectively, and the size of SZ was wider in the WJ combinations of DP1000 than DP800. The tensile test fractures were located at the base material (BM) for all DP800 weldments, while the fractures occurred at the fusion zone (FZ) for the weldments with DP1000 and those with dissimilar sheet thicknesses. The DP800-DP1000 weldment presented similar yield strength (YS, 747 MPa) and ultimate tensile strength (UTS, 858 MPa) values but lower elongation (EI, 5.1%) in comparison with the DP800-DP800 weldment (YS 701 MPa, UTS 868 MPa, EI 7.9%), which showed similar strength properties as the BM of DP800. However, the EI of DP1000-DP1000 weldment was 1.9%, much lower in comparison with the BM of DP1000. The DP800-DP1000 weldment with dissimilar thicknesses showed the highest YS (955 MPa) and UTS (1075 MPa) values compared with the other weldments, but with the lowest EI (1.2%). The fatigue fractures occurred at the WJ for all types of weldments. The DP800-DP800 weldment had the highest fatigue limit (348 MPa) and DP800-DP1000 with dissimilar thicknesses had the lowest fatigue limit (<200 MPa). The fatigue crack initiated from the weld surface.

Effects of Postweld Heat Treatment on Microstructure and Properties of Laser-Welded Ti-24Al-15Nb Alloy Joint

The microstructure evolution and mechanical properties of laser-welded joints of Ti-24Al-15Nb alloy under different postweld heat treatment (PWHT) conditions were systematically investigated. The results show that the microstructure is very sensitive to PWHT temperatures. The weld zone consists of B2 and O phases after PWHT. With the PWHT temperatures increasing from 850 to 1000 °C, the amount of O phase decreases gradually, while the grains of O precipitates become coarser. After PWHT, there are some thin acicular O precipitates in heat-affected zone (HAZ), and the decomposition of α2-phase caused by niobium diffusion can be observed in the HAZ. The PWHT can significantly increase the microhardness of joints, resulting from O phase precipitation hardening effect. The tensile strength and elongation of joints can be remarkably improved after PWHT, which was closely related to the strengthening effect of O precipitates and slip transmission between O and B2 phases. In addition, the results indicate that the best mechanical properties can be achieved only when the number and size of O phase and B2 phase are the best match.

Microstructure and Mechanical Properties of Adding Adhesive-Layer Laser-Welded Joints of DP590 Dual-Phase Steel and 6061 Aluminum Alloy

In this study, the experiments of laser weld bonding for DP590 dual-phase steel and 6061 aluminum alloy were carried out. Plasma shape and welding spectra were collected and analyzed, and the effect of adding adhesive layer on the microstructure and mechanical properties of laser-welded joints was also investigated. Some defects, such as pore, crack and softening in heat-affected zone, can be avoided in the weld, the increased weld penetration and width are obtained by adding adhesive layer, and the average tensile–shear strength of joint exceeds than that of no-added joint. In laser weld bonding, the Al content reduces weld penetration depth, but increases both sides of weld, Fe-rich Fe–Al ICMs with better ductility are formed in steel/aluminum interface, the Al diffusion from the lower aluminum to the upper steel side is suppressed, and transverse area of joining steel to aluminum is increased due to increasing Al volume fraction on weld width. In addition, the uniform-sized grains are distributed in the weld, shear load is shared by adhesive layer and weld, and hence, mechanical properties of laser-welded joints are improved.

Microstructural evolution and mechanical properties of laser-welded joints of medium manganese steel

The cold-rolled 5% medium Mn steel was butt-welded using a fiber laser. The microstructure, distribution of microhardness, and tensile properties of the base metal (BM) and welded joint were investigated. The results showed that the fusion zone of the welded joint had the highest microhardness due to the formation of 100% martensite. A finely mixed microstructure of martensite, ferrite, and austenite was formed in the heat-affected zone, and there was no softened zone in this area. The tensile test results indicated that the ultimate tensile strength and yield strength were higher for the joint than for BM. The joint efficiency was approximately 100%. All samples of the welded joint failed at the location of BM during tensile deformation. The fracture surfaces of the BM and welded joint were mainly ductile fractures. The BM and welded joint exhibited strain rate independence of the tensile strength and yield strength at strain rates of 0.01–1 s−1, while the yield strength of the BM and welded joint increased rapidly when the strain rate reached 5 s−1 due to changes in the dislocation movement mechanisms. The uniform elongation of the BM and welded joint decreased with increasing strain rate.

Influence of Welding Parameters on the Mechanical Properties of a Laser-Welded Joint

Laser welding is a well-known and relatively widely-used joining process in the engineering industry. Laser welding is also a proven suitable welding method for the joining of modern, high-strength structural steels. Welding of these steels can be challenging when using traditional manual fusion welding, since limits are set for the minimum and maximum welding energy and the cooling time, to retain the original properties of the base material. In the laser welding process, constant welding parameters are used and the movement is performed mechanically, to achieve a high and even processing speed, so that the welding values set can be fulfilled. In the study, the mechanical properties of laser-welded joints were researched in respect of the welding energy used and the cooling time resulting from different combinations of laser power and travelling speed. Several welds with a variable laser power and travelling speed were joined. The thicknesses of the test materials were 3, 4 and 6 mm and the welding energies used for each thickness were 0.05, 0.07 and 0.15 kJ/mm, respectively. The test material was thermo-mechanically rolled structural steel with 500 MPa of yield strength. The joint configuration used was bead-on-plate. Various destructive testing was performed for welded joints. For example, the transversal tensile strength results only showed just minor differences between the values, whereas the hardness values showed clearer differences between the joints.

Microstructure and mechanical properties of laser-welded joints of Ti-22Al-25Nb/Ti-6Al-4V dissimilar titanium alloys

Laser welding was applied to join 2-mm-thick dissimilar titanium alloys, Ti-22Al-25Nb (at. %) and Ti-6Al-4V (wt. %). Defect-free joints were obtained. The microstructures and mechanical properties of the welded joints were investigated. The results showed that the fusion zone mainly consisted of B2 phase and martensite α′ phase. The heat-affected zone microstructures of Ti-22Al-25Nb and Ti-6Al-4V were different because of the different base metal. The heat-affected zone of Ti-22Al-25Nb mainly consisted of B2 phase, O phase, and α2 phase, whereas the heat-affected zone of Ti-6Al-4V mainly consisted of α phase, β phase, and martensite α′ phase. EDS (Energy Dispersive Spectroscopy) results showed that the element distribution of each titanium alloy heat-affected zone and fusion zone was similar without an obvious change. Due to severe agitation, the element distribution was even. Different microstructures in different heat-affected zones resulted in an unsymmetrical hardness distribution. The hardness of fusion zone was the lowest of around 300 HV when the hardness of Ti-6Al-4V heat-affected zone near fusion zone was the highest of around 400 HV due to the influence of martensite α′ phase. In tensile tests performed at room temperature and 650 °C, the tensile strength of the joint reached 1057 and 363 MPa, respectively. Both these joints fractured at the fusion zone.

Effects of niobium addition on the microstructure and mechanical properties of laser-welded joints of NiTiNb and Ti6Al4V alloys

The dissimilar joining of shape memory alloys to Ti alloys is critically important in many industrial applications. However, joining these two alloys is a considerable challenge due to the formation of brittle intermetallics. In this paper, the dissimilar laser welding of 200-μm-thick NiTiNb and Ti6Al4V alloys was successfully achieved by the addition of a niobium filler wire with a diameter of 300 μm. The morphology, microstructure, phase composition and mechanical behavior of the obtained joints were studied by optical microscopy, scanning electron microscopy, transmission electron microscopy, micro X-ray diffraction, and tensile testing. Variations in the laser power changed the degree of Nb dilution in the weld metal, which affected the amount of intermetallic compounds (IMCs), NiTi, and Ti2Ni formed. All the welds consisted of a dendritic (Nb, Ti) solid solution and varied IMCs. The tensile strength of the joints depended on the amount of IMCs, which decreased with increasing Nb addition. All the tensile specimens fractured along the continuous IMC region in the weld. The highest tensile strength of the joint reached 82% that of the weaker base metal, Ti6Al4V, when the niobium filler was melted completely.

Microstructure characteristics and mechanical properties of laser-welded joint of γ-TiAl alloy with pure Ti filler metal

γ-TiAl alloy was successfully welded using pure Ti filler metal by laser. The microstructures, element distribution and phase composition of the joint were investigated by SEM, EDS and XRD, and the mechanical properties of the joints were evaluated by nanoindentation and tensile strength tests. Crack-free joints were obtained by using Ti filler metal. The weld zone mainly contained of α2-Ti3Al phase and a small amount of Ti2Al phases. The hardness values in the weld zone were higher than that of base metal (BM) due to the formation of α2-Ti3Al phase, but for the modulus values were just the reverse. The tensile strength and elongation of the joints were 288MPa and 2.19%, respectively, accounting for 74.8% and 94.0% of the BM, respectively. The joint fracture surface exhibited typical brittle fracture morphology, and Ti2Al and TiAl2 particle phases can be seen on the fracture surface.

Morphology, microstructure, and mechanical properties of laser-welded joints in GH909 alloy

The experimental laser welding of GH909 alloy was conducted in this study. The morphology, microstructure, and mechanical properties of laser-welded joints were analyzed by scanning electron microscopy, energy diffraction spectroscopy, and other techniques. Results revealed that the microstructure of the welded joints mainly consisted of tiny cellular structures, dendritic structures, and equiaxed crystals. Pores appeared in the interdendritic regions because of the insufficient local feeding of molten metal during solidification. Nb segregation in the heat-affected zone caused liquation cracking, whereas C segregation further induced the formation of carbide precipitates along the grain boundaries during the welding thermal cycle. The instability of the keyhole significantly promoted the escape of the metal vapor/plasma from the hole; as a result, porosity defects formed in the weld. The average tensile strength of the test joints was 756 MPa, which is 93.1 % of that of the base metal. The average microhardness of the weld zone (250 HV) was higher than that of the GH909 alloy substrate (208 HV), peaking at 267 HV. Microcracks appeared along the grain boundaries, proving that the grain boundaries were the weakest areas in the joint.

Investigation of the structure and properties of titanium-stainless steel permanent joints obtained by laser welding with the use of intermediate inserts and nanopowders

Results of an experimental study of the structure, the phase composition, and the mechanical properties of laser-welded joints of 3-mm thick titanium and 12Kh18N10T steel sheets obtained with the use of intermediate inserts and nanopowdered modifying additives are reported. It is shown that that such parameters as the speed of welding, the radiation power, and the laser-beam focal spot position all exert a substantial influence on the welding-bath process and on the seam structure formed. In terms of chemical composition, most uniform seams with the best mechanical strength are formed at a 1-m/min traverse speed of laser and 2.35-kW laser power, with the focus having been positioned at the lower surface of the sheets. Under all other conditions being identical, uplift of the focus to workpiece surface or to a higher position results in unsteady steel melting, in a decreased depth and reduced degree of the diffusion-induced mixing of elements, and in an interpolate connection formed according to the soldering mechanism in the root portion of the seam. The seam material is an over-saturated copper-based solid solution of alloying elements with homogeneously distributed intermetallic disperse particles (Ti(Fe, Cr)2 and TiCu3) contained in this alloy. Brittle fracture areas exhibiting cleavage and quasi-cleavage facets correspond to coarse Ti(Fe, Cr)2 intermetallic particles or to diffusion zones primarily occurring at the interface with the titanium alloy. The reported data and the conclusions drawn from the numerical calculations of the thermophysical processes of welding of 3-mm thick titanium and steel sheets through an intermediate copper insert are in qualitative agreement with the experimental data. The latter agreement points to adequacy of the numerical description of the melting processes of contacting materials versus welding conditions and focal-spot position in the system.

Effects of the thickness of Cu filler metal on the microstructure and properties of laser-welded TiNi alloy and stainless steel joint

Dissimilar welds of TiNi shape memory alloy (SMA) wires and stainless steel (SS) wires were produced via a laser welding process. Different thickness of Cu was used as filler metals. The current study details the effects of the thickness of Cu filler metal on the microstructure and mechanical properties of laser-welded joints. The results indicated that the Cu filler metal thickness had great effects on the chemical composition, microstructure, and mechanical properties of laser-welded joints. With an increase of Cu filler metal thickness, the weld Cu content increased, the soft Cu solid solution phases became more and coarser, and the brittle intermetallic compounds (TiFe2, TiNi3, etc.) decreased, and the tensile property of the welded joints had an increased tendency. The maximum tensile strength and elongation of the joints reached the maximum values (521MPa and 5.1%), and the corresponding joint fracture surface exhibited dimples features. However, too much Cu addition resulted in decreasing the joint properties because of forming TiNi3 phase and more Cu–Ti phases at weld/TiNi interface. The brittle intermetallic compound layer at the weld/TiNi interface was the weak zone where cracks inclined to derive and propagate during tensile testing. Furthermore, the performances of bending and shape memory effect of the welded joints have been analyzed and detailed.