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A 3D finite-element thermal model has been developed to estimate weld geometry and analyze thermal gradients and cooling rates in pulsed laser welding of Inconel 718 superalloy. Since Inconel 718 superalloy is sensitive to hot cracking, the extent of hot crack sensitive areas including heat affected zone, mushy zone and partially melted zone is very important in the investigation of weldability for this alloy. Because the welding process is very fast and the welding width is very thin, the numerical modeling approach can be very helpful to overcome the difficulties and limitations of experimental procedure. In this study, a finite element model is presented to simulate the process of pulsed laser welding of Inconel 718 and to predict the temperature distribution and shape of the weld pool and finally to estimate the extent of the hot cracking sensitive regions. Heat source was modeled by the combination of conical Gaussian and ellipsoid Goldak. Then, the results of numerical model were compared to the experimental results at preheat temperatures of 308, 393, 473 Kelvin. The proposed numerical model exhibits good agreements with the experimental results including the internal and external weld pool geometries and the extent of the hot cracking susceptible regions. Furthermore, detailed pulsed temperature distribution and thermal cycle containing solidification latent heat are obtained. Accordingly, this pulsed temperature history is correlated to the internal and external geometry of pulsed solidified weld pool. [ABSTRACT FROM AUTHOR]

• Dissimilar laser welding of Kovar to AISI 304L has been investigated. • Solidification crack and lack of complete mixing in the weld metal are seen. • The heat input must have an optimum value equaling 19.1 J/mm. • At heat input less than 15 J/mm, non-mixing is clearly observable. Kovar, due to its close thermal expansion coefficient to glass, can be used as a mediating alloy in joint of glass to metals such as AISI 304L. In the present research, the effect of parameters of pulsed laser on dissimilar welding of Kovar to AISI 304L has been investigated. For observation and investigation of the results, metallography, Energy dispersive X-ray spectroscopy (EDS) and microhardness tests have been used. Results obtained from microstructural examinations, indicated the existence of solidification crack and lack of complete mixing in the weld metal. According to the results, the heat input must have an optimum value equaling approximately 19.1 J/mm. At higher levels such as 32.6 J/mm, the TCL (total crack length) value reached 760 μm. Increasing the heat input lead to increasing thermal stresses and formation of crack in the weld metal. On the other hand, at heat input levels less than the value of 15 J/mm, cracks are eliminated and non-mixing is clearly observable because sufficient time is not provided for mixing and, then, solidification for the weld metal. The weld metal hardness is also higher than the base metals due to the fine dendrite structure and the presence of hardening elements. [ABSTRACT FROM AUTHOR]

A 700 W pulsed Nd:YAG laser welding was employed to weld 5456 aluminum alloy plates with thickness of 5 mm. The weld penetration and tendency of liquation cracking in the heat affected zone and solidification cracking in the weld metal are investigated. It is found that hot cracks can be avoided by pulsed laser in preheated status. Increasing the laser average power in fixed pulse frequency and pulse duration resulted in a significant reduction in hot cracking tendency in pulsed laser welding. Also, the hot cracking susceptibility is reduced with increasing pulse frequency up to 50 Hz in fixed average power and pulse duration. The hot cracking tendency can be decreased by pulse duration decrement to 6 ms in fixed average power and pulse frequency. The factors contributing to the prevention or reduction of hot cracks in laser welding of aluminum alloy 5456 are the reduction in cooling rate causing change in solidification mode from columnar to equiaxed grains, and backfilling of liquated grain boundaries. With a 0.4 mm beam diameter Nd:YAG pulsed laser at 300 W average power, 6 ms pulse duration, 20 Hz pulse frequency, 5 mm/s travel speed and 150 °C preheating temperature, hot crack-free weld cross sections were achieved. [ABSTRACT FROM AUTHOR]

Titanium alloys are commonly used in orthopedic devices due to their good corrosion resistance, high specific strength and excellent biological response. The direct contact between the implant surface and the host tissue results in notable effect of surface properties such as surface topography on the biological responses. The aim of this study is to investigate the effect of frequency of pulsed Nd-YAG laser on Ti6Al4V alloy surface topography and its influence on the improvement of biocompatibility while other laser parameters kept constant. The range of applied frequency values were selected from 1 to 20 Hz. The range of surface roughness was found between 452 nm and 3.37 μm. The untreated sample and also samples with the highest and the lowest average surface roughness parameter were subjected to the further analyses. Characterization of the samples was performed with surface roughness tester, Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM). The high rate of melt and solidification during the laser treatment led to the martensite formation and consequently an increase about 12–25% in hardness. Furthermore, in vitro study was carried out using MG-63 osteoblast like cells. The analyses of cell viability for 3 culture times, cell morphology and cell spreading area revealed that sample with the highest average surface roughness parameter is more biocompatible. 10 Hz frequency was found as the optimum parameter which led to the highest surface roughness and thus the biocompatibility enhancement. In conclusion, the pulsed Nd-YAG laser with an optimum value of applied frequency can be utilized as an effective technique to improve the biological characteristics. • Laser treated samples had 2.8–7.5 times higher roughness untreated sample. • Martensitic phase in fusion zone caused 12–25% increase in the hardness. • The best results of biological analyses were observed in R a = 3.37 μm. • This sample had the highest amount of cell viability and cell spreading area. • The best cell viability, cell attachment, obtained in frequency of 10 Hz. [ABSTRACT FROM AUTHOR]

• 17-4 PH SS welded by pulsed Nd:YAG laser was free of crack. • Aging at 550 °C for 30 min increased hardness of weld metal and HAZ. • Yield and ultimate strengths, were 995 MPa, 1185 MPa for as-welded samples. • Microstructure of weld metal in aged condition was δ-ferrite in martensite. Pulsed Nd:YAG laser welding of 17-4 martensitic precipitation hardening (PH) stainless steel (SS) is investigated. In order to achieve optimum laser welding parameters, bead on plate (BOP) welding was conducted on 17-4 PH SS. No crack was detected in the weldments, and microstructure of weld metal was δ-ferrite in the martensite matrix. Resistance to cracking of weldment was correlated to the presence of δ-ferrite in the weld metal, and using pulsed Nd:YAG laser as a low heat input welding method. Four HAZs were distinguished and microstructure evaluation in each zone is discussed. In the second part of this research, pulsed Nd:YAG laser welding of similar 17-4 PH SS in butt joint configuration was performed. Microstructure characterization and mechanical properties of weldment in as-welded and aged in 550 °C for 30 min conditions were investigated and compared. Aging caused precipitation of copper-rich clusters and increasing hardness of weld metal and homogenizing the hardness in HAZ. Tensile test results demonstrated that the failure occurred in the base metal and showed features of ductile fracture. [ABSTRACT FROM AUTHOR]

• In laser welding of aluminum alloys, the weld penetration is dependent on Mg content of the alloy. • The presence of Mg vapor in the plasma plume over the weld pool affects the laser absorption, and it is through this phenomenon that the penetration is affected. • Increasing the laser pulse energy, Mg evaporation is increased, and that, in turn, increased the effective laser absorption coefficient. It is observed that in laser welding of aluminum alloys, magnesium can evaporate, and the weld penetration is dependent on Mg content of the alloy and Mg loss from the weld pool. In this research, it is proposed that the presence of Mg not in the base metal alloy, but rather the presence of Mg vapor in the plasma plume over the weld pool affects the laser absorption, and it is through this phenomenon that the weld profile and penetration is affected. Numerical simulation was performed to determine the relationship between the weld profile to estimate the effective laser absorption coefficient of four Al alloys and in parallel EPMA technique was used to determine the Mg losses of the weld metals. The combined analysis of the results showed that increasing the laser pulse energy (decreasing laser pulse frequency), Mg evaporation is increased, and that, in turn, increased the effective laser absorption coefficient. However, more laser power absorption does not necessarily mean more weld penetration. Laser absorption results in weld penetration, once the threshold Mg evaporation rate of 200 × 10 - 6 g/cm2 is passed. [ABSTRACT FROM AUTHOR]