Process characterization of laser-arc hybrid welding of pure copper was studied. The morphological features and the porosity of the welds were measured by optical microscope and X-ray non-destructive test, respectively. Process stability was characterized by the numbers in interruption and short circuit of the arc, which were counted from the waveform of arc current. The dynamic behaviors of molten pool, keyhole and droplet transfer were observed by high speed video camera. The results showed that the process was unstable in single laser welding and single arc welding, but could be easily stabilized by hybrid welding. Accepted weld and stable process could be obtained under the optimized parameters, which were the laser power around 4 kW, the arc current higher than 120 A and the welding speed lower than 1.5 m/min. The process stabilization was attributed to two reasons. Firstly, laser keyhole was stabilized by homogenizing and decreasing of dynamic vapor pressure on keyhole rear wall. Secondly, the arc wandering was avoided by keyhole fixation. Both of the reasons prevented the fast heat loss caused by the high heat conductivity of pure copper.

A series of experiments of high-power fiber laser-arc hybrid welding of pure copper was carried out. It could be seen that the microstructure of weld metal was obviously coarsened, and the columnar grain spacing at fusion zone and the massive grain size at heat-affected zone were both linearly increased with the increase of heat input. The weld conductivity decreased with the increase of heat input because the widening weld increased the microstructure nonuniformity of test samples. The heat input as well as welding parameter has no obvious effect on the ultimate tensile strength (UTS) of cross-weld but has obvious effect on the elongation. The UTS of all the welds was 200 MPa or so. The elongation was bigger than 20% when the heat input was in the optimization range from 250 to 380 J/cm. The decrease of the elongation was attributed to either high porosity at insufficient heat input or coarser grain at excessive heat input.

High cycle fatigue performance and the microstructures of laser-arc hybrid welded 8 mm-thick heavy plate of AA6082 aluminum alloy were studied firstly. The microstructures and the fatigue fracture surfaces were characterized by scanning electron microscope, electron back-scattered diffraction and transmission electron microscopy. The effects of the microstructures on different fatigue process were discussed theoretically. The fatigue limit at 107 cycles was about 120 MPa, far superior to the corresponding values for pure laser or pure arc welds. The fatigue process consisted of crack initiation and three crack propagation stages. The hybrid welding produced the welds with low percent porosity, grain refinement, and increased static strength. It improved the fatigue life during crack initiation, and then prolonged the total fatigue life. The theoretical analysis suggested that the microstructures played a key role in improving the fatigue properties of hybrid weld.

High-power fiber laser cutting of AA2219 aluminum alloy was studied. The cutting surface morphologies were examined by optical stereoscopic microscope, and the characteristic sizes were measured by a Vernier caliper. The results showed that the cutting surface was grouped into three zones featured by different striation patterns from the upper to the bottom, which are horizontal striation zone at the upper, vertical striation zone at the middle, and oblique striation zone at the lower. The oblique striation zone was the roughest zone. An optimal range of laser line energy to get an accepted cutting surface both with a small fraction of oblique striation zone and less dross was found, which was determined as 25–35 J/mm. The formation mechanisms of surface features were summarized by the interaction between the gas flow inside the cut and the forces exerted on the molten pool.

•The metallurgical bonding area increases with increasing layer thickness.•The size of crystal grain firstly increases and then tends to be stable.•The microhardness varied differently for the horizontal and vertical sections.•All the UTS and σ0.2 are greatly exceeded the ASTM A473-13 standard level.•The layer thickness in high-power SLM cannot be increased without limit.High-power selective laser melting (HP SLM) technology has been used to build 1Cr18Ni9Ti stainless steel samples with 60–150 μm thick powder layers. The relative density, metallurgical bonding mechanisms, microstructure and mechanical properties of the samples are presented. It is found that full density cannot be obtained at thicker powder layers due to the residual micropores. With increasing layer thickness from 60 μm to 150 μm, the primary dendrite spacing first increases from about 0.5 μm to 1.5 μm and then stabilizes around 2.0 μm. The microhardness of the fabricated samples by HP SLM shows directional dependent due to the anisotropy of microstructure and grain coarsening in the bonding area. Tensile strengths of the HP SLMed 1Cr18Ni9Ti samples are much higher than those of wrought 1Cr18Ni9Ti regardless of layer thickness and building direction. Importantly, the powder layer thickness cannot be increased without limit to ensure the comprehensive performance of the HP SLMed samples.High-power selective laser melting (HP SLM) technology has been used to build 1Cr18Ni9Ti samples with 60–150 μm thick powder layers as shown in the above figure. With increasing layer thickness from 60 μm to 150 μm, the primary cellular dendrite spacing first increases from about 0.5 μm to 1.5 μm and then stabilizes around 2.0 μm. The microhardness of the HP SLMed samples shows directional dependent due to the anisotropy of microstructure and grain coarsening in the bonding area. Tensile strengths of the HP SLMed 1Cr18Ni9Ti are much higher than those of wrought 1Cr18Ni9Ti regardless of layer thickness and building direction. Importantly, the powder layer thickness cannot be increased without limit to ensure the comprehensive performance of the HP SLMed samples.

Laser-arc hybrid welding with Cu3Si filler wire was employed to join dissimilar Ti6Al4V titanium alloy and AISI316 stainless steel (316SS). The effects of welding parameters on bead shape, microstructure, mechanical properties, and fracture behavior were investigated in detail. The results show that cross-weld tensile strength of the joints is up to 212 MPa. In the joint, obvious nonuniformity of the microstructure is found in the fusion zone (FZ) and at the interfaces from the top to the bottom, which could be improved by increasing heat input. For the homogeneous joint, the FZ is characterized by Fe67−xSixTi33 dendrites spreading on α-Cu matrix, and the two interfaces of 316SS/FZ and FZ/Ti6Al4V are characterized by a bamboo-like 316SS layer and a CuTi2 layer, respectively. All the tensile samples fractured in the hardest CuTi2 layer at Ti6Al4V side of the joints. The fracture surface is characterized by river pattern revealing brittle cleavage fracture. The bead formation mechanisms were discussed according to the melt flow and the thermodynamic calculation.

•Pulsed arc is more effective to improve the stability of laser-arc hybrid welding.•LCHW has the highest fraction of acicular ferrite and high-angle grain boundaries.•Grain refinement depends on effective current of the arc.•LSHW has the most apparent vestige of texture components.•The microstructure and microtexture formation mechanisms were summarized.Arc mode plays an important role in joint characterizations of arc welding, but it has been seldom considered in laser-arc hybrid welding. This paper investigated the role of arc mode on laser-metal active gas (MAG) arc hybrid welding of mild steel. Three arc modes were employed, which were cold metal transfer (CMT), pulsed spray arc and standard short circuiting arc. Microtexture of the joints were observed and measured via electron back scattering diffraction (EBSD) system to reveal the effect of arc mode on microstructure. Mechanical properties of the joints were evaluated by tensile and Charpy V-notch impact tests. It was found that both the stability and mechanical properties of laser-CMT hybrid welding (LCHW) is the best, while those of laser-standard short circuiting arc welding (LSHW) is the worst. OM and EBSD results showed that the fraction of acicular ferrite and high-angle grain boundaries in fusion zone decreases gradually in the sequence of LCHW, laser-pulsed spray arc welding and LSHW, while the mean grain size increases gradually. Finally, the microstructure formation mechanisms and the relationship between microstructure and mechanical properties were summarized by the loss of alloying element and the stirring effect in molten pool.

Laser–cold metal transfer arc hybrid welding of 6061 Al alloy and AISI304 stainless steel (304SS) was carried out. Bead morphologies and intermetallic compound (IMC) layer characterizations of the joints were studied in detail. The optimal parameter range for accepted bead appearances (OPRBA) without surface and interface defects was obtained, and the growth mechanism of the IMC layer was summarized. The results showed that the nonuniformity in the thickness and shape along the fusion zone/304SS interface from the top surface to the bottom increases with increasing heat input and is more sensitive to laser power because the interface temperature is dominated by a high-temperature laser keyhole throughout the molten pool. As the welding parameters are within the OPRBA and the heat input is within the range of 80–110 J/mm, the joints are stronger than 130 MPa and the corresponding IMC layer thickness is at the range of 3–6.5 μm. The kinetic analysis showed that a controlling interface temperature no more than 1,120 °C may limit the growth of the IMC layer.

Amorphous Fe nanoparticles are always difficult to prepare by physical gas-phase methods though rapid cooling rates are applied. Here we report a physical preparation of pure amorphous Fe nanoparticles by laser ablation of a 0.5-mm-diameter Fe wire and the investigation of their formation mechanism. Amorphous Fe nanoparticles with a shell of γ-Fe2O3 and the sizes of 1–3 nm are obtained at the laser power densities above the ablation threshold. Finally, the as-prepared nanoparticles are characterized by XRD, TEM, XPS and VSM to discover the structure, morphology, surface composition, crystallization and magnetic property in detail. We find that the holistic explosive evaporation induced by the small-size target not by the processing parameters determines the nature of the amorphous Fe nanoparticles. The as-prepared amorphous Fe nanoparticles are crystallized at 400 °C with an increase of particle size to about 10 nm.Amorphous Fe nanoparticles with a shell of γ-Fe2O3 are obtained by laser ablation of an iron wire. The holistic explosive evaporation induced by the small-size target not by the processing parameters determines the nature of the amorphous Fe nanoparticles. The as-prepared amorphous Fe nanoparticles are crystallized at 400 °C.Highlights► We report a novel approach to prepare amorphous Fe nanoparticles by laser ablation. ► Amorphous Fe nanoparticles with a shell of γ-Fe2O3 were obtained. ► Ablation mechanism changed by target size determines the nature of nanoparticles. ► The as-prepared amorphous Fe nanoparticles are crystallized at 400 °C.

A novel and cheap direct writing method based on the micropen has been developed to fabricate fluorinated polyimide stripe optical waveguides on Si/SiO2 wafers. The overall design, starting material, micropen direct writing system and fabrication processes of the stripe optical waveguides are presented. The effects of the key direct writing parameters, such as the tip-to-substrate distance, extrusive gas pressure, writing speed and viscosity of the polyamic acid, on the dimension and morphology of the stripe optical waveguides are discussed in detail. After deposition by the micropen system and baking process, the fluorinated polyimide stripe optical waveguides with good morphology and surface quality can be fabricated using the optimal parameters. The propagation losses at the wavelength of 1.55 μm are in the range of 1.4–3.5 dB cm−1 as characterized by different length combinations of the strip optical waveguides.Research highlights► Deposit fluorinated polyimide stripe optical waveguides on Si/SiO2 wafers using the direct writing method based on a designed micropen. ► Key direct writing parameters, such as tip-to-substrate distance, extrusive gas pressure, writing speed and viscosity of the polyamic acid, have been investigated to control the dimension and morphology of the stripe optical waveguides. ► Stripe optical waveguides with good morphology, surface quality and low losses can be fabricated cheaply and easily at the optimal parameters. ► Except for the simple stripe optical waveguides, optoelectronic devices with complex patterns are promising to be directly fabricated at low cost using this method only or by combining with other micro-manufacturing methods in the future.

A 6 kW fiber laser is used to weld AZ31B wrought magnesium alloy and the characterization of welded joints are studied by the observations of bead size, microstructure and mechanical properties. The accepted joints without macro-defects can be obtained when the laser power is in the range of 2.5 to 4.0 kW. Typical hexagonal dendrites are observed in the fusion zone, whose average semi-axis length increases with increasing heat input or decreasing welding speed. The minimum ultimate tensile strength of welded joints reaches 227 MPa, 94.6% of the base metal. And when the heat input reduces to 48 J/mm or lower, the joints are fractured in the base metal, showing stronger failure strength compared to the base metal. For the joints ruptured in the weld metal, the fracture surface is characterized by a ductile-brittle mixed pattern consisting of both dimples and cleavages. Finally, the formation mechanism of pore in the welds is discussed and summarized by the pore morphologies on the fracture surface.Highlights► Accepted joints of AZ31B Mg alloy are produced by high power fiber laser. ► Optimal welding parameters are summarized by experimental observations. ► Obvious hexagonal dendrites are observed in the fusion zone. ► The joints are stronger than base metal as the heat input is lower than 48 J/mm. ► Pore formation mechanism of welded joints is discussed and summarized.

For improving the weldability of ultra-fine grained (UFG) steel, detailed experiments of laser-tungsten inert gas (TIG) hybrid welding were carried out on this material to investigate the effects of welding parameters on weld shape, microstructure, grain growth in heat-affected zone (HAZ) and mechanical performance. For the hybrid welds, increasing energy ratio of laser to arc (ERLA) can narrow weld width, reduce the tendency of grain growth and ferrite grain coarsening in HAZ and also make the microstructure of fusion zone gradually change from coarse pearlite to fine martensite and bainite. The hybrid weld with low ERLA has obvious softening zone in HAZ; while that with high ERLA has no softening zone because of the low line energy. Compared to laser weld, under appropriate welding parameters hybrid weld with high ERLA can obtain higher welding speed, better weld shape and more sound mechanical performance including similar tensile strength and higher toughness. These results demonstrated that laser-TIG hybrid welding is an effective process for UFG steel.

•Temperature at cut front edge of fiber laser cutting of Al alloy, Tce is studied firstly.•Optimal Tce range for accepted kerf quality is found, which is 1800–1950 °C.•The mechanism is discussed by the calculation of the forces acting on molten pool.•Kerf quality is dependent on the angle of the resultant to downward direction.The characteristics of the temperature at cut front edge (Tce) during fiber laser cutting of AA2219 aluminum alloy were studied. A temperature monitoring system employing an infrared laser thermometer was established to record the time-domain Tce. A series of experiments was carried out to explore the relationship between the Tce and kerf quality. There was an optimal Tce range of 1800–1950 °C for accepted kerf. By calculating the forces acting on molten pool, the existence of the optimal Tce range was demonstrated, and the effect of the Tce on kerf quality was discussed. The kerf quality was mostly dependent on the angle of the resultant to the downward (θ). Only when the θ was around the minimum of 38° that is corresponding to optimal Tce range, the kerf was accepted because the liquid metal was pushed away smoothly, although the resultant value was small. When increasing or decreasing the Tce outside the optimal range, the kerf was bad because the increased θ intended to push the liquid metal backward to attach on the kerf rather than push them away.

•The inter- and intra-granular carbides in CGHAZ play a big role in the toughness.•The variations of CGHAZ microstructure and Erichsen value are consistent.•An equivalent grain size is proposed to establish a new Hall-petch relationship.•The threshold of equivalent grain size is calculated to be 11 μm.•Below the threshold, the weld toughness is acceptable to meet rolling requirement.Laser-arc hybrid welding of AISI 420 martensitic stainless steel (MSS) was carried out, and the microstructure-toughness relationship was discussed and established. The coarse-grained heat affected zone (CGHAZ) is the weakest area of the weld. The CGHAZ microstructure and the toughness assessed by Erichsen cupping test had a good agreement with the increase of heat input, indicating that the toughness was not only dependent on the grain size but also the contents of inter- and intra- granular carbides within the CGHAZ. By introducing the effects of the carbides, an equivalent grain diameter (De) was proposed to establish a new Hall-petch relationship between the CGHAZ microstructure and the toughness, whose fitting degree was increased from 0.677 to 0.974. The weld toughness was acceptable to meet the rolling requirement when the De was smaller than 11 μm, under which the weld fractured along the base metal rather than the CGHAZ.

•Weld corrosion resistance increases with the increase of laser-arc distance, DLA.•Weld corrosion resistance is close to the baser metal as the DLA increases to 9 mm.•The passive film is thickened with the DLA by increasing weld Cr content.•The damage of passive film is slowed with the DLA by homogenizing microstructure.Single pass laser-arc hybrid welding of stainless clad steel plate was first carried out to develop alternatives to multi-pass welding with low efficient or single pass welding with poor corrosion resistance. The results demonstrated that the weld corrosion resistance increased with the increase of laser-arc distance (DLA), and the corrosion morphologies changed from big and deep ‘nail’ pits to small and shallow hemispherical pits. The weld with nearly equal corrosion resistance to the base metal (BM) could be obtained at the DLA of 9 mm, where the charge transfer resistance increased to 89% of the BM and the corrosion current density decreased by ten times to close to the BM, only 0.13 μA·cm− 2 lower. The corrosion resistance improvement had a good agreement with the microstructure homogenization and the increase of Cr content. The microstructure formation was discussed by the molten pool behavior and the laser-arc interaction. The corrosion resistance improvement was explained by the formation and damage mechanism of the passive film.Download high-res image (384KB)Download full-size image