Laser beam welding was conducted on GH909 alloy in a solution treatment and aging conditions. The microstructure, mechanical properties, and softening behavior of welded joints were carefully studied using scanning electron microscopy, energy diffraction spectroscopy, and other techniques. Results indicated that γ′-Ni3(TiAl)-precipitated phases were produced inside the matrix, and the presence of Laves at grain boundaries restricted grain growth. A sharp reduction in γ′-precipitated phases evidently weakened the average tensile strength and microhardness of the welded joints. Moreover, fractures occurred in the weld zone. Microcracks in the fracture of the joint mainly depended on the competition result between thermal stress and cohesion forces among the grains.

In this paper, we describe a method employing a suction device adapted to provide a negative pressure zone on the surface of the keyhole to allow the highly-pressurized zinc vapor to escape. The microstructure and properties of the lap joint were studied, and the distribution of zinc element in the joints was analyzed using synchrotron radiation X-ray. A high-speed video camera was used to record the dynamic behavior of the laser-induced plasma plume and the zinc vapor. Experimental results demonstrated that this suction method can not only facilitate venting of the high pressure zinc vapor from the molten pool and the keyhole, but also stabilizes the laser-induced plasma plume during the welding process. A lap joint with a good surface quality and excellent mechanical strength was obtained using this method.

This paper details a new method that combines laser autogenous welding, laser wire filling welding and hybrid laser-GMAW welding to weld 30 mm thick plate using a multi-layer, multi-pass process. A “Y” shaped groove was used to create the joint. Research was also performed to optimize the groove size and the processing parameters. Laser autogenous welding is first used to create the backing weld. The lower, narrowest part of the groove is then welded using laser wire filling welding. Finally, the upper part of the groove is welded using laser-GMAW hybrid welding. Additionally, the wire feeding and droplet transfer behaviors are observed by high speed photography. The two main conclusions from this work are: the wire is often biased towards the side walls, resulting in a lack of fusion at the joint and the creation of other defects for larger groove sizes. Additionally, this results in the droplet transfer behavior becoming unstable, leading to a poor weld appearance for smaller groove sizes.Highlights• It is a new method to weld 30mm thick plate by small power laser.• It presents the mechanism of wire melting and droplet transfer mode.• A good joint without obvious defects can be achieved by a proper groove size.

In this study, we describe the effect of aging heat treatment on the microstructural response in laser welded joints of low thermal expansion superalloy GH909. This investigation was conducted using an optical microscope, scanning electron microscope, energy diffraction spectrum and other methodologies. The results demonstrated that part of strengthening phase γ' was transformed into ε phase in the interior of the grains and more precipitated particles were formed in the grain boundary after aging heat treatment of the welded joint, resulting in bending of the grain boundary. Laves phase formed in the grain boundary is rich in Si and Nb elements, which helps to restrict grain growth. The second-phase particles generated at the bottom of dimples enhance the mechanical properties of the welded joint. Microhardness and strength of the welded joints can be significantly improved due to the occurrence of the precipitated phase in the grain boundary and interior of the grains. However, the occurrence of liquated GB can weaken binding between the grains, with the result that the heat affected zone becomes the weakest zone in the joint.

In this paper, we describe experimental laser welding of low-thermal-expansion superalloy GH909. The main welding defects of GH909 by laser in the weld are liquation cracks and porosities, including hydrogen and carbon monoxide porosity. The forming mechanism of laser welding defects was investigated. This investigation was conducted using an optical microscope, scanning electron microscope, energy diffraction spectrum, X-ray diffractometer and other methodologies. The results demonstrated that porosities appearing in the central weld were related to incomplete removal of oxide film on the surface of the welding samples. The porosities produced by these bubbles were formed as a result of residual hydrogen or oxygenium in the weld. These elements failed to escape from the weld since laser welding has both a rapid welding speed and cooling rate. The emerging crack in the heat affected zone is a liquation crack and extends along the grain boundary as a result of composition segregation. Laves–Ni2Ti phase with low melting point is a harmful phase, and the stress causes grain boundaries to liquefy, migrate and even crack. Removing the oxides on the surface of the samples before welding and carefully controlling technological parameters can reduce welding defects and improve formation of the GH909 alloy weld.Highlights► It is a new process for the forming of GH909 alloy via laser welding. ► The forming mechanism of laser welding defects in GH909 has been studied. ► It may be a means to improve the efficiency of aircraft engine production.

In order to investigate the mechanism of stability for the wire feed laser welding process, systematic experiments are carried out in this study for 5A06 aluminum alloy. By using high speed camera, the melting dynamics of filler wires with different feed positions and feed rates are studied. The results indicate that these two factors mainly influence the characteristics of the filler wire melting dynamics and determine the stability of the welding process. The melting dynamics of filler wire can be generally characterized by three different forms: explosion, big droplet and molten metal bridge. When the filler wire melts and transits to the molten pool in the forms of explosion or big droplet, the stability of the welding process is strongly disturbed, resulting in an undesirable weld quality. In contrast, when it is in molten metal bridge form, the welding process is more stable and a uniform weld bead is achieved.

High power fibre lasers have recently received much attention because of their inherent advantages such as high output power, high beam quality, compact size, and flexible fibre delivery. Studies on the mechanism behind fibre laser welding systems may further promote their practical application. In this paper, high speed video observations were used to study the characteristics of the plasma/vapour induced during the bead-on-plate welding of ZL114 using a high power CW fibre laser. We also analysed the cause of the periodic oscillation of the plasma/vapour. The results revealed that plasma/vapour induced from high power lasers oscillate periodically at 450–600 μs cycles above the weld pool surface. The use of a shielding gas has little effect on the oscillation cycle. The plasma/vapour absorption is not the main reason for the periodical oscillation of plasma/vapour induced during fibre laser welding. The periodic oscillation of the plasma/vapour can be attributed to the oscillation of the keyhole.Highlights► High speed video observations were used to study the characteristics of the plasma. ► Plasma can be divided into two parts: the keyhole plasma and the floating plasma. ► Plasma oscillates periodically at 450–600 μs cycles above the weld pool surface. ► Use of a shielding gas has little effect on the oscillation cycle. ► Reason of the periodic oscillation of plasma is the oscillation of the keyhole.

Hybrid plasma is an important physical phenomenon in fiber laser-MIG hybrid welding. It greatly affects the stability of the process, the quality of the weld, and the efficiency of energy coupling. In this paper, clear and direct proofs of these characteristics are presented through high-speed video images. Spectroscopic analysis is used to describe the characterization of hybrid plasma. The hybrid plasma forms a curved channel between the welding wire and the keyhole during the fiber laser-MIG hybrid welding process. The curved channel is composed of two parts. The laser-induced plasma/vapor expands due to the combined effect of the laser and the MIG arc, forming an ionization duct, which is one part of the curved channel. The resistance of the duct is smaller than that of other locations because of the rise in electrical conductivity. Consequently, the electrical arc is guided through the duct to the surface of the material, which is the other part of the curved channel. The spectral intensities of metal elements in laser-MIG hybrid welding are much stronger than those in MIG-only welding, whereas the spectral intensities of shielding gas element in laser-MIG hybrid welding are much weaker.

The mechanism of porosity formation and its suppression methods in laser formation of aluminum alloy have been studied using a 4kW fiber laser to weld 5A06 aluminum alloy with SAl-Mg5 filler. It was found that the porosity formation is closely related to the stability of the keyhole and fluctuation of the molten pool in the laser welding aluminum alloy. The filling wire increased the instability of the keyhole and weld pool, thus further increasing the amount of gas cavities in the joint. Prefabrication of a suitable gap for the butt joint can provide a natural passage for the flow of the liquid metal, which can weaken, and even completely eliminate the disturbance of the filling wire on the formation of keyhole. The gap can also provide a passage for the escape of the bubble. Thus, this method can greatly decrease the sheet’s susceptibility to porosity. Moreover, for a thin sheet, if the power of the laser is sufficient to form a keyhole with stable penetration through the weld sheet, a weld bead without porosity can also be obtained because closing the keyhole is almost impossible.

•A parameter field is proposed to lower the susceptibility to HAZ cracking.•The HAZ cracking is mainly attributable to the coarseness of the microstructure.•Different fracture morphologies are studied to confirm the effect of thermal stresses.•The stress field is simulated to verify the correctness of experimental results.In this paper, we describe influence of heat input on HAZ liquation cracking in laser welded GH909 alloy. The results demonstrated that more cracks were produced using high laser power and welding speed. The presence of cracks greatly weakened the hot ductility of this material and the binding force between the adjacent grains, resulting in reducing the tensile strength of welded joints. The occurrence of HAZ cracking was mainly attributable to the coarseness of microstructures and large tensile stresses. A new method was proposed to prevent HAZ liquation cracking using low laser power and welding speed at a constant heat input. The simulated results were consistent with the experimental results, verifying the correctness and feasibility of the method.