Stable Keyhole Research Articles

Numerical simulation on porosity formation and suppression in oscillation laser welding of Ti-6Al-4V alloy

Porosity formation and suppression in oscillation laser beam welding of Ti-6Al-4V alloy is simulated in this study. The results show that, in nonoscillation welding, slender keyholes with a depth and width radio of about 3.66 are continually formed. In oscillation welding, shallow and wide keyholes with a depth and width ratio of about 0.97 are formed. In nonoscillation welding, a positive pressure appears at the front of the keyhole, which gives the keyhole a wasp-waist structure and makes it easily break into small parts. In oscillation welding, the positive pressure appears at the bottom of the keyhole, and the average value decreases from 5.23 to 1.04 MPa when the laser beam oscillating frequency increases from 50 to 200 Hz. This kind of pressure distribution results in a more stable keyhole and suppresses the formation of porosity. In the meantime, with the increase in oscillation frequency, the interval of porosity formation becomes longer. The results of this study are helpful in understanding the porosity formation mechanism in oscillating laser beam welding.

Microstructural evolution in laser powder bed fusion of water-atomized high-carbon low-alloy steel: Analysis of melting mode effects

Medium/High‑carbon steels are considered susceptible to cracking during rapid cooling in laser powder bed fusion (LPBF) applications. Also, water-atomized (WA) steel powders, containing oxides, have been associated with porosity defects during this process. In this study, LPBF was employed to process WA high‑carbon low-alloy steel powders under three operational regimes – conduction, transition, and keyhole modes. Analyses based on the scaling law of keyhole stability, melt pool dimensions, and x-ray computed tomography (XCT) suggested that processing under the transition mode closely resembled a stable keyhole, enabling the successfully printing of water-atomized powders to a density of 99.93 %. Subsequent heat treatment, hardness, and residual stress assessments presented a prominent structural softening and relief of compressive stress under the transition mode. This was attributed to the intense in-situ tempering and re-austenitization that occurred within each layer. Under this condition, with no post heat treatment, an ultimate tensile strength of 1250 MPa with >2.6 % elongation was achieved. Finally, the ball-on-disk test showed that wear performance of the printed steels was primarily governed by the tribo-oxides, with a limited influence from the microstructural variation.

Free arc heating effect in double-layers hybrid arc source

Keyhole welding process is easily achieved with plasma arc heat source, but the welding speed for a stable keyhole is hard to increase because of the low-level heat input. Higher welding current will increase the heat input, but the induced heavy arc pressure acts to damage the keyhole stability. The narrow window of welding current creates limited usable range of heat input. Double-layers hybrid arc heat source is developed by compositing an outer layer of free arc into the center plasma arc jet. Set the plasma arc current level to create enough arc pressure to open a keyhole, the outer layer arc is used to control additional heat power into the weld pool but has little effect to influence the arc pressure. The free arc heating effect in the hybrid arc is figured out: (1) much faster welding speed can be achieved, it increases from 3 mm/s to 4.75 mm/s after additional 80 A/80 A free arcs are used, and (2) the increase of welding speed is highly related to the total heat input in the hybrid arc and is unrelated to the free arc number; (3) the calculated efficiency factor is selected as 0.75 for the added free arc. The developed hybrid arc is proved to be a robust method to increase the manufacturing efficiency.

Experimental study on the keyhole dynamic evolution and porosity formation during laser welding of Ti-6Al-4V alloy

Keyhole dynamic evolution significantly affects laser welding quality. For observing the keyhole’s dynamic behaviors directly, a sandwich workpiece that consists of a piece of quartz glass and a piece of Ti-6Al-4V alloy is used in experiments of this study, and the dynamic behaviors of the keyhole are recorded with a high-speed camera. The experimental results show that, in the first 5 ms of welding, the depth and width of the keyhole increase linearly and, at about 70 ms, a relatively stable keyhole is formed. A keyhole typically exhibits the shape of a “7” character in a short period after it forms. At this stage, the laser beam is reflected by the wall of the keyhole repeatedly until it reaches the bottom, which results in some tiny pore formations in the middle part of the keyhole. In subsequent welding, the collapsing and rebuilding process of the keyhole are observed, and big pores are formed at the bottom of the keyhole. The results of this research are helpful to understand the mechanism of the keyhole dynamic evolution and porosity formation during titanium alloy laser welding.

Role of Stable Keyhole Mode in the Formability, Microstructure, and Mechanical Properties of High Layer Thickness in Laser Powder Bed Fusion of Ti-6Al-4V Alloy

Role of Stable Keyhole Mode in the Formability, Microstructure, and Mechanical Properties of High Layer Thickness in Laser Powder Bed Fusion of Ti-6Al-4V Alloy

Keyhole Welding with Hybrid Plasma-Free Arc Source

Improving the keyhole welding process window is challenging work because the force and thermal state in the weld pool are hard to control. A hybrid plasma-free arc source was developed based on a plasma arc torch, sided tungsten was added close to the constraint orifice to form a free arc, and hybrid arcs formed after the free arc was fully absorbed into the constraint arc. In such a one-anode, twocathode hybrid arc system, the added free arc acts in an assistant role to adjust arc heat output without influencing the arc pressure peak, and the constraint arc acts as the guiding arc to control the arc pressure and slightly incline to the free arc side. Bead-on welding experiments were done to test keyhole welding process behavior with the hybrid arc source, including arc column, keyhole state, weld surface performance, and melting state in the weld pool. Results showed that (1) the hybrid arc can enlarge the stable keyhole welding process window in aspects of welding current and welding speed, and a more-obvious improving effect is obtained with the leading-sided tungsten; (2) the hybrid arc with the rear-sided tungsten has a smooth front weld surface; (3) the increased heat will enlarge the weld cross-section area with no obvious influence to the backside weld width; and (4) the leading keyhole wall is related to the outer tungsten position. The research gives a new method for improving keyhole welding stability.

Keyhole Welding with Hybrid Plasma-Free Arc Source

Improving the keyhole welding process window is challenging work because the force and thermal state in the weld pool are hard to control. A hybrid plasma-free arc source was developed based on a plasma arc torch, sided tungsten was added close to the constraint orifice to form a free arc, and hybrid arcs formed after the free arc was fully absorbed into the constraint arc. In such a one-anode, twocathode hybrid arc system, the added free arc acts in an assistant role to adjust arc heat output without influencing the arc pressure peak, and the constraint arc acts as the guiding arc to control the arc pressure and slightly incline to the free arc side. Bead-on welding experiments were done to test keyhole welding process behavior with the hybrid arc source, including arc column, keyhole state, weld surface performance, and melting state in the weld pool. Results showed that (1) the hybrid arc can enlarge the stable keyhole welding process window in aspects of welding current and welding speed, and a more-obvious improving effect is obtained with the leading-sided tungsten; (2) the hybrid arc with the rear-sided tungsten has a smooth front weld surface; (3) the increased heat will enlarge the weld cross-section area with no obvious influence to the backside weld width; and (4) the leading keyhole wall is related to the outer tungsten position. The research gives a new method for improving keyhole welding stability.

Harmonizing sound and light: X-ray imaging unveils acoustic signatures of stochastic inter-regime instabilities during laser melting

Laser powder bed fusion (LPBF) is a metal additive manufacturing technique involving complex interplays between vapor, liquid, and solid phases. Despite LPBF’s advantageous capabilities compared to conventional manufacturing methods, the underlying physical phenomena can result in inter-regime instabilities followed by transitions between conduction and keyhole melting regimes — leading to defects. We investigate these issues through operando synchrotron X-ray imaging synchronized with acoustic emission recording, during the remelting processes of LPBF-produced thin walls, monitoring regime changes occurring under constant laser processing parameters. The collected data show an increment in acoustic signal amplitude when switching from conduction to keyhole regime, which we correlate to changes in laser absorptivity. Moreover, a full correlation between X-ray imaging and the acoustic signals permits the design of a simple filtering algorithm to predict the melting regimes. As a result, conduction, stable keyhole, and unstable keyhole regimes are identified with a time resolution of 100 µs, even under rapid transitions, providing a straightforward method to accurately detect undesired processing regimes without the use of artificial intelligence.

Keyhole stability, arc behavior, and molten pool flow in narrow-gap oscillating laser-arc hybrid welding of titanium alloy

In the study, the keyhole stability, arc behavior and molten pool flow were investigated to clarify the welding process stability in narrow-gap oscillating laser-arc hybrid welding of titanium alloy. At an oscillating frequency of 300 Hz and amplitude of 1 mm, a concave weld with low porosity was produced. Compared with non-oscillating laser, the keyhole opening size increased about twofold, and the standard deviation of arc deflection angle differences decreased by approximately 87.6 %. Owing to the stirring effect of beam oscillation, a stable keyhole with larger opening size was formed, which facilitated the escape of bubbles from the keyhole. More excitation electrons were generated and erupted outward from the keyhole, and both plasma temperature and density increased by approximately 55.2 %. An enhanced electric channel was formed between the welding wire tip and groove center. As a result, the arc burnt stably at the groove bottom, which could melt the groove bottom synchronously, and suppress sharp corners and lack of fusion. The impact caused by droplets was cushioned as the liquid metal flowed towards the rear molten pool orderly with a circular path. Consequently, the liquid metal near the keyhole fluctuated slightly, indicating that the keyhole stability was enhanced. Besides, dendrites were fragmented and more equiaxed grains were produced due to the strong disturbance of dendrites by liquid metal in the whole mushy zone.

Fundamental understanding of open keyhole effect in plasma arc welding

The keyhole arc welding technique has the advantage of improving welding efficiency by utilizing a stable keyhole mode. Accurate understanding of the keyhole effect is necessary to enhance the welding quality. Due to the high temperature and strong arc force involved, the complex gas–liquid–solid interactions in the complete keyhole process need to be explored. In order to fully demonstrate open keyhole mode welding, a three-tier sandwiched model based on multiphysics and multiphase effects was developed. The top layer of the model is filled with plasma arc, which gradually fuses and penetrates through the middle metal layer. Finally, it enters the third layer, resulting in an open keyhole mode. Multiphysics phenomena due to the plasma arc are fully included in the model, and the gas–liquid–solid interactions are calculated by combining the Volume of Fluid technique and the Enthalpy-porous technique. Arc ignition and dynamic open keyhole effect are demonstrated, and an arc discharge is shown from the open keyhole exit. The arc reflection phenomenon is observed as the arc is blocked by the weld pool frontier. The electric current path varies with the welding movement, and most of the current comes from the weld pool frontier. An experiment was conducted to obtain weld pool and keyhole images, which basically agree with the calculated results. Additionally, the calculated open keyhole time and electric potential drops also coincide well with experimental data.

Study of spatter net forming mechanism and penetration mode under flexible ring mode laser welding

Flexible ring mode (FRM) laser technology can generate different powers by adjusting the power ratio of the inner and outer rings, and has great potential for welding copper and its alloys with high reflectivity. This paper used a high-speed photographic camera to monitor the behavior and spatter-forming process of molten pool on the front of red copper plate. The spattering behavior in different penetration modes and net forming mechanisms under FRM are revealed. The results show that full penetration causes the high-temperature vapor to escape from the upper and lower openings. More energy is absorbed by the molten pool and keyhole in the semi-penetration mode, forming an expansion zone and increasing the pool's instability. The outer ring laser of FRM reduces the temperature gradient, which can lower the surface tension gradient, making the flow of the molten pool smoother and the height and formation times of the liquid column lower and less, thereby reducing the spatter. Moreover, FRM laser produces a larger and more stable keyhole, making the escape of metal vapor easier and minimizing the kinetic energy. The reasons above bring the sound result of high yield rate, improved by 75%, comparing with pure Gaussian laser. This paper provides theoretical guidance for engineering researchers to understand the role of shaping laser beams in the welding of highly reflective materials.

High Reflectivity and Thermal Conductivity Ag–Cu Multi-Material Structures Fabricated via Laser Powder Bed Fusion: Formation Mechanisms, Interfacial Characteristics, and Molten Pool Behavior

Ag and Cu have different advantages and are widely used in key fields due to their typical highly electrical and thermal conductive (HETC) properties. Laser powder bed fusion (LPBF), an innovative technology for manufacturing metallic multi-material components with high accuracy, has expanded the application of Ag-Cu in emerging high-tech fields. In this study, the multi-material sandwich structures of Ag7.5Cu/Cu10Sn/Ag7.5Cu were printed using LPBF, and the formation mechanism, interface characteristics, and molten pool behavior of the Ag7.5Cu/Cu10Sn (A/C) and Cu10Sn/Ag7.5Cu (C/A) interfaces were studied to reveal the influence of different building strategies. At the A/C interface, pre-printed Ag7.5Cu promoted Marangoni turbulence at a relatively low energy density (EA/C = 125 J/mm3). Due to the recoil pressure, the molten pool at the A/C interface transformed from a stable keyhole mode to an unstable keyhole mode. These phenomena promoted the extensive migration of elements, forming a wider diffusion zone and reduced thermal cracking. At the C/A interface, the molten pool was rationed from the conduction mode with more pores to the transition mode with fewer defects due to the high energy density (EC/A = 187.5 J/mm3). This work offers a theoretical reference for the fabrication of HETC multi-material structures via LPBF under similar conditions.

Quantifying Equiaxed vs Epitaxial Solidification in Laser Melting of CMSX-4 Single Crystal Superalloy

The competition between epitaxial vs. equiaxed solidification has been investigated in CMSX-4 single crystal superalloy during laser melting as practiced in additive manufacturing. Single-track laser scans were performed on a powder-free surface of directionally solidified CMSX-4 alloy with several combinations of laser power and scanning velocity. Electron backscattered diffraction (EBSD) mapping facilitated identification of new orientations, i.e., “stray grains” that nucleated within the fusion zone along with their area fraction and spatial distribution. Using high-fidelity computational fluid dynamics simulations, both the temperature and fluid velocity fields within the melt pool were estimated. This information was combined with a nucleation model to determine locations where nucleation has the highest probability to occur in melt pools. In conformance with general experience in metals additive manufacturing, the as-solidified microstructure of the laser-melted tracks is dominated by epitaxial grain growth; nevertheless, stray grains were evident in elongated melt pools. It was found that, though a higher laser scanning velocity and lower power are generally helpful in the reduction of stray grains, the combination of a stable keyhole and minimal fluid velocity further mitigates stray grains in laser single tracks.

Multiphase and multi-physical simulation of open keyhole plasma arc welding

Plasma arc welding (PAW) is an important technique in the manufacturing industry. To improve the welding efficiency, the special keyhole mode has attracted considerable attention because it has the potential to achieve full penetration without a groove. However, rigorous process conditions are required to maintain a stable keyhole mode. For better and more widespread application in industry, a multiphase and multi-physical model is established to reveal the complicated electro-magneto-thermo-hydro-mechanical phenomena and gas-liquid-solid interactions in the keyhole PAW process. Arc backward reflection and deviation of the heating center occur because of the relative movement in the system, and molten liquid metal is pushed backward and upward to form a salient metal layer at the rear. The maximum height reaches 1.45 mm. Outflow phenomenon occurs through the open keyhole, and the outflow temperature and velocity are approximately 6000 K and 60 m/s by the calculation, respectively. The predicted weld pool and keyhole basically coincide with measured results. This study provides a comprehensive understanding of the multiphase and multi-physical interactions in the PAW process, which may help improve the associated techniques.

Effect of oscillation modes on weld formation and pores of laser welding in the horizontal position

The present paper systematically investigated the effect of oscillation modes on weld formation and pores of laser welding for high-strength low alloy (HSLA) steel in the horizontal position. The laser energy distributions with different oscillation modes were calculated and analyzed. The vapor plume, the molten pool characteristic, and the keyhole behavior with different oscillation modes were observed. The experimental results show that compared with non-oscillation laser welding and lateral oscillation laser welding, circular oscillation laser welding has more uniform energy distribution, less fluctuation of laser spot motion velocity, and more stable plume and keyhole. Circular oscillation laser welding decreases the depth-to-width ratio of the welds, reduces welding spatters, and improves weld formation. Circular oscillation of the laser beam increases keyhole size, and the oscillation keyhole formed is more likely to trap the bubbles in the molten pool, thus reducing the probability of pore formation. Finally, circular oscillation laser welding with an oscillation amplitude of 4 mm and an oscillation frequency of 60 Hz obtains high-quality weld with expected morphology and free of pores.

Study on droplet transfer behavior during medium and low power laser assisted pulsed MAG hybrid welding

This paper comparatively investigates the arc plasma morphology and the droplet transfer behavior and Characteristic of electrical signal between the pulsed MAG welding and the medium and low power laser assisted pulsed MAG hybrid Welding. Compared with the pulsed MAG welding, the assisted laser acts on the substrate before the arc to generate a stable keyhole and mass metallic vapor is generated near the keyhole leading the expansion of arc plasma morphology. The arc plasma appears obvious compression phenomenon under the action of medium power assisted laser. In addition, the keyhole has an attractive effect on the discharge channel and the height of the discharge channel becomes shorter, which makes the arc initiation stage more stable. The contact area between the arc root and the workpiece is expanded, in addition the laser has a compression effect on the arc, making the arc energy density higher and the energy use more efficient. The acting of assisted laser increased the droplet transfer frequency with the power increases, which essentially changes the electromagnetic force acting on the droplet. Others, the range of welding voltage distribution and the probability of high voltage distribution are reduced, and the transition is more stable.

Reduced pressure laser weld comparison to electron beam welds in Ti-6Al-4 V

Reduced pressure laser welds were made using a 6-kW commercial fiber-laser system on Ti-6Al-4 V and compared to electron beam welds of the same beam diameters as measured by beam diagnostics. The laser welds showed keyhole characteristics under easily achievable mechanical pumped vacuum levels of 1 mbar pressure that nearly matched the electron beam weld penetrations made at 9 × 10–5 mbar vacuum. Ti-6Al-4 V alloys were used to represent refractory metals such as vanadium, tantalum, zirconium, or molybdenum that require vacuum or highly protective inert gas protection systems to prevent adverse interactions with air and can be difficult to weld under non-vacuum conditions. Results show that laser weld depths of 20 mm with aspect ratios of 17:1 can be made under what appears to be stable keyhole behavior as the result of reduced pressure. The effect of fiber diameter was examined using 0.1-, 0.2-, and 0.3-mm fibers, showing that small spot sizes can easily be achieved at long focal length lenses of 400 and 500 mm. The 0.1- and 0.2-mm fibers produced keyhole welds with minimal amounts of porosity, which was only present at 2 kW or higher, while the 0.3-mm fiber produced keyhole welds with more rounded roots that were porosity free as shown by radiography up to the maximum power of 6 kW. Correlations between weld depth and processing conditions are presented for the reduced pressure laser. These results are directly compared to electron beam welds, facilitating design of future reduced pressure laser systems targeted for deep weld penetrations historically developed for electron beams.

Dynamic behavior of keyhole and molten pool under different oscillation paths for galvanized steel laser welding

This study investigated the dynamic behavior of a keyhole and molten pool under circular and zig-zag oscillating paths for laser welding on galvanized steel. A large variation in the length of the keyhole and a bulge in the molten pool resulted in more spatters in the case of a circular oscillating path, whereas a little spatter was observed in the case of a zig-zag oscillating path because of the relatively stable keyhole and molten pool observed by a high-speed camera. Numerical simulation results showed that zinc vapor entered the keyhole with a velocity of 93 m/s because of a large amount of zinc evaporation in the first half of the circular oscillating path, leading to a rapid increase in the keyhole size. In this case, the fluid near the rear wall of the keyhole accelerated to 1.7 m/s and gushed out of the molten pool in a spatter. In the zig-zag oscillating path, the zinc vapor continually entered the keyhole at a lower velocity, leading to a relatively stable keyhole. In this case, the maximum liquid velocity of the molten pool was 0.9 m/s, not reached the critical spattering velocity (1.36 m/s), thus no spatter occurred. These numerical results sufficiently explain the dynamic behavior of a keyhole and molten pool induced by zinc vapor, and an effective spatter suppression method is proposed.

Effect of beam oscillation frequency on spattering in remote laser stitch welding of thin-gage zinc-coated steel with keyhole penetration

In the paper, the effect of beam oscillation frequency on zinc-induced spatter behavior was investigated for remote laser welding of thin-gage zinc-coated steel. The experimental study showed that welding with zigzag beam oscillation of 5 cycles/mm exhibited significantly improved weld aesthetics and less spatter-induced mass loss compared to the baseline condition by conventional straight-line welding. In addition, the weld quality varied with the oscillation frequency. To understand the above-mentioned phenomena, numerical simulations of welding processes by straight line and zigzag oscillations were carried out to study process physics including keyhole opening, zinc evaporation rate, molten pool flow and keyhole wall temperature. It was found that with keyhole penetration mode, variation of keyhole opening exhibited consistent frequency of 135 Hz across welding with the straight line and welding with various beam oscillation frequencies. Welding with beam oscillation frequency larger than the keyhole oscillation of 135 Hz was observed to have a relative stable keyhole and reduced process spatter as long as the frequency is not too high to cause reduced laser energy absorption and inadequate recoil pressure to hold keyhole stable. Further analysis showed that beam oscillation led to a wide and less dense laser energy distribution with low energy gradient, and an adequate oscillation frequency resulted in consistent laser energy input along the feed direction, such as 5 cycles/mm. Together they contributed to large molten pool mass, few hot spots in the laser keyhole rear wall, stable zinc evaporation rate and reduced peak magnitudes of recoil pressure. All these are beneficial to reduction of keyhole opening variation amplitude and prevention of spatter. The application of spatter occurrence measures Zout and Zp proposed in our previous work further confirmed the observation that the welding with oscillation frequency higher than the keyhole oscillation frequency has lower possibility of spattering.

Laser beam welding setup for the coaxial combination of two laser beams to vary the intensity distribution

Deep-penetration laser beam welding is highly dynamic and affected by many parameters. Several investigations using differently sized laser spots, spot-in-spot laser systems, and multi-focus optics show that the intensity distribution is one of the most influential parameters; however, the targeted lateral and axial intensity design remains a major challenge. Therefore, a laser processing optic has been developed that coaxially combines two separate laser sources/beams with different beam characteristics and a measuring beam for optical coherence tomography (OCT). In comparison to current commercial spot-in-spot laser systems, this setup not only makes it possible to independently vary the powers of the two laser beams but also their focal planes, thus facilitating the investigation into the influence of specific energy densities along the beam axis. First investigations show that the weld penetration depth increases with increasing intensities in deeper focal positions until the reduced intensity at the sample surface, due to the deep focal position, is no longer sufficient to form a stable keyhole, causing the penetration depth to drop sharply.