Analysis of laser welding process and porosity suppression based on controllable adjustment of energy and distribution

The porosity produced in the 304 stainless steel laser welds was investigated. The laser welding process was conducted at a laser power of 10 kW. Different welding speeds and shielding gases were used. The porosity in the welds was characterized. The welding processes were observed using a high-speed video camera. The mechanism of the elimination of the porosity was revealed. The suppression of the porosity was made in N2 rather than Ar or He shielding gases. The porosity was mainly produced at the root and most of the porosity was less than 0.02 mm3. No significant differences of the metallic plume, liquid melt pool, and laser keyhole were found when Ar or N2 shielding gas was employed. The solubility of N2 in the liquid melt pool contributed to the reduction or elimination of the porosity in the 304-L laser welds.

A review porosity in aluminum welding

Porosity in fibre laser welds of two thixomolded heat resistant magnesium alloys AE42 and AS41 was investigated in detail, and porosity formation mechanism was discussed in terms of gas compositions in porosity. It is found that the area percentage of porosity in welds decreases with increasing welding speed, and can be correlated to width of weld metal. Microstructure observation and gas composition analysis in porosity show that the porosity in welds is mainly attributed to the micropores pre-existing in base metals during melting of AE42 and AS41 alloys by fibre laser welding, which are formed due to air entrapment during thixomolding process. Hydrogen rejection and Ar shielding gas entrapment are also the possible reasons for the porosity formation; however, their contribution is much smaller than that of pores in base metals. Furthermore, the addition of rare earth element may probably decrease porosity amount in the thixomolded Mg alloys and their welds.

During Underwater Wet Welding (UWW), the water that surrounds the arc decomposes liberating large amount of hydrogen and oxygen. As a consequence of the presence of these gases in the arc atmosphere and weld pool, porosity in the weld metal occurs. In the past years, many research programs had been carried out with the objective to reduce or eliminate porosity in wet welds. A simple way to accomplish this goal is using chemical elements or ingredients to promote or avoid certain chemical reactions in the weld pool. In conventional stick (shielded metal arc - SMA) electrodes, it is possible to add alloying elements or other ingredients through the external covering. A tubular covered electrode (TCE) (a hybrid process between SMA and flux cored arc - FCA welding) allows the addition of reactive elements in the hollow rod, separate from the other ingredients used in the flux covering. This way, it is possible to use exothermic elements, placed inside the tube, to control the oxidation reactions, but limiting these reactions to the arc plasma and in the weld pool. Exothermic additions in welding consumables can promote desirable oxidation reactions, change the metal transfer mode, reduce the cooling rate, and decrease the electrical dependence of the welding process. Theoretically, the application of flux cored shielded metal arc (FC-SMA) welding with exothermic additions will permit better control the weld metal composition and reduce the porosity in wet welds. This paper describes underwater wet welding with tubular covered electrodes that contain exothermic additions such as (CaC2) and aluminum (A1), and the influence of these ingredients on weld metal composition and porosity.

The application bottleneck of laser welding is being gradually highlighted due to a high prevalence of porosity. Although laser welding technology has been well applied in fields such as vehicle body manufacturing, the suppression of weld porosity in the laser welding of stainless steel containers in the pharmaceutical industry is still challenging. The suppression of bottom porosity was investigated by applying ultrasonic vibration, changing welding positions and optimizing shielding gas in this paper. The results indicate that bottom porosities can be suppressed through application of ultrasonic vibration at an appropriate power. The keyhole in ultrasound-assisted laser welding is easier to penetrate, with better stability. No obvious bulge at the keyhole rear wall is found in vertical down welding, and the keyhole is much more stable than that in flat welding, thus eliminating bottom porosity. The top and bottom shielding gases achieve the minimal total porosities, without bottom porosity.

The laser welding of Aluminum-Lithium alloy (Al-Li) alloy were conducted to investigate the weld porosity features in this paper. The results show that there are two kinds of weld porosity existing for laser welding Aluminum-Lithium alloy sheet. They are can be divided into metallurgical porosity and unstable keyhole porosity according to their different reason causing them. The mechanism of unstable keyhole porosity occurring was discussed according to the results of YAG laser welding processing. The methods how to reduce weld porosity for laser welding of Aluminum-Lithium alloy was also described.

Porosity is defined as cavity-type discontinuities formed by gas entrapment during solidification. Causes of porosity in fusion welds are the dissolved gases in weld metal and welding process variables that control the solidification rate. To study the mechanisms of porosity formation in weld metal, single-pass gas tungsten-arc weld metal was produced using the bead-on-plate technique on three nickel-copper alloys (80 wt pct Ni-20 wt pct Cu, 65 wt pct Ni-35 wt pct Cu, 35 wt pct Ni-65 wt pct Cu). Four different welding speeds were used under various amounts of nitrogen content in argon-shielding atmosphere. A qualitative model was proposed to characterize the effect of welding variables and solidification substructure on bulk and interdendritic porosity formation. Increasing amounts of nitrogen gas (from 0.2 pct to 6.0 pct in volume) introduced in argon-shielding atmosphere increased the amount of porosity in weld metal. The amount of bulk and total porosity increased as the solubility of nitrogen in the weld metal alloy decreased. The solidification rate of the weld pool is the most important factor controlling the mechanism of porosity formation. The observed amount of bulk pores in this study increased with the increase of welding speed; that is, if the time is insufficient for dissolved and evolved gases to escape during solidification, porosity will result. However, a decrease in the amount of interdendritic pores was observed with increasing welding speed in the 80Ni-20Cu and 35Ni-65Cu alloys. This decrease can be related to the effect of solidification rate on the balance between the disjoining pressure, resistance of the liquid film to be disrupted, repulsion of the bubble from the solidification front, and the hydrodynamic force resisting the movement of the bubble. This balance determines the ability of the cellular solidification front to “equilibrium” capture the pores. Furthermore, the observed decrease of interdendritic porosity with increasing welding speed (80Ni-20Cu and 35Ni-65Cu alloys) can also be related to the time for nucleation and growth of pores in the molten weld metal and their entrapment in the interdendritic channels of a dendritic solidification front. This phenomenon is considered a “nonequilibrium capture” of pores. On the other hand, the 65Ni-35Cu alloy that exhibited a structural transition in solidification substructure with the variation of welding speed showed a slight increase in the amount of interdendritic pores. This increase was correlated to the change of pore-capture mechanism from an equilibrium to a nonequilibrium mode as the solidification substructure changed from cellular to cellular dendritic. To substantiate that the controlling mechanism of interdendritic porosity formation is the nonequilibrium capture, a good correlation between the measured mean pore radius and the interdendritic arm spacing was found.

Effect of welding stability on process porosity in laser arc hybrid welding of dissimilar steel

The welding process has a wide range of applications in aerospace, military manufacturing, machinery, and other fields, and with the continuous improvement of welding technology requirements, the inhibition of porosity in the welding process is also increasing. This study, through the narrow gap laser welding characteristics and porosity formation mechanism of the depth of analysis, for the existing narrow gap laser welding method exists in the side wall of the weld fusion is poor porosity, poor uniformity of organizational properties and other issues, proposed ultrasonic-based welding method to achieve effective inhibition of porosity. The experimental analysis of ultrasonic vibration on the formation of porosity is carried out based on narrow-gap laser welding. The peak sound pressure of narrow gap welding increases when the ultrasonic current amplitude changes from 24A to 32A. Under different ultrasonic vibration conditions, the number of welds without applied ultrasound was as high as 95, while the number of porosities gradually decreased to 48 with the increase of applied ultrasound amplitude from 12% to 37%. Furthermore, as the ultrasonic amplitude increased, the residual height decreased by 0.45 ml. In addition, at fixed ultrasonic amplitude, with the increase of laser power, the porosity increased from 0.89% to 2.21%, and the average diameter of the porosity increased from 0.29 mm to 0.43 mm. The porosity for the porosity diameter of less than 200 μm was reduced to 0.020%. The percentage of stomata with a diameter greater than or equal to 200μm increased to 2.098%. This study analyzes the inhibition effect of ultrasonic vibration on porosity to a certain extent. Moreover, the inhibition effect of ultrasonic vibration in narrow gap laser welding is significant and the smaller the laser power under the same amplitude, the better the inhibition effect, which provides a valuable reference for the research of the narrow gap laser welding process.

Image processing has been widely adopted as an effective technology for analyzing weld forming quality which is greatly affected by the welding process parameters. In this paper, an L25(53) orthogonal experiment is designed to investigate the effects of welding process parameters on the weld forming quality in laser welding of aluminum alloy. The weld characteristics including the weld width (WW), weld penetration (PD), weld area (WA) and weld porosity (WP) under the conditions of the different welding process parameters consisting of the laser power (LP), welding speed (WS) and defocus distance (DD) are extracted from the laser welding experiment based on image processing. The effectiveness of the weld characteristics extraction method is verified by comparing the extracted results with the measured results. It is found that the WW, PD and WA are all significantly influenced by the LP among the three welding process parameters while the influences of the three process parameters on the WP are insignificant. The DD has a significant influence on the PD and the WS has a significant influence on the WA. The corresponding significance of influence is lower than the significance of influence of LP. The analysis results are conducive to the optimization of laser welding process parameters and improvement of welding quality.

In the recent past great study has been dedicated to porosity formation in laser welding. Using impressive apparatus for the experimentation Japanese authors have pointed out the main phenomenological causes of porosity produced by laser keyhole welding. Moreover, they have evidenced the effect of process parameters and bead configuration on porosity formation. Keyhole oscillation and consequently local vaporisation of the molten pool have been found to be present in all metals (stainless steel, carbon steel, aluminium and magnesium alloys) and in all process conditions (CO2 or Nd:YAG systems, continuous wave or pulsed lasers). Since keyhole porosity can not be totally avoided, the laser welding process should be accurately selected and process parameters designed in such a way as to minimise pore formation. The paper is aimed at studying the effect of laser configuration, single beam or dual beam, on the keyhole porosity. A dual beam system, allowing the laser beam to be divided in two beams, both of them working in keyhole mode, has been studied and compared to the traditional single beam welding process. In order to compare the two laser configurations, quantitative features of pores have to be individuated. Since the pores in the bead are supposed to have a negative effect on fatigue strength of the welded component, the main characteristics of investigated porosity were the porosity percentage and the pore diameter. The experimental results show that the dual beam configuration is effective in pore area reduction, if the related process parameters (inter-beam distance and feed rate) are accurately selected.

Welding within forty eight (48) hours of joint preparation will help to prevent porosity in weld deposited at a Gas Tungsten Arc Welding (GTAW) process. This research aims at preventing weld porosity and improving weld quality using diagnostic case statistics for percentage dilution. Poor process conditions due to faulty operator error, use of wrong techniques, use of incorrect consumables and the use of bad weld grooves, are the common causes of porosity in weld deposits. In order to achieve the objectives of this research, the response: percentage dilution is used with four input process parameters such as current, voltage, speed and gas flow rate to optimize percentage dilution. Eradication of weld porosity was achieved using a current of 160.5 Amperes, voltage 17.5Volts, speed 100.6mm /min and gas flow rate of 16.07 Lit/min with a value of 46.9% percentage dilution. An established result from literature survey corresponds with the results in this study.Keywords: Weld Porosity, diagnostic case statistics, Percentage dilution (D), Gas Tungsten Arc Welding process

In recent years, to meet society’s increasing demands to reduce environmental pollution, the industry’s attention has been drawn to the subject of lightweight construction. Especially die-cast aluminium alloys offer their user a variety of design options for targeted weight reduction. However, a problem with the use of this material is the cohesive connection using welding technology. The main problem are the release agent residues as well as gas and oxide residues, trapped in the die-cast, which diffuse during the welding process in the direction of the liquid aluminium melt and result in a weld porosity. Due to the high heating and cooling rates resulting from the laser beam welding and the resulting outgassing possibilities of the liquid melt, the process of laser beam welding is particularly susceptible to the joining of aluminium die-cast components and, despite its many advantages, can currently only be used by complex and expensive additional measures. This is where the research activities of the Department for Cutting and Joining Manufacturing Processes (tff) start. The department deals with a specific influence on the prevailing melt flow through a sound wave superposition during the welding process. For the coupling of the sound waves, flexibly positionable piezoshakers are used, which allow greater degrees of freedom with respect to the welding geometry.

Fiber laser-cold metal transfer arc hybrid welding of pure copper was studied. Weld porosity was tested by X-ray nondestructive testing. Microstructure and fracture features were observed by scanning electron microscopy. Mechanical properties were evaluated by cross weld tensile test. Full penetrated and continuous welds were obtained by hybrid welding once the laser power reached 2 kW, while they could not be obtained by laser welding alone, even though the laser power reached 5 kW. The ultimate tensile strength (UTS), the yield strength (YS), and the elongation of the best hybrid weld material were up to 227, 201 MPa, and 21.5 pct, respectively. The joint efficiencies in UTS and YS of hybrid weld were up to 84 and 80 pct of the BM, respectively. The fracture location changes from the fusion zone to the heat-affected zone with the increase of laser power. Besides, the mechanisms of process stability and porosity suppression were clarified by laser–arc interaction and pool behavior. The strengthening mechanism was discussed by microstructure characteristics.

Keyhole-induced porosity as one of the serious defects in deep-penetration laser welding, negatively affected the mechanical properties of welded structures. Keyhole laser welding is an effective method to fully penetrate the face and web plate for T-joint steel structures used in ship building and offshore platform. It is found that shielding gas parameters play an important role in laser welding process; different flow rates, blowing angle, and distance of shielding gas are considered to study the porosity number and distribution in order to make a systematic investigation on keyhole-induced porosity formation and its influencing factors. The result shows that appropriate shielding gas flow rate, blowing angel, and distance to molten pool could help to achieve less porosity for this T-joint. Also, a high-speed photography of 1,000 fps was applied to observe the plasma over the keyhole and fluid flow of molten pool to better understand the formation process of porosity.