Porosity Free Welds Research Articles

Weld zone porosity elimination process in remote laser welding of AA5182-O aluminum alloy lap-joints

Abstract A laser beam with a travelling speed of 43.5 m/min along circular oscillation paths was utilized to weld AA5182-O aluminum alloys in a lap joint configuration for automotive application. Experimental results showed that the porosities formation in the weld zone can be dramatically reduced compared with the case following the straight welding path and the welding speed of 3.0 m/min. The porosity-free welds were achieved and the corresponding tensile shear strength was significantly enhanced. The relationship between overall area percentage of the porosity in the weld zone and the circular oscillation parameters, which include amplitude A and period length L, was investigated. It was found that a combination of A and L with the value of A larger than the laser beam focal diameter was crucial for eliminating the porosity in the weld zone. A welding mode that combines the keyhole mode with conduction mode was proposed to explain the porosity elimination mechanism. A designated experiment was conducted to validate the hypothesis.

Optimization of Percentage Dilution in Butt Welds to Prevent Weld Porosity Using Response Surface Methodology

A detailed analysis to optimize percentage dilution in butt welds in order to prevent weld porosity has been carried out in this study. The response surface steepest ascent was used to optimize values of percentage dilution in a butt weld using gas tungsten arc welding process. Predictions for percentage dilution were made using the second order mathematical model for response surface methodology and results obtained showed that percentage dilution value of 45.73% was able to eradicate weld porosity in butt welds. Overall results of percentage dilution analyzed, recorded an optimal value of 45.73% with values of welding current, welding voltage and welding speed as 131.5amperes, 17.4 volts and 100mm/min respectively. A defined quantity of percentage dilution value of 45.73% established in this study should be used at a GTAW process to provide sufficient amount of dilution for controlled solidification of molten metal during welding operation in order to produce porosity-free welds. This work presented results that revealed percentage dilution values that will prevent weld porosity in butt welds with no gas traps or voids in welded joints produced.

Sinusoidal oscillating laser welding of 7075 aluminum alloy: Hydrodynamics, porosity formation and optimization

This paper reports a mechanism understanding of how the sinusoidal beam oscillation reduces the porosity defects during laser welding of 7075 aluminum alloy. In this study, the coupled time dependent keyhole and weld pool dynamics as well as the porosity formation during the oscillating laser welding process were simulated. Also, the keyhole and weld pool behaviors were experimentally investigated using high speed imaging and the porosity rates of the welds with or without beam oscillations were characterized. The results indicated that the beam oscillation modulated the fluid flow in the weld pool into a swinging mode and stabilized it. In this mode, the average flow velocity exhibited periodical oscillation with the double frequency of that of the laser beam oscillation. The velocity oscillating amplitude decreased as the frequency of beam oscillation increased. When the oscillating frequency of the fluid flow increased to 100 Hz, the swinging flow with a small oscillation amplitude could be obtained. The results also indicated that with the beam oscillation the keyhole stability was improved and the porosities were reduced in the welds. This is mainly because the keyhole diameter was significantly increased and the surface tension effect, the major cause responsible for the keyhole oscillations, was weakened. However, in cases with beam oscillation, there were still some porosities in the welds. An optimization method to reduce the porosities by limiting the time of laser staying in unstable area was proposed. Porosity free welds were successfully made. This study could help improving the understanding of the weld pool dynamics in laser welding and guide the selection of the process parameters.

Low Temperature Embrittlement studies on Stainless Steel 304 LN TIG Welds

Austenitic stainless steel 304LN (SS 304LN) is widely used in nuclear industry. TIG welding is selected here to weld the stainless steel plates because it offers the possibility of producing high quality and porosity free welds. However, weld metal experiences hot cracks during welding of austenitic stainless steels. Due to this, austenitic stainless steel welds are designed to have 3-8% delta ferrite in the weld metal to avoid hot cracks during solidification. With the presence of delta ferrite, when these materials are exposed to elevated temperature (300 - 500°C) the weld transforms into embrittlement phases such as iron rich α and chromium rich α, G phase and M23C6. These embrittlement phases deteriorate the mechanical properties of the weld. In this study, an attempt is made to study the low temperature embrittlement on SS 304LN TIG Welds. The low temperature embrittlement behaviour on aged samples at (300, 350, 400°C) was evaluated for 10000 hours by mechanical property testing and micro structural examination. Mechanical testing involves Hardness test and Impact test. Delta ferrite measurement was done to see any transformation of delta ferrite. Fracture surfaces were characterized using SEM. Increase in hardness with increase in aging temperature and decrease in delta ferrite content at 350°C and 400 °C clearly indicate that aging temperature affects kinetics of transformation of delta ferrite. Scanning Electron Microscopy did not show any transformation of delta ferrite. It was found that aging treatment has a strong influence on impact toughness. Decreases in impact toughness value especially at sub – zero temperatures indicate kinetics of transformation of delta ferrite is affected by aging temperature during low temperature aging.

Method for producing porosity-free joints in laser beam welding of maraging steel 250

Laser beam welding of 4.5-mm-thick sheets of maraging steel 250 using a 4-kW ytterbium fiber laser was studied. Maraging steel 250 is widely used in the aerospace industry in applications that require high strength. Therefore, in order to join two parts made of maraging steel 250, it is essential to produce a high-quality weld that meets weld bead geometry requirements, without any porosity formation in the weld that may lead to premature fatigue failure. In order to achieve a reliable and repeatable welding process, we investigated the influence of welding parameters on the welded process responses, i.e., weld penetration depth, bead width, and potential formation of porosity. A complete model was developed using multiple regression analysis. Based on this model, we developed a method for selecting welding parameters that assure production of porosity-free welds with the required geometry. We used this method to automate the industrial welding process of a part-family of maraging steel 250 cylinders.

Porosity formation mechanisms in cold metal transfer (CMT) gas metal arc welding (GMAW) of zinc coated steels

The porosity formation in cold metal transfer (CMT) gas metal arc welding (GMAW) of zinc coated steel is studied over a wide range of the heat inputs (160–250 J mm− 1), which shows low porosity in weld bead ( < 2% of bead area) in low ( < 250 J mm− 1) and high (>350 J mm− 1) heat inputs and maximum at medium (250–350 J mm− 1) heat inputs. The high speed imaging of weld pool shows that the highest frequency of zinc vapour escapes at high heat inputs compared to other conditions. Numerous experiments show that size and location of pores along with escaping of zinc vapour are the results of competition of viscosity of weld pool against buoyancy and vapour pressure within the time required to reach solidification temperature. Based on this concept, mechanisms involved in porosity formation, growth and escape phenomena are disclosed, which can help select the optimised welding conditions to obtain porosity free welds in CMT-GMAW of zinc coated steels.

Tensile Fracture in Laser Welding Joints of Al-Cu-Li-Mg-X Alloy Bearing Gaseous Porosity

This article measures the impact of gaseous porosity on the mechanical performance of the laser welded thin sheet of an aerospace-grade Al-Cu-Li-Mg-X material. The laser fabricated joints were characterized by optical microscopy and scanning electron microscopy to reveal the metallurgical features. A uniaxial tensile test was carried out to measure the mechanical strength of the porosity free welds, and porosity bearing welds. The results showed that the welds bearing gaseous porosity performed nearly 24% lower mechanically than the porosity free welds. Fractography exposed the presence of fine spherical holes in porosity bearing welds, which acted as crack nucleation sites, and their further growth lead to a sudden failure at higher strain rates. Microstructural observations revealed that the laser processing has transformed the superior age-hardened microstructure of the base metal into a new as-cast microstructure. The freshly evolved microstructure was held responsible for the low mechanical properties of the joints with respect to the base metal.

Self-optimization in tool wear for friction-stir welding of Al 6061+20% Al2O3 MMC

Tool wear and the rate of wear for hardened, steel, right-hand screws rotating at 1000 rpm in the friction-stir welding of Al 6061+20 vol.%Al2O3 particles were observed to decrease for increasing weld or traverse speeds. When sufficiently long traverse distances were reached, tool wear became small or negligible, and an optimized tool shape emerged. This shape was slightly different at 6 and 9 mms−1 weld speeds but in each case a self-optimized tool shape emerged. This self-optimizing wear phenomena and tool shape result by counter motions of solid-state flow regimes which depend upon both tool rotation speed and actual weld traverse speed. Although sound, porosity-free welds are obtained with both the unworn, threaded pin tool and the worn, unthreaded pin tool, microstructures vary and the worn pin tool produced a narrower heat affected zone with less drop in hardness than the threaded pin tool.

Filler Metal Selection for Welding a High Nitrogen Stainless Steel

Cromanite is a high-strength austenitic stainless steel that contains approximately 19% chromium, 10% manganese, and 0.5% nitrogen. It can be welded successfully, but due to the high nitrogen content of the base metal, precautions have to be taken to ensure sound welds with the desired combination of properties. Although no matching filler metals are currently available, Cromanite can be welded using a range of commercially available stainless steel welding consumables. E307 stainless steel, the filler metal currently recommended for joining Cromanite, produces welds with mechanical properties that are generally inferior to those of the base metal. In wear applications, these lower strength welds would probably be acceptable, but in applications where full use is made of the high strength of Cromanite, welds with matching strength levels would be required. In this investigation, two welding consumables, ER2209 (a duplex austenitic-ferritic stainless steel) and 15CrMn (an austenitic-manganese hardfacing wire), were evaluated as substitutes for E307. When used to join Cromanite, 15CrMn produced welds displaying severe nitrogen-induced porosity, and this consumable is therefore not recommended. ER2209, however, outperformed E307, producing sound porosity-free welds with excellent mechanical properties, including high ductility and strength levels exceeding the minimum limits specified for Cromanite.

High pwder Nd-YAG laser welding of SiC particle reinforced aluminium alloy 2124

The effects of different Nd-YAG laser output waveforms on the weldability of a SiC particle reinforced aluminium alloy 2124 have been studied. The results show that although the square waveform can produce the greatest depth-of-penetration among the three different waveforms studied, i.e. continuous wave, sine wave and square wave, a high level of porosity was observed in the weld. Alternatively, porosity free welds with a reasonable depth-of-penetration can be obtained by using a sine waveform operated at high peak powers. However, the results also show that it would be difficult to stop theformation of aluminium carbides entirely in the fusion zone simply by varying the laser output parameters and the waveform. In order to stop the, formation of carbides, a new laser joining technique has been developed. This involved brush plating of nickel on the joining surfaces before laser welding. The results of the corrosion tests of the weld zone show that welding with a nickel coating would result in a much lower corrosion current than that without nickel coating.

CO2 shielding gas effects in laser welding mild steel

The most widely used shielding gases for laser welding of steels are helium and argon. Helium produces significantly more penetration than argon in penetration-mode laser beam welding. Another gas that has been proposed as an alternative to these inert gases is carbon dioxide. The benefits of using carbon dioxide as a shielding gas for laser welding are that it costs less than helium and argon and that it provides nearly the same penetration as helium. The major drawback to its use as a shielding gas for mild steel is that it can cause porosity and other weld discontinuities. In this investigation helium and carbon dioxide were investigated as shielding gases in the laser welding of a fully killed 1018 steel, a semikilled 1018 steel, and an unkilled 1018 steel. Helium was generally found to produce discontinuity-free welds, although small quantities of very fine centerline porosity were observed in the fully killed and semikilled steels. Carbon dioxide was found to provide an average of 70% of the penetration obtained with helium. The surface appearance and formation of porosity in welds made using carbon dioxide shielding was found to be dependent on the amount of deoxidizing element in the weld pool. While porosity-free welds were formed in the semikilled steel (0.22% Si), low silicon or other deoxidizer content resulted in poor surface appearance and entrapped porosity in both the fully killed and unkilled steels in this investigation, presumably due to the formation of carbon monoxide in the weld pool. The addition of ferrosilicon powder to the weld pool effectively controlled the formation of carbon monoxide in these welds and resulted in porosity-free welds with carbon dioxide shielding. The results of this investigation indicate that carbon dioxide can be used as a cost-effective shielding gas for penetration-mode welding of carbon steels. It was found that sufficient levels of deoxidizing elements must, however, be available in the molten pool to control porosity.

電子ビーム溶接部の性質に関する二,三の研究

Chemical compositions, porosities and mechanical properties of electron-beam welds were studied of ractive and refractory metals, aluminium alloy, stainless steel and carbon steel by comparing with controlled atmosphere TIG welds and the following conclusions were obtained :(1) The contents of oxygen and nitrogen in titanium and zirconivm were changed neither by electron-beam welding (EBW) nor by TIG.By EBW, contents of carbon and oxygen in sintered niobium sheets, that of carbon in a sintered tantalum sheet and that of nitrogen in a type 304 stainless steel plate were considerably reduced.(2) X-ray radiographs of both EBW and TIG welds in sintered niobium, molybdenum and tungsten sheets showed usually many porosities in weld metal near the fusion line, while porosityfree welds were obtiand in vacuum melted sheets.Repeated electron-beam bead welding in sintered sheets was very effective in eliminating porosities in weld metals.(3) An element of which the partial pressure at melting point of a base metal is higher than about 10-2mmHg is likely to be vaporized during EBW in a vacuum of (1-5) × 10-5mmHg.(4) Subsize V-charpy impact values of commercially pure titanium, zirconium and Zircaloy-2 plates were generally far greater in EBW welds than in TIG welds.(5) Brittle behavior of arc welds of molybdenm sheets was not improved by EBW, probably because of grain coarsening in weld zone.(6) The joint efficiency of EBW, welds of a 1 mm thick 24 S aluminum alloy sheet was about 87% and much greater than 50 to 70% in TIG welds, owing to the very narrow width of weld metal and heat-affected zone.(7) The joint efficiency of EBW weld of a 6 mm thick 17-7 PH plate was 100% and was greater than 95% in TIG weld.

ON THE WELDABILITY OF ANP. PLATE

We have previously reported1) the weldability of Al-Mg alloys. In this experiment, we studied the weldability of ANP plate, using Sigma, Heliarc and oxyacetylene welding.The results obtained are summarized as follows:(1) Porosity-free welds are obtained using cleaned filler wire and mother plate.(2) In Sigma welding, low arc voltage and current produce porosity weld.(3) Argon arc welding (Sigma and Heliarc welding) are recommended for ANP plate, for mechanical properties welded by these processes are the best. The tensile efficiencies are over 90%.(4) Oxyacetylene welding is not recommended for thick ANP. plate. Mechanical properties of this weld are extremely inferior to that of argon arc welding.(5) The electro-chemical potential of weld zone is nobler than that of mother plate, and weld zone is anticorrosive in sea water.