Weld Pore Research Articles

Microstructure transformation and pore formation mechanism of Mo-14Re alloy weld by vacuum electron beam welding

The vacuum electron beam welding (EBW) properties of Mo-14Re alloys were studied in this work. The weld morphology and microstructure were observed by optical microscope (OM) and scanning electron microscope (SEM). By using 3D Measuring Laser Microscope, energy dispersive spectroscopy (EDS), electronic probe micro analysis (EPMA) and transmission electron microscope (TEM), the weld pore was further characterized. In addition, its formation mechanism was analyzed. The results showed that the grains in fusion zone (FZ) of the powder metallurgy (PM) alloy weld obviously grew up to about 226.6 ± 75.7 μm and there were a large number of 12∼80 μm spherical pores. The element oxygen was enriched in the pores and a layer of MoO2 was formed on the surface. The formation of pores may be closely related to the micropores and high oxygen content in the materials. Under the strong stirring of the molten pool, the generated carbon monoxide (CO) and molybdenum monoxide (MoO) bubbles couldn't escape and remained in the weld to form pores. However, no pores were found in the melted alloy weld. It was difficult to generate bubbles by homogeneous nucleation for the melted alloy. Due to the grain coarsening, the microhardness value of the weld significantly decreased.

Study on Microstructure and Properties of Mechanical Vibration-Assisted MIG Welded Joint of Aluminium Alloy Car Body

The aluminium alloy MIG welding of car body has problems such as joint softening and porosity in the weld, which directly affects the welding quality of the body structure. In this study, mechanical vibration was introduced into the aluminium alloy MIG welding process as an external excitation by improving the joint microstructure and performance of aluminum alloy MIG welding. In the research process, optical microscope and scanning electronic microscope were used to observe the microstructure of the joint, observe the size of grains, detect the number of pores, test hardness, take tensile test, and study the influence of frequency of external vibration excitation on weld forming, pore distribution, microstructure and mechanical properties. Studies have shown that mechanical vibration can effectively refine the weld grain, reduce the number of weld pores, increase the penetration depth, improve the strength and hardness of the joint, and improve the welding quality of aluminum alloy. And with the increase of vibration frequency, the grain size and number of pores of the weld gradually decrease, and the penetration depth and tensile strength and hardness of the joint gradually increase.

Study on Corrosion Fatigue Properties of the Welded Joints of SMA490BW Weathering Steel

The polarization curve test and corrosion fatigue test of SMA490BW weathering steel welded joints for high-speed train bogies were carried out by electrochemical workstation and fatigue testing machine, respectively, and the fatigue fractures were observed and analyzed by scanning electron microscope (SEM). The results show that: The mechanical properties of the SMA490BW weathering steel welded joints welded by MAG welding are good, the tensile strength is 519MPa, and the elongation after fracture is 26.67%. In 3.5wt% sodium chloride solution, the self-corrosion potential of the heat-affected zone is the lowest, the self-corrosion current density is the largest, and the corrosion resistance is the worst, the weld is in the middle, and the base metal is the best. The corrosion fatigue limit of SMA490BW weathering steel welded joints in 3.5wt% sodium chloride (NaCl) solution is 236.55MPa, which is much lower than the fatigue limit of 335.22MPa in air. SEM fracture analysis showed that the fatigue cracks of the joints in the air originated from the weld pores, while the corrosion fatigue cracks originated from the pitting pits on the surface of the specimens.

Fatigue life prediction for Al–Zn–Mg alloy butt welded joints based on a fatigue damage model accounting for the influence of interior porosity

AbstractThis paper proposed a high‐cycle fatigue life prediction method for Al–Zn–Mg alloy welded joints considering the thermal process of welding and the effect of welding pores. First, the welding process was simulated by a numerical thermal–mechanical analysis. The residual stresses were then obtained and applied as the initial stress field of the subsequent fatigue life calculation. Second, a fatigue damage model was constructed by introducing a dimensionless quantity to equivalently describe the effect of interior pores. Further, a corresponding damage mechanics–finite element numerical method was numerically implemented by the ABAQUS platform to calculate the high‐cycle fatigue life for Al–Zn–Mg alloy welded joints. The calculated lives have good consistency with test data, which validates the feasibility and applicability of the proposed model and the calculation method. Finally, the curves of the cyclic stress versus fatigue life of Al–Zn–Mg alloy welded joints with different interior pore sizes were calculated to explore the influence law of the welding pore.

Effect of pulse mode and frequency on microstructure and properties of 2219 aluminum alloy by ultrahigh-frequency pulse Metal-Inert Gas Welding

Based on self-developed ultra high frequency pulse Metal-Inert Gas Welding (UFP-MIG) power source, the ultra-high frequency pulse (UFP) current was studied as an effective way for improving the microstructure and properties of 2219 aluminum alloy joints. The welding pores reduction performance, microstructures, microhardness, and tensile properties of conventional pulsed MIG(CP-MIG) welds and the UFP-MIG welds with different modes and frequencies were investigated. The results indicated that the UFP-MIG process could almost completely eliminate welding porosity in the joints of 2219 aluminum alloy. Compared with CP-MIG welding process, the grain size of the UFP-MIG welding zone was reduced and gradually decreased as the frequency of UFP increased. Similarly, the grain refinement effect also appeared in the heat affected zone (HAZ) near the welds, and the grain size could be reduced by up to 14.85%. Meanwhile, the morphology and uniformity of θ phases and α+θ eutectics along grain boundaries of UFP-MIG welds were also improved. The microhardness in the weld zone and partially melted zone (PMZ) of UFP-MIG weld joints with different modes and frequencies were significantly improved. The highest joint average microhardness occurred at peak pulse mode (PP) with 40 kHz(75.17HV), which was 27.9% higher than that of CP-MIG (58.74HV). The strength and ductility of welded joints were simultaneously improved by UFP-MIG weld process. Fracture analysis results showed that the large reduction of weld pores, the refinement of weld grains, and the lower segregation of Cu element in the eutectic were responsible for the joint tensile properties improvement.

Slip-driven and weld pore assisted fatigue crack nucleation in electron beam welded TC17 titanium alloy joint

High cycle and very high cycle fatigue (VHCF) crack initiation mechanism of TC17 alloy joint with heterogeneous microstructures in FZ has been studied at the stress ratio of 0.1. Intrinsic deficiencies of slip deformation in β grains with few martensites and cracking coarse grain boundary between prior β grains with martensite colonies account for the high cycle fatigue (HCF) of joints. The average diameter of intergranular welding pores regarding VHCF crack nucleation is measured to be ∼36 μm. The driving force governing the crack nucleation from intrinsic deficiencies in HCF to porosity defects in VHCF is ∼1.66 MPa·m1/2.

Insight into the feed/permeate flow velocity on the trade-off of water flux and scaling resistance of superhydrophobic and welding-pore fibrous membrane in membrane distillation

The primary problem with membrane distillation (MD) for concentrating hypersaline wastewaters is its severe membrane scaling induced by increasing salt concentration of wastewater. Herein, we developed a superhydrophobic and welded-pore polyimide fibrous membranes (SH-welded PI FM) combined with optimizing feed/permeate flow velocity to alleviate membrane scaling and improve membrane water flux simultaneously. The experiments reveal that the SH-welded PI FM functionalized with welding pores avoids membrane pore deformation induced by increasing flow velocity and superhydrophobicity reduces the overall adhesive interaction between crystal and membrane surface to against membrane scaling. Comparative experiments and simulation results reveal that increasing flow velocity is also beneficial for vapor transmission by partially overcoming membrane temperature and concentration polarization and alleviating membrane scaling by producing asymmetrical disturbation on membrane two surfaces. However, the progressive increase will aggravate membrane scaling because of the excessive disturbance of flow velocity to fibers enlarging membrane pores. Therefore, designing welded-pore and superhydrophobic fibrous membrane combined with optimizing flow velocity is extraordinarily effective in simultaneously improving membrane water flux and mitigating membrane scaling for concentrating hypersaline wastewaters. These findings are significant to break the trade-off of membrane water flux and scaling for MD to concentrate hypersaline wastewaters for practical application in industries.

Effects of Sc on laser hot-wire welding performance of 7075 aluminum alloy

In order to clarify the effects of Sc modification on laser hot-wire welding performance of 7075 high strength aluminum alloys, the changing rules of weld formation, microstructure, element loss, mechanical performances and pores of weld joints were investigated during laser fusion welding, laser hot-wire welding and Sc-modified laser hot-wire welding. Comparing with laser fusion welding, the filler wire in laser hot-wire welding brought welding material and hot wire energy, resulting in better weld formation and sufficient fused weld joint. In laser hot-wire welding, tensile strength and elongation of weld joint were enhanced, element loss of weld joint was dramatically reduced and weld pore was reduced slightly. However, grains in fusion zone were relatively coarse, which caused lower microhardness in laser hot-wire welding. In Sc modified laser hot-wire welding, Sc modification led to smooth and continuous weld joint. During solidification of weld joints, Sc could combine with base metal to form Sc-containing alloy phase, which could lead to the formation of refined and equiaxed grains, thereby enhancing the yield strength of microstructure and increasing the microhardness, tensile strength and elongation of weld joint. In addition, Sc modification could increase the fluidity of molten pool, which thus promoted the escape of bubbles from the molten pool and hindered the formation of weld pores.

Mechanism to Reduce the Porosity during Argon Arc Welding of Aluminum Alloys by Changing the Arc Angle

A mechanism to reduce the porosity by changing the arc angle during aluminum alloy welding was studied. Industrial computed tomography was used to scan the welds with different arc angles, and the scanned model was processed by a specific software package to obtain the digital size and position of weld pores. The forces acting on the pores in the molten pool explained the test results that the number of pores decreases and the average size increases. As the inclination angle of the arc increased, the vertical component that prevented the bubble from rising decreased, and the horizontal component that pushed the molten metal flow and promoted the nucleation and growth of the bubbles increased. A horizontal movement during the droplet transition as the arc inclination was produced, which was conducive to the growth and overflow of bubbles. The theoretical analysis and temperature field measured by a far-infrared with different torch angle showed that when the arc was tilted from 0, the shape of the molten pool changed from the circle to the ellipse. The long axis of the ellipse increased as the bevel angle of the arc increased. This showed that the molten metal existed a longer time for the bubbles to escape from the molten pool when the angle of the arc increased. The paper provides fundamental insights into a mechanism for porosity reduction during the welding of Al alloys.

Experimental Research on The Distribution Law and Formation Reasons of As-cast Magnesium Alloy Laser Welding Porosity

Characteristics and distribution of the weld porosity are observed by using OM and SEM after as-cast magnesium alloy AZ91D is welded by CO2 laser. And then the formation model of two types of porosity is established by use of organization composition and distribution of metal base. Besides, the mechanism of pores in welding is explained on three aspects including microscopic porosity, macroscopic porosity and escape of bubbles. Based on these models and mechanisms, the shape characteristics and distribution of a welding porosity in a welding seam is explained and the formula of bubbles escape is also given. In addition, the measures to prevent welding porosity are discussed preliminarily. And based on the preliminary research on this topic, when (0.2%Ca+0.4%Y) is added to AZ910D magnesium alloy and high strain rolling is used, the laser weld porosity ratio of magnesium alloy can be effectively reduced.

Tensile and fatigue behavior of electron beam welded TC17 titanium alloy joint

The tensile fracture was originated from the strain concentration in equiaxed grain region adjacent to the fusion center, where a lower micro-hardness occurred when compared to the columnar grain region in the fusion zone. A higher stress level was more potential to fabricate a fatal fatigue crack initiating from the intrinsic metallic deficiencies. At a lower stress of 180 MPa, fatigue crack appeared to initiate from the intergranular welding pores other than the intrinsic metallic deficiencies. Furthermore, intergranular welding pores induced fatal crack initiation became the major fatigue crack initiation mechanisms when the fatigue stress amplitude down to 140 MPa.

Ultrasonic vibration assisted tungsten inert gas welding of dissimilar magnesium alloys

The effects of ultrasonic vibration assisted (UVA) treatment on the microstructures and mechanical properties of MB3/AZ31 dissimilar magnesium (Mg) alloy joints were studied by microstructural characterization, micro-hardness testing and tensile testing. Results indicate that the welding pores are eliminated and coarse ￎﾱ-Mg grains of fusion zone are refined to 26 ￎﾼm, owing to the acoustic streaming effect and cavitation effect induced by the UVA treatment with an optimal ultrasonic power of 1.0 kW. In addition, Mg17Al12 precipitation phases are fine and uniformly distributed in the whole fusion zone of weldment. Micro-hardness of fusion zone of the Mg alloy joints increases to 53.5 HV after UVA process, and the maximum tensile strength with optimized UVA treatment increases to 263 MPa, which leads to fracture occurrence in the Mg alloy base plate. Eventually, it is experimentally demonstrated that robust MB3/AZ31 Mg alloy joints can be obtained by UVA process.

Underwater local dry cavity laser welding of 304 stainless steel

The relationship between the gas rates and the shielding condition of the double-layer gas curtain nozzle utilized in underwater laser beam welding (ULBW) was observed. The welding morphology, weld porosity and mechanical properties of butt joints produced in different process parameters were investigated. With the increase of the heat input, the welding porosity decreased firstly and then increased. Higher welding porosity degraded the mechanical properties significantly and the formation of the welding pores could be attributed to the water vapor generating on the bottom surface of the joint and the solidification rate of the molten pool. The high-quality butt joint of ULBW without welding pores was obtained under optimal process parameters, whose tensile strength, impact toughness and microhardness profile approached to these of the in-air welded joint. Compared with in-air joint, the lathy ferrite appears in the ULBW bead and some equiaxed crystals were replaced by the columnar dendrites in the weld center region owing to the cooling effect of water.

Effect of Porosity on the Fatigue Behavior of Gas Metal Arc Welding Lap Fillet Joint in GA 590 MPa Steel Sheets

Weld defects such as blowholes and surface pores occur due to zinc vaporization during the gas metal arc welding of lap fillet joints of Zn-coated steel. In this study, the effect of porosity on fatigue behavior was investigated. A Zn-coated steel sheet with the strength of 590 MPa and a thickness of 2.3 mm was used as the base material. Three kinds of specimens with weld pore defects, such as blowholes and surface pores, were prepared and the tensile shear strength, hardness and fatigue behavior were investigated. The pore defects in the welds reduced the tensile shear strength. In the fatigue test, at higher load stresses between 122 and 366 MPa, pore defects reduced the fatigue life of the weld. However, the pore defects in the welds did not significantly affect the fatigue life of the welds at stresses below 92 MPa.

Quantitative Analysis of Weld-Pore Size and Depth and Effect on Fatigue Life of Ti-6Al-2Zr-1Mo-1V Alloy Weldments

Titanium-alloy weldments have been extensively used in engineering structures, and accurate estimation of the fatigue life is beneficial for avoiding catastrophic failures in titanium-alloy structures. The weld pore is a common defect in these weldments that significantly influences the fatigue process. In this study, the weld-pore size and depth and its effect on the fatigue life of Ti-6Al-2Zr-1Mo-1V (TA15) alloy weldments are investigated by adopting fatigue tests and fracture observations. The results show that fatigue crack initiations occurred at the weld pores for all specimens. The diameter and depth of all weld pores on fracture surfaces were then measured, and an indicator, P, was proposed, defined as a combination of pore diameter and depth. It was found that all the fatigue cracks initiated from the pore have the smallest P indicator, which suggests that P can be used to judge the location of crack initiation in an individual sample. Moreover, a model was developed based on P to estimate the fatigue life of weldments, considering the effects of weld-pore size and depth. Finally, analogous fatigue tests were carried out for model verification, and results show that the proposed model has a higher accuracy compared with several typical models. The findings of this study can be helpful in estimating the fatigue life and fatigue design of titanium-alloy weldments.

Study on Pores in Ultrasonic‐Assisted TIG Weld of Aluminum Alloy

Ultrasonic‐assisted tungsten inert gas welding was carried out on a thin plate of 2195 Al‐Li alloy, and the characteristics of the weld pores were analyzed in terms of their size and porosity. The effects of welding speed and ultrasonic power on the porosity and size of the pores were investigated. The pores were found to occur primarily adjacent to the surface of the weld. The porosity decreased and the size increased with a decrease in welding speed. The effect of ultrasonic power on the characteristics of the pores was different from that of the welding speed. The porosity and size of the pores decreased and then increased with an increase in ultrasonic power. A relationship was found between the transient cavitation intensity and the characteristics of pores. An increasing transient cavitation intensity results in a decrease in the porosity and size of pores when the transient cavitation intensity is lower. However, it can result in an increase in the porosity and pore size when the transient cavitation intensity further increases. Finally, the influencing mechanism of cavitation on welding pores was discussed.

TIG Dressing Effects on Weld Pores and Pore Cracking of Titanium Weldments

Weld pores redistribution, the effectiveness of using tungsten inert gas (TIG) dressing to remove weld pores, and changes in the mechanical properties due to the TIG dressing of Ti-3Al-2.5V weldments were studied. Moreover, weld cracks due to pores were investigated. The results show that weld pores less than 300 μm in size are redistributed or removed via remelting due to TIG dressing. Regardless of the temperature condition, TIG dressing welding showed ductility, and there was a loss of 7% tensile strength of the weldments. Additionally, it was considered that porosity redistribution by TIG dressing was due to fluid flow during the remelting of the weld pool. Weld cracks in titanium weldment create branch cracks around pores that propagate via the intragranular fracture, and oxygen is dispersed around the pores. It is suggested that the pore locations around the LBZ (local brittle zone) and stress concentration due to the pores have significant effects on crack initiation and propagation.

Effect of the quality of filler wire on the formation of pores in welding of titanium alloys

(2008). Effect of the quality of filler wire on the formation of pores in welding of titanium alloys. Welding International: Vol. 22, No. 12, pp. 853-858.

Repairability of plastic automobile bumpers by hot gas welding

AbstractProblems associated with the repair of fractured plastic car bumpers by traditional hot gas welding have been identified. In particular, interfacial cracks arising due to contraction stresses, weld pores, root notches, and voids in filler rods have been observed. These result in welds with inferior mechanical properties. Even so the impact properties and strength of plastic bumpers which were fractured and have been repaired by hot gas welding are likely to be acceptable for the purposes that the bumpers are intended. We have considered three materials: a polycarbonate/polyester blend, poly(butylene terephthalate), and an ethylene‐propylene‐diene rubber. Improvements in weld strength and interfacial bonding arise if a heated roller immediately following the welding torch is used to consolidate and compress the molten plastic. Porosity is reduced if the hot gas can escape easily from beneath the weld.