Porosity Defects Research Articles

Analysis of microstructure and mechanical strength of lap joints of TZM alloy welded by a fiber laser

The application of molybdenum alloys to structural components is severely limited due to their poor weldability with serious defects of porosity and joint embrittlement after welding despite their high melting temperature, hot strength and creep resistance. A systematical experimental study has been conducted to explore the potential of laser welding of 0.5 mm-thick Titanium-zirconium-molybdenum (TZM) alloy in a lap welding configuration. Porosity was found to be the most serious problem in the TZM laser lap welding process. Introducing an interface gap of 0.09 mm had the most positive effect in reducing the porosity compared to using helium gas, different shielding gas flow rates, adding alloy element and different heat input rate. With the use of 0.09 mm-interface gap, the porosity of the weld joint was reduced to 3%. The tensile stress of the bead on plate (BOP) welded joint could achieve about 60% that of the base metal. The fracture stress of the lap welded joint obtained by using 0.09 mm-interface gap in tensile-shear test was about 142 MPa. The porosity and embrittlement were responsible for the reduction of the strength and ductility of the welded joint.

A CFD-FEM Model of Residual Stress for Electron Beam Welding Including the Weld Imperfection Effect

Residual stress of electron beam welding (EBW) is associated with the heat transfer, fluid flow, and keyhole dynamics occurred in the process. In engineering, it was commonly simulated by pure heat conduction-based thermomechanical modeling with an experimentally calibrated heat source model. This method, however, is difficult to be used to consider the effect of weld imperfections such as the spiking and porosity defect. In this study, we proposed a novel combined computational fluid dynamics and finite element method (CFD-FEM) model to predicate the residual stresses of EBW. This model allows us to include the integral effect of heat transfer, fluid flow, and keyhole dynamics as well as process defects on residual stresses for the first time. A phenomenological heat source model dependent on the dynamic keyhole was considered in the CFD model part. A mapping algorithm was developed to build a new heat source model of FEM modeling part from the CFD simulations. Parametrical simulations and experiments of EBW of a 10-mm-thickness Ti-6-Al-4-V alloy plate were carried out and good agreements were obtained by the comparisons. It was shown that for deep penetration EBW the keyhole oscillates in radial and depth directions and the spiking defect occurs, particularly under large heat input parameters. Interestingly, the equivalent and principle residual stresses exhibits strong oscillations in the bottom part of welds along the welding direction, in which the amplitudes are around 9 pct (73.69 MPa) of the equivalent stress and 24 pct (121.84 MPa) of the principle stress in penetration direction, respectively, when the power of electron beam is 1.5 kW. As the power of electron beam increases, the effect of spiking on the residual stress enhances. When the welding power increases to 2.25 kW, the amplitudes of equivalent residual stress increase to 12 pct (119.2 MPa). Our results provide a quantitative method connecting the weld pool dynamics and process defects with the residual stress of EBW, which previously could be understood only in a qualitative way, and demonstrate the possibility of residual stress predications only from the information of materials and process parameters before welding, without the knowledge of post-weld profiles.

Microstructure and mechanical properties of welding–brazing of Ti/Al butt joints with laser melting deposition layer additive

Laser welding–brazing of Ti/Al butt joints was performed with coaxial Al–10Si–Mg powders feeding. The experimental results indicated that a sound Ti/Al butt joint could be obtained by an additive layer approach. The influence of the laser melting deposition layers on the weld appearance, interfacial microstructure and tensile properties were investigated. High-quality joints were produced when five and seven layers were deposited. The morphology and thickness distributions of the interfacial intermetallic compounds (IMC) at the brazing interface along the thickness direction of the joint varied with the number of deposition layers. Continuous serrated IMC was obtained in joints produced by seven deposition layers, and the IMC layer was distributed homogenously along the thickness direction. The maximum thickness difference of the IMC was only approximately 0.12 μm. The microstructure of the IMC layer was composed of a nanosized granular Ti7Al5Si12 phase and serrated Ti(Al, Si)3 phase. The maximum tensile joint strength reached 240 MPa, 80% of that of the aluminum base metal, and the lower tensile strength of the other joints was caused by insufficient IMC layer or a porosity defect.

Investigation of porosity in rotating laser-MIG hybrid welding A6N01 aluminum alloy

In this study, rotating laser was employed to reduce the porosity defect in laser-MIG hybrid welding A6N01 aluminum alloy. The influence of laser rotating frequency and rotating radius on the weld were researched respectively. The images of the molten pool were acquired by the high-speed camera, and X-ray images of the weld were calculated by binarization. The result shows that rotating laser-MIG hybrid welding has a dramatic effect on suppressing the generation of porosity, which indicates the possibility to obtain the A6N01 aluminum alloy weld with few pores. With proper parameters about rotation frequency (50 Hz) and rotation radius (2 mm), the porosity decreased obviously from 8.41% to 5.68%, and there was hardly distinct change in the weld penetration. The keyhole stability of rotating laser-MIG hybrid welding was improved, compared with the conventional laser-MIG hybrid welding. On account of laser beam stirring the metal molten pool, it was beneficial for the pores to escape from the molten pool. The rotating laser-MIG hybrid welding shows a good prospect.

Criticality of porosity defects on the fatigue performance of wire + arc additive manufactured titanium alloy

This study was aimed at investigating the effect of internal porosity on the fatigue strength of wire + arc additive manufactured titanium alloy (WAAM Ti-6Al-4V). Unlike similar titanium alloys built by the powder bed fusion processes, WAAM Ti-6Al-4V seldom contains gas pores. However, feedstock may get contaminated that may cause pores of considerable size in the built materials. Two types of specimens were tested: (1) control group without porosity referred to as reference specimens; (2) designed porosity group using contaminated wires to build the specimen gauge section, referred to as porosity specimens. Test results have shown that static strength of the two groups was comparable, but the elongation in porosity group was reduced by 60% and its fatigue strength was 33% lower than the control group. The stress intensity factor range of the crack initiating pore calculated by Murakami’s approach has provided good correlation with the fatigue life. The kink point on the data fitting curve corresponds well with the threshold value of the stress intensity factor range found in the literature. For predicting the fatigue limit, a modified Kitagawa-Takahashi diagram was proposed consisting of three regions depending on porosity size. Critical pore diameter was found to be about 100 µm.

The influences of Ar-He shielding gas mixture on welding characteristics of fiber laser-MIG hybrid welding of aluminum alloy

During laser or laser-arc hybrid welding process of aluminum alloy, laser power waste and keyhole fluctuation are drastic due to the shielding effect of dense plasma. Consequently, small weld penetration depth and numerous weld porosity defects are produced. In the present work, Ar-He shielding gas mixture was used to improve the weld quality of fiber laser-metal inert gas (MIG) hybrid welded aluminum alloy. The influences of Ar-He shielding gas mixtures with various He volume ratios on plasma temperature, electron density, weld penetration depth and porosity defect were investigated, respectively. The He volume ratio enough for improving the weld penetration depth and suppressing the porosity defect was 50%. The effective laser power density increased with the increasing He volume ratio, which contributed to the increase of weld penetration depth. The weld porosity defects were suppressed efficiently, since the stability of the keyhole was improved while using Ar-He shielding gas mixture.

Enhancing the Permeability and Properties of Ceramic Shell in Investment Casting Process Using ABS Powder and Needle Coke

In the investment casting process, initially, a wax pattern is made, around which repeated slurry/stucco coatings are applied. The slurry is made up of very fine ceramic ingredients like zircon flour, etc. Once the ceramic slurry is dried out, a ceramic shell is formed around the wax pattern. After drying out of the ceramic shell, the wax is drained out and the shell is fired in an oven for inducing additional strength. Molten metal is then poured into the ceramic shell. Because of the fine ingredients of the ceramic shell, the investment casting gets a very fine surface roughness, compared to a conventional sand casting. However, the ceramic shell exhibits poor permeability, which often results in porosity defects. Hence, in the present work, an attempt was made to enhance the permeability of ceramic shell by mixing certain fillers like acrylonitrile–butadiene–styrene (ABS) powder and needle coke to the ceramic slurry. Fine ABS powder was added to the inner coat slurry in a very small proportion. The ABS powder, upon heating, got burnt and left micropores inside the inner coating of the shell. In a similar way, needle coke was added to the outer coat slurry in a very small proportion. During the firing of the shell, the needle coke got burnt and, consequently, macropores were created in the outer coating of the shell, which would enable escape of undesirable gases that may generate inside the shell. This ultimately would reduce the porosity defects in the final cast parts. Tests conducted on the ceramic shell modified with ABS powder and needle coke reveal that the modified shell possesses higher permeability compared to the conventional one. The modified shell also possesses adequate mechanical properties like flexural strength, hot tensile strength, etc., and fair surface roughness.

Numerical and experimental study on keyhole and melt flow dynamics during laser welding of aluminium alloys under subatmospheric pressures

Porosity defects was highly related to the keyhole and melt flow dynamic during laser welding process. In this paper, a novel 3D numerical model was developed to describe the keyhole dynamic and melt flow behaviors during laser welding of 5A06 aluminium alloy under subatmospheric pressures. The effect of ambient pressure on laser welding process was taken into consideration by optimizing the boiling point of aluminium alloy and recoil pressure of evaporated metallic vapor jets based on vapor–liquid equilibria calculation and Wilson equation. A moving hybrid heat source model was employed to describe the laser energy distribution under subatmospheric pressures. Numerical results indicated that a wider and deeper keyhole with less humps was produced under subatmospheric pressure comparing with that of atmospheric pressure. The vortices in the rear keyhole wall became unapparent or even disappeared with the decrease of ambient pressures. The melt flow velocity on the keyhole wall was larger under a lower pressure. A smaller difference between boiling point and melting point was produced and this led to the formation of a thinner keyhole wall and improved the stability of molten pool. Larger recoil pressure produced under subatmospheric pressure was responsible for the weakened vortices and enhanced melt flow velocity. Bigger keyhole opening size, larger melt flow velocity, thinner keyhole and the weakened vortices all resulted into the reduction of porosity defects during laser welding of aluminium alloys. Based on the simulation results, the plasma distribution, weld formation and porosity defects had been demonstrated. The compared results showed that the simulation results exhibited good agreements with the experimental ones.

Cobalt (oxy)hydroxide nanosheet arrays with exceptional porosity and rich defects as a highly efficient oxygen evolution electrocatalyst under neutral conditions

Highly porous and defective cobalt (oxy)hydroxide nanosheet arrays rank the best cobalt-based noble-metal-free oxygen evolution electrocatalyst under neutral conditions.

Synthesis of Mesoporous Co₃O₄/NiCo₂O₄ Nanorods and Their Electrochemical Study.

Mesoporous Co₃O₄/NiCo₂O₄ nanorods were obtained by a hydrothermal reaction with the assistance of Ni foam and subsequent annealing treatment. The characterization of this composition by X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, energy dispersive spectra and Brunauer-Emmett-Teller analysis revealed that the nanorods consisted of Co₃O₄ and NiCo₂O₄ phase, exhibiting high porosity and rich crystal defects. The electrochemical data showed a specific capacitance of 1173 mF cm-2 and 606 mF cm-2 at 2 mV s-1 and 1 mA cm-2, respectively. Its cycling performance was 83.9% at 3 mA cm-2 after 4000 cycles. Furthermore, the asymmetric supercapacitor Co₃O₄/NiCo₂O₄//AC delivered an energy density of 11.7 W h kg-1 and power density of 760 W kg-1.

GISS Technology: Principle and Applications in Die Casting

In the past, there have been a lot of effort to solve gas and shrinkage porosity defects in die casting. The common solutions are vacuum technology, jet cooling technology, and application of squeeze pins. However, these solutions often increase the die casting production costs. A new solution that has recently been introduced worldwide is GISS Technology. This technology applies the superheated slurry casting process. Gas and shrinkage porosity defects can be reduced. Furthermore, the production costs are lowered due to die life extension, cycle time reduction, melting energy reduction, and lubrication usage reduction. This paper describes the principle of GISS Technology, and selected applications and case studies are also be presented.

Porosity Characteristics and Effect on Tensile Shear Strength of High-Strength Galvanized Steel Sheets after the Gas Metal Arc Welding Process

In this study, lap joint experiments were conducted using galvanized high-strength steel, SGAFH 590 FB 2.3 mmt, which was applied to automotive chassis components in the gas metal arc welding (GMAW) process. Zinc residues were confirmed using a semi-quantitative energy dispersive X-ray spectroscopy (EDS) analysis of the porosity in the weld. In addition, a tensile shear test was performed to evaluate the weldability. Furthermore, the effect of porosity defects, such as blowholes and pits generated in the weld, on the tensile shear strength was experimentally verified by comparing the porosity at the weld section of the tensile test specimen with that measured through radiographic testing.

Microstructure and surface analysis of friction stir processed Ti-6Al-4V plates manufactured by electron beam melting

In this paper, the feasibility and capability of friction stir processing to enhance material structure and surface properties of additively manufactured Ti6Al4V plates is investigated. The base material was produced by electron beam melting process and then friction stir processed using a pin less tool to compensate the provides poor surface finishing, reduced mechanical properties and internal porosity defects affecting the outer layer. A reduction by 80% of surface roughness was achieved. Internal porosity decreased as well. Microstructural analysis evidenced that the heating and stirring effects promoted the formation of distinct metallurgical zones, namely a stirred zone (SZ), a transition zone (TZ) and a heat affected zone (HAZ). The SZ exhibits a fully recrystallized microstructure, whereas highly deformed but not recrystallized grains and coarser grains were detected in the TZ and HAZ, respectively. Within the treated zone the hardness increased up to 450 HV with respect to 380 HV of the base material.

An improved model of porosity formation during pulsed GTA welding of aluminum alloys

Porosity defect has been a serious issue in the welding of aluminum alloys. In this paper, the mechanism of porosity formation was discussed from the perspective of metallurgical physical chemistry, and two types of bubble nucleation model were set up respectively. Given the welding current has a significant influence on porosity defect, the effect of welding current on the critical nucleation radius was investigated. The results indicated that critical nucleation radiuses in two models are the same, which were slightly affected by welding current. It means current plays an important role in the stage of bubble growth. Furthermore, bubble growth model was established under the assumption that the ratio of growth speed to solidification speed is constant. Morphology of porosity was obtained by analyzing the bubble radius varying with time. Finally, experiments were conducted and results showed the calculated morphological appearance agreed well with metallographic observation.

Mechanical property of thin walled selective laser melted parts and the effect of heat treatment

The variability in mechanical properties in selective laser melted (SLM) maraging steel parts was captured by testing miniature size tensile coupons that were extracted from different locations in the part. Tensile testing provides better mechanical response to defects such as pores and inclusions as compared to hardness testing. The effect of heat treatment on the tensile property variation with different section thickness was also studied. It was observed that tensile property of miniature samples was more sensitive to porosity defects in maraging steel samples. The elongation values in the tensile specimens exhibited more variability as the section thickness of the part was reduced. This was due to the presence of more pore defects in thin wall sections. Heat treatment did not influence the tensile properties with respect to the wall thickness except for an increase in the strength due to hardening.

Reducing Porosity in 35crmo Steel Sand-Cast by Ultrasonic Processing

The influence of ultrasonic processing on porosity defects of 35CrMo steel sand-cast ingot was studied. A self-developed ultrasonic-introduced method was applied to carry out the ultrasonic experiment. The acoustic pressure and melt velocity field were calculated through numerical simulation, which plays an auxiliary role in the analysis of the influence of ultrasound on the molten steel. Comparing the porosity of ultrasonic castings and direct castings, the results reveal that ultrasonic processing could reduce the porosity defects of ingot significantly, thus the mechanical properties of castings could be significantly improved. The ultrasound tends to attenuate when propagating in the steel melt. The porosity at the central area of ingot reduced more apparent than that at the edge area. In addition, the mechanism of the effect of ultrasound on porosity reduction was also discussed.

Microstructure characterisation of a friction stir welded hemi-cylinder structure using Ti-6Al-4V castings

Friction stir welding (FSW) was applied to manufacture a spacecraft propellant tank from cast Ti-6Al-4V with the aim to reduce costs and lead time compared to conventional manufacturing routes. These tanks are usually produced via forging and a combination of tungsten inert gas (TIG) and electron beam (EB) welding. FSW is a solid-state process and produces joints of high quality and fewer internal defects, given the use of appropriate welding parameters, and is expected to become an attractive alternative for future spacecraft tank manufacturing.The study presented in this paper investigates the FSW region of a cast cylinder and hemisphere shell, joined by a single curved track friction stir weld of ~1.3 m length. The weld parameters, optimised in a separate development study, resulted in a stable and consistent weld around the circumference of the tank with good mechanical properties, no porosity and few defects in the microstructure. Particular emphasis was placed on the variation of the microstructure from the base material, cast Ti-6Al-4V, to the refined stir zone. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were employed to visualise these regions. Furthermore, periodic bands of tungsten contamination were detected in the stir zone.

Fracture Failure Analysis on Main Wheel Hole of Escalator Step

The escalator steps used in a building fractured during applications. The cause for escalator step fracture has been investigated systemically. The supporting rod main wheel hole of escalator step was studied by means of chemical composition analysis, scanning electron microscope and optical microscope.The results indicate that there are some porosity defects in the support rod of escalator step.The fracture mode of main wheel hole is overload brittle fracture.

Failure Analysis of Escalator Step

The escalator step fractured during applications. The cause for escalator step fracture was investigated by means of scanning electron microscope, macroscopic fractography and optical microscope. The results indicate that some porosity defects distributed in the escalator step support. The fracture mode of escalator step is overload fracture.

Effects of Different Parameters on Porosity Defects Between the Horizontal and Vertical Shot Sleeve Processes

The effects of different casting parameters on porosity defects between the horizontal and vertical shot sleeve processes are quantitatively studied based on experiments and simulations. The castings made using the two processes are characterized in terms of porosity defects by X-ray radiographic inspection and blistering formation by T4 heat treatment process. Experimental results indicate that much fewer porosities and blisters are formed in the castings made using the vertical process than those made using the horizontal one. Meanwhile, numerical simulations have been performed to quantitatively investigate the flow dynamics behavior of molten metal in the horizontal and vertical shot sleeves. The simulation results demonstrate that compared with the conventional horizontal process, the vertical shot sleeve process is more efficient in elimination of waves and reduction of air entrapment and oxide formation. Moreover, the vertical process needs a lower pouring temperature which can decrease hydrogen solubility in the molten metal; it has a smaller temperature gradient of molten melt which can reduce the formation of cold flakes and pre-solidified materials. Therefore, the vertical shot sleeve process is more competitive in improving the internal integrity of die castings.