Mechanism Of Porosity Formation 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.

Analysis of the Role of Aquatic Gases in the Formation of Sea-Ice Porosity

The porosity of freshwater ice and sea ice is one of the main parameters that determine their strength. The strength of ice varies over a wide range of values, and the differences in the intensity of the mechanisms of ice porosity formation in different water areas can be one of the possible reasons for these variations. The water mass contains gases in two forms: gases dissolved in the water mass, as well as gas bubbles that are formed when wind waves break up, and bubbles that float up from the seabed. This article presents the results of an analysis of the role of each of these forms in the formation of gas inclusions (pores) in the crystal structure of ice. The results showed that the main source of gas pores in ice crystals is the gas bubbles coming to the surface from the bottom, formed during the decomposition of bottom sediments or during gas leaks from near-bottom oil and gas fields. The possibility of gas bubbles occurring and rising to the ice–water boundary depends on the presence of bottom sources of the gases, the intensity of dissolution of the bubbles and the depth of the water area. Therefore, the variation in the porosity and the strength of ice over the space of the water areas can be associated with the changes in their depths, and the presence and location of the natural gas sources.

Mechanical properties of hierarchical porous lattices with microscopic porosity fabricated by combination of lattice design and HSSH process

The hierarchical porous lattice implant with microscopic porosity, which resembles the original human bone structure, has significant potential application value. The microscopic porosity in the hierarchical porous lattice implant plays a crucial role in transporting nutrients and promoting tissue regeneration. However, it is difficult to fabricate microscopic pores smaller than 200 μm by powder bed fusion (PBF) processes due to the limitations of the laser spot size. In this study, the preparation of a hierarchical porous lattice implant with microscopic porosity is proposed by coupling structure design with the high scanning speed - short hatch spacing (HSSH) process formation. The structure-versus-process relationship is used to control the size and distribution of the microscopic pores, which meets the requirements of modulus and high biocompatibility. This proposed process breaks the inherent law where the higher the porosity, the higher the modulus of the lattice for the same lattice type. The modulus and porosity of the hierarchical porous implants could be flexibly tuned over a wide range (0.1–1.8 GPa and 63–84 %, respectively). The microscopic porosity is mainly distributed near the contour of the core rod, and the macroscopic porosity and microscopic porosity are approximately 73.70 % and 8.22 %, respectively. The problem of large errors in fitting the hierarchical porous lattice modulus calculation model according to the Gibson-Ashby equation has been solved. The mechanism of microscopic porosity formation was also analyzed. Furthermore, this method has the advantage of universal applicability, providing a feasible way for the development and application of porous materials.

Experimental analysis of the effect of ultrasonic vibration on porosity suppression in narrow gap laser welding

Abstract 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.

DBN-based online identification of porosity regions during laser welding of aluminum alloys using coherent optical diagnosis

As a typical serious defect during laser welding of aluminum alloys, porosity can significantly impair the performance of the weld seam. Real-time monitoring of porosity defect can provide guidance for feedback adjustment of welding quality to prevent the subsequent continued formation of porosity, which has attracted increasing attention. This paper implements the deep learning-based online identification of porosity regions using coherent optical diagnosis. Considering the formation mechanism of porosity, a monitoring platform including a coherent optical sensor is established to measure the keyhole 3D morphology and extract the keyhole depth as the coherent optical signal. The acquired keyhole depth signal is processed by ensemble empirical mode decomposition, and it is discovered that there is a segmentation threshold in the reconstructed signal to separate the porosity regions and the non-porosity regions. A deep belief network is constructed to complete the reception of the reconstructed keyhole depth signal and the response of the prediction state, realizing the online identification of the porosity regions. The results show that the identification model has high accuracy, and the validity verification demonstrates the reliability of the proposed method.

Porosity Reduction in 15CDV6 Steel Manufactured by L-DED

The Laser Directed Energy Deposition (L-DED) is an Additive Manufacturing process based on the direct injection of the filler material into the desired areas of a substrate. Damaged components can be repaired or coated in order to enhance their properties and increase their lifespan. However, this process is highly material dependent and to assure the quality of the deposited material, the process parameters need to be optimised. In the present work, a novel material for the L-DED is developed, the 15CDV6 aeronautical steel. The material was atomized ad hoc and the obtained powder was characterized (particle shape, size distribution, and chemical composition) in order to ensure its quality. Once the powder was analysed, the suitability of the 15CDV6 steel for the L-DED process was studied through a methodology consisting of three sequential steps: single clad tests, layer deposition and wall construction. The main process parameters (laser power, feed rate and powder feed rate) were controlled during these tests. Nevertheless, the initial tests showed high levels of porosity, which required considering additional process parameters to reduce them, such as the carrier and shielding gas flows, preheating, or the laser beam diameter. Consequently, the porosity formation mechanisms were identified and the most relevant process parameters established, which for the 15CDV6 steel were the gas flows. The effect of the shielding gas flow in the powder distribution below the nozzle was studied experimentally by digital image analysis to better interpret the obtained results and determine the optimum process parameters for achieving defect free parts. The quality of the deposited material was studied through metallographic analysis and an Acicular Ferrite structure was observed, ensuring good mechanical properties. To summarise, this research work proves the viability of employing 15CDV6 steel in the L-DED process and has determined the main process parameters to obtain porosity-free parts.

Operando tomographic microscopy during laser-based powder bed fusion of alumina

Laser-based Powder Bed Fusion (LPBF) of oxide ceramics enables fabrication of objects with complex three-dimensional shapes. However, mechanical properties of dense LPBF-manufactured ceramics are poor due to large amount of structural defects. Here, we perform the operando tomographic microscopy during LPBF of a magnetite-modified alumina to gain a deeper understanding of the underlying mechanisms. The effect of the laser energy density on the surface roughness, powder denudation zone and porosity formation mechanisms are investigated. Increasing laser power results in significant increase of the melt pool width, but not its depth and no melt pool depression is observed. Forces due to the recoil pressure are not seen to significantly influence the melt pool dynamics. Increasing power allows to avoid fusion porosity but enhances formation of spherical porosity that is formed by either reaching boiling point of liquid alumina, or by introducing gas bubbles by injection of hollow powder particles into the liquid.

Numerical and experimental study on keyhole dynamics and pore formation mechanisms during adjustable-ring-mode laser welding of medium-thick aluminum alloy

Keyhole induced-pore is one of the biggest challenges in laser welding of aluminum alloys. A shaped laser controlling strategy, ring-mode laser, has shown the possibility of reducing porosity in welding due to its potential to enlarge keyhole openings and improve the stability of the keyhole. However, few attempts have been made to reveal the influence of ring-mode laser on keyhole dynamics and pore formation. This paper quantitatively investigated porosity improvement during adjustable-ring-mode laser welding (ARM-LW) of aluminum alloys and analyzed keyhole dynamics and porosity formation mechanisms using a validated model. Experimental results indicate that porosity can be dramatically reduced when compounding a ring laser beam over the Gaussian beam. As the core/ring power ratio adjusts from 10:0 to 5:5, the porosity decreases significantly and remains lower (1.73%∼2.58%) while ensuring a similar melt depth (5∼6 mm). Simulation results demonstrate that two mechanisms work synergistically to suppress porosity: (1) The ring-mode laser contributes to decreasing keyhole collapse and regulating melt pool flow, hence reducing pores created at the Gas-Liquid interface (G/L-pore); (2) The ring-mode laser facilitates reducing the frequency and depth of keyhole into the melt pool, thus minimizing pores created at the Liquid-Solid interface (L/S-pore). This work can guide the porosity suppression during laser welding of aluminum alloys and is of significant theoretical meaning and application value.

Investigation on the Morphology and Formation Mechanism of Porosity in Different Heat Source Regions of Nickel-Based Alloy Laser Hybrid Welding

Investigation on the Morphology and Formation Mechanism of Porosity in Different Heat Source Regions of Nickel-Based Alloy Laser Hybrid Welding

Additive manufactured parts produced by selective laser sintering technology: porosity formation mechanisms

Abstract Additive manufacturing represents a powerful tool for the fabrication of parts with complex shapes by the deposition and the consolidation of materials as opposed to subtractive manufacturing methodology. Selective laser sintering (SLS), one of the most popular powder bed fusion (PBF) technologies for thermoplastic part production, has demonstrated extensive applications in various industrial sectors. The process involves the deposition of homogeneous powder layers and employs a laser source to selectively melt a powder bed according to a CAD model. Due to its layer-by-layer nature, voids and pores are inevitably introduced in the fabricated thermoplastic parts. Porosity represents one of the major limitations of this technology being one of the main causes of the variation of the mechanical properties. With the intention of providing support for reducing the porosity and thus increasing the quality and performance of the final product, in this paper, a brief review was carried out focusing on the SLS process parameters and their interaction with the porosity of the product. In addition, an in-depth look was given to the mechanisms of formation and consolidation of pores within parts made of polymeric material.

Porosity formation mechanisms, microstructure evolution and mechanical performance of AlMgScZr alloy fabricated by laser powder bed fusion: Effect of hatch distance

The present work studied the effect of the hatch distance, on densification, surface quality, melt pool configuration, residual stress, microstructure and mechanical performance of the laser powder bed fusion (LPBF) fabricated AlMgScZr alloy. Four hatch distances, i.e., 60, 80, 100 and 120 μm, were subjected to investigation and comparison. The sample fabricated with a 60 μm hatch distance demonstrated a near full relative density of 99.80 %, which was attributed to a favorable compromise between nucleation, growth and outgassing of pore during the LPBF process. The specimen processed with a 120 μm hatch distance demonstrated a relatively smooth surface with Ra of about 15 μm owing to the adequate lifetime of the melt and the reduced Plateau-Rayleigh capillary instability. The influence of hatch distance on microhardness was not evident. This work demonstrates that the relative density, surface quality, residual stress, grain size, texture and dislocation densities can be well regulated by optimizing hatch distance.

Formation mechanism of large porosity in solid oxide cell electrode coatings fabricated using ethylene glycol-based precursor solutions and its impact on the electrochemical performance

Recently, a novel composite solid oxide fuel cell electrode fabrication method based on ethylene glycol (EG)-based polymeric precursors, which yielded long triple phase boundaries and thus, high performance, was developed. However, some of these coatings were reported to contain randomly formed, large pores. Since porosity observed in the microstructure may impact the electrochemical performance directly, the origin of its formation and ways to eliminate it were investigated in the present study. We hypothesized that the large pores detected in microstructure were related to incomplete polymerization which resulted in instant discharge of carbonaceous gases and that promoting further polymerization by adding extra nitric acid would eliminate these pores. Thermal analyses revealed that when liquid La 0.8 Sr 0.2 FeO 3 –Ce 0.8 Sm 0.2 O 2 (LSF-SDC) precursors without additional nitric acid were heated, an abrupt weight loss took place at 200 °C in the case of polymeric solutions. Addition of nitric acid resulted in a weight loss that took place over a temperature range of 200–400 °C. Microstructural analyses showed large pores in LSF-SDC films, especially in the case of thick (ca. 4–4.5 μm) samples. These large pores were mostly eliminated upon nitric acid addition. Electrochemical impedance spectroscopy measurements revealed ca. 60% reduction in the polarization resistance of the thick electrodes upon nitric acid addition, due to the elimination of large pores and thus enhancement of the triple phase boundary length.

Study on the distribution, element characteristics, and formation mechanism of porosity during laser welding for Ti-6Al-4V bottom-locking joint

The bottom-locking joint is widely used in the aerospace manufacturing field as an imperative connection structure. The laser welding experiment for the Ti-6Al-4V titanium alloy bottom-locking joint is carried out in the present paper. Meanwhile, porosity in the weld seam including the process porosity and metallurgical porosity is investigated. Furthermore, it is novel that the distribution, element characteristics, and formation mechanism of process porosities and metallurgical porosities in the bottom-locking joint are comprehensively analysed based on the porosity morphology, microstructure, and element analysis of the resultant bottom-locking joints. The process porosity, which mainly appeared in the weld seam centre, is formed by the combined effect of two mechanisms which closely related to the dynamic behaviour of the molten pool and the stability of the keyhole. Besides, a large number of micropores with a diameter of 2–5 µm are observed on the surface of the process porosity, whose formation is related to the evaporation of elements. Moreover, the tensile stress generated during the solidification and shrinkage of the molten metal is the main factor leading to microcracks in the micropores. The nucleation characteristics of bubbles cause metallurgical porosities to be mainly distributed at the bottom of the weld seam.

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

Welding porosity is an important factor affecting the mechanical properties of welded joints. This study investigated the effect of welding stability on weld porosity to solve the problem of welding porosities in laser-arc hybrid welding of dissimilar steels. The type and formation mechanism of welding porosity were analyzed. The study results showed that the porosities generated during the welding process were process welding porosities. The generation of bubbles was closely related to the instability of the steam pressure inside laser keyhole. The droplet transfer form was an essential factor affecting the stability of the laser keyhole. When the droplet was in the projected transfer mode, the weld porosity could be effectively reduced. Penetration weld was also greatly helpful in reducing weld porosity.

Laser Joining of CFRTS and Steel by Interfacial Pressure Control

The carbon fiber-reinforced polymer (CFRP) metal structure is widely used in various industries to reduce the weight and cost of the structure without compromising performance. The main challenge for manufacturing the CFRP metal structure comes from the lack of flexible and robust joining processes. In this study, a new laser joining process was developed that used a thin layer of polyamide 6 as an interlayer material lying between carbon fiber-reinforced thermosets and a quenching-partition (QP) 980 steel to achieve the joint since the laser can’t directly join the steel to the thermoset composite. It was found that the joint’s strength was seriously degraded by the porosity’s formation. The porosity formation mechanism was studied by online-recording the thermal history inside the joint during the laser joining process. Experimental results demonstrated that the emerging of the porosities in the joint was mainly caused by the cooling shrinkage instead of pyrolysis gas release. Furthermore, a new approach of controlling and optimizing the interfacial pressure was developed to suppress the porosity formation in the joint, which can significantly reduce the porosity rate from 25.8 to 1.2% and dramatically improve the joint shear strength from 10.69 to 18.6 MPa by 73.99%.

Microstructure evolution and defect characteristics of multilayer Fe-Cr alloy coatings fabricated by laser melting deposition

To clarify the microstructure evolution, defect characteristics and suppression methods for multilayer cladding by laser melting deposition (LMD), experimental research on multilayer LMD of Fe-Cr alloy with different laser energy densities was carried out. The phase composition and the evolution rules of the phase during the melting-solidification process were analyzed. The crystal growth trend, dendritic transition and dendrite spacing of multilayer Fe-Cr coatings were investigated. The formation mechanism of crack, porosity and surface spheroidization defects were revealed, the effects of laser energy densities on the defects were analyzed, and the defect suppression methods were proposed and verified. The results showed that the primary and secondary remelting zones of multilayer Fe-Cr coatings were fine equiaxed dendrites and irregular cellular structures, respectively. When the laser energy density increased from 22.74 J/mm2 to 55.70 J/mm2, the dendrite spacing increased gradually, and the porosity and spheroidization defects showed a trend of improvement and then deterioration. The cracks were considered cold cracks with transgranular characteristics. Meanwhile, when the residual stress exceeded the theoretical fracture strength of 1.6 GPa, the multilayer cladding experienced brittle fracture and formed cracks. Selecting an appropriate laser energy density and preheating treatment can effectively improve the forming quality and inhibit the formation of defects.

Three-dimensional MXenes heterostructures and their applications

Due to their unique surface chemistry, highly adjustable metal components, hydrophilicity, and high carrier concentrations, MXenes are applied in a variety of scenarios. Similar to other two-dimensional (2D) materials, building heterostructures with additional materials to form a 3D porous architecture for MXenes can significantly enhance their functionality and reactivity. Notably, the open structures and well-defined pathways of these 3D structured MXenes can improve ionic and electronic transport, thereby promoting their applications in electrochemical energy storage, sensing, catalysis, and environment. In this review, the recent efforts made on preparing 3D porous MXenes with heterostructures, focusing on MXenes/C, MXenes/inorganics, and MXenes/polymers were summarized. The discussion covers aspects ranging from the design to synthesis of 3D porous MXenes, and their applications in photocatalysis, environmental monitoring and electrochemical energy storage. This review is concluded by presenting the prospects and insights on exploring the relationships between the porosity formation mechanisms, properties and applications of the 3D porous MXenes heterostructures. This review can provide meaningful guidance for the design, fabrication and application of 3D porous MXenes in high-performance materials and devices.

Numerical investigation of gas entrapment in metal additive manufacturing using a quasi-sharp-interface particle method

Porosity is a key issue for the parts fabricated by selective laser melting. However, the porosity formation mechanisms have not been fully clarified. In particular, the formation of small pores in fabricated parts from the gas among granules is controversial. The existence of sharp corners among molten granules is a key numerical challenge because the conventional diffusive interface model (DIM) might trigger spurious gas entrapment. To solve the problem, a quasi-sharp-interface model (QSIM) was proposed for the moving particle semi-implicit (MPS) method in this study. A new approach to calculate the interface area based on the local triangularization was proposed to accurately impose the surface-tension force in the QSIM. The bubble-rising simulation verified the capability of the QSIM to simulate topological changes of interfaces. The sharp-corner deformation problem demonstrated that the QSIM could effectively prevent unphysical breakup and the spurious gas entrapment. Thus, the QSIM was indispensable for reliably simulating the gas entrapment from the gaps among granules in SLM. Finally, the QSIM results indicated that the gas among granules was completely discharged when the granules melted. This suggested that the gas among granules did not cause small pores in the fabricated parts. This observation correlated with an experimental composition analysis of gas in pores.

Laser-beam powder bed fusion of cost-effective non-spherical hydride-dehydride Ti-6Al-4V alloy

Hydride-dehydride (HDH) Ti-6Al-4V powders with non-spherical particle morphology are typically not used in laser-beam powder bed fusion (LB-PBF). Here, HDH powders with two size distributions of 50–120 µm (fine) and 75–175 µm (coarse) are compared for flowability, packing density, and resultant density of the LB-PBF manufactured parts. It is shown that a suitable laser power-velocity-hatch spacing combination can result in part production with a relative density of > 99.5% in LB-PBF of HDH Ti-6Al-4V powder. Size, morphology, and spatial distribution of pores are analyzed in 2D. The boundaries of the lack-of-fusion and keyhole porosity formation regimes are assessed and results showed parts with a relative density of > 99.5% could be LPBF processed at a build rate of 1.5–2 times of the nominal production rates in LPBF machines. The synchrotron x-ray high-speed imaging reveals the laser-powder interaction and potential porosity formation mechanism associated with HDH powder. It is found that lower powder packing density of coarse powder and high keyhole fluctuation result in higher fractions of porosity within builds during the LB-PBF process.

Study on the molten pool behavior and porosity formation mechanism in dual-beam laser welding of aluminum alloy

Based on the mechanism of dynamic coupling between molten pool and keyhole, a three-dimensional (3D) transient model for the dual-beam laser welding of aluminum alloy is established by considering the surface tension, Marangoni force, and recoil pressure. The morphology of the molten pool, porosity formation process, and the heat transfer mechanism during the process of laser welding under different parameters are analyzed. A double rotating 3D Gaussian heat source is used to represent the laser beam, and the volume of fluid method is used to track the gas-liquid free surface, and the gas-liquid interface force is transformed by the continuous surface force model. The results show that in the process of dual-beam welding, the interaction between the two keyholes enhances the fluid flow perpendicular to the spot line, and the shape of weld pool surface changes from elliptical to circular. Furthermore, welding speed and spot spacing have a significant effect on the shape of the molten pool. Furthermore, it is observed that dual-beam welding can improve the stability of the keyhole and reduce the maximum oscillation amplitude and the number of keyhole breakups. At a specific spot spacing, a unique process of separation and fusion is discovered in addition to the common stages of growth, maintenance, breakup, and shrinkage of the keyhole. The simulation results are in good agreement with the existing experimental results. Overall, this paper provides useful insights into the dynamics of the molten pool in dual-beam welding and reveals the molten pool behavior and porosity formation mechanism.