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Laser powder bed fusion (LPBF) is an additive manufacturing process that has gained interest for its material fabrication due to multiple advantages, such as the ability to print parts with small feature sizes, good mechanical properties, reduced material waste, etc. However, variations in the key process parameters in LPBF may result in the instantiation of porosity defects and variation in build rate. Particularly, volumetric energy density (VED) is a variable that encapsulates a number of those parameters and represents the amount of energy input from the laser source to the feedstock. VED has been traditionally used to inform the quality of the printed part but different values of VED are presented as optimal values for certain material systems. An optimal VED value can be maintained by changing the key process parameters so that various combinations yield a constant value. In this study, an optimal constant VED value is maintained while printing SS316L with variable key processing parameters. Porosity analysis is performed using optical microscopy, as well as X-ray computed tomography, to reveal the volume density and distribution of those pores. Two primary defect categories are identified, namely lack of fusion and porosity induced by balling defects. The findings indicate that, even at optimal VED, variations in process parameters can significantly influence defect type, underscoring the sensitivity of defect formation to the variation of these parameters. Furthermore, a minor change in the build rate, driven by adjustments in process parameters, was found to influence defect categories. These findings emphasize that fine tuning the process parameters and build rate is essential to minimize defects. Finally, fiducial marks have been identified as a source of unintentional porosity defects. These results enable the refinement of process parameters, ultimately optimizing LPBF to achieve enhanced material density and expedite the printing. [ABSTRACT FROM AUTHOR]

This study aims to investigate the role of oxygen in optimizing the Pore-Free Die Casting (PFDC) process to enhance the quality of aluminum castings by minimizing porosity defects. The effects of oxygen levels on the integrity of high pressure die casting specimens was investigated by injecting oxygen at different durations (1 s, 3 s, and 5 s) through air jet valves installed at the mold cavity. The CT results indicate that increasing the oxygen injection time significantly reduces the porosity volume from 0.9 to 0.18%, with smaller defects in size as well. Notably, after applying the PFDC process, the elongation improved from 2.23 to 4.58%, suggesting that replacing atmosphere in the cavity space with oxygen plays a crucial role in enhancing the mechanical properties of the HPDC specimens. The improvement is believed to be caused by promoting oxidation reactions with the high concentration of oxygen, which leads to a decrease in gas entrapment during the casting process. [ABSTRACT FROM AUTHOR]

Copyright of Automobile Technology & Material is the property of Automobile Technology & Material Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The fatigue performance of laser powder bed fusion-fabricated Ti-6Al-4V alloy was investigated using four-point bending testing. Specifically, the effects of keyhole and lack-of-fusion porosities along with various surface roughness parameters, were evaluated in the context of pore circularity and size using 2D optical metallography. Surface roughness of Sa = 15 to 7 microns was examined by SEM, and the corresponding fatigue performance was found to vary by 102 cycles to failure. The S–N curves for the various defects were also correlated with process window examination in laser beam power–velocity (P–V) space. Basquin's stress-life relation was well fitted to the experimental S–N curves for various process parameters except keyhole porosity, indicating reduced importance for LPBF-fabricated Ti-6Al-4V alloy components. [ABSTRACT FROM AUTHOR]

This study focuses on the influence laws of porosity defects on the performance of the joint formed in the CFRTP/Al6061 laser direct joining process. Image extraction and finite‐element modeling methods are introduced to investigate the porosity defects, so that the model of joint with porosity defects of real characteristics is established. With the model, the failure process of the joints formed under different laser direct joining processes are analyzed, and the change law of the maximum failure displacement and the maximum failure load of the joints are revealed. The maximum error is within 17%. Results suggest that if and only if the porosity defects are distributed in the zone of the joint with the biggest stress under load, the defects will significantly decrease the maximum failure load of the joint; otherwise, the porosity defects will facilitate the cracking propagation process after the joint failure, thereby decreasing the maximum failure displacement. Highlights: Porosity defects are identified from CFRTP background by image extraction.Local porosity is defined to illustrate the real distribution of defects.Joints with defects distributed by real characteristics are modeled.Max failure load hinges on relative position of defects and max stress zone.Porosity defect quantity influences cracking propagation rate after failure. [ABSTRACT FROM AUTHOR]

To explore the size, morphology, and distribution patterns of internal pore defects in WE43 magnesium alloy formed by laser powder bed fusion (LPBF), as well as their impact on its mechanical properties, computer tomography (CT), metallographic microscopy, and scanning electron microscopy were used to observe the material's microstructure and the morphology of tensile test fractures. The study revealed that a large number of randomly distributed non-circular pore defects exist internally in the LPBF-formed WE43 magnesium alloy, with a defect volume fraction of 0.16%. Approximately 80% of the defects had equivalent diameters concentrated in the range of 10∼40 μm, and 56.2% of the defects had sphericity values between 0.65∼0.7 μm, with the maximum defect equivalent diameter being 122 μm. There were a few spherical pores around 20 μm in diameter in the specimens, and unfused powder particles were found in pore defects near the edges of the parts. Under the test conditions, the fusion pool structure of LPBF-formed WE43 magnesium alloy resembled a semi-elliptical shape with a height of around 66 μm, capable of fusion three layers of powder material in a single pass. Columnar grains formed at the edge of individual fusion pools, while the central area exhibited equiaxed grains. The "scale-like pattern" formed by overlapping fusion pool structures resulted in the microstructure of LPBF-formed WE43 magnesium alloy mainly consisting of fine equiaxed grains with a size of 2.5 μm and columnar grains distributed in a band-like manner. [ABSTRACT FROM AUTHOR]

Adjustable ring mode laser beam welding (ARMLBW) has become a promising welding method due to its stable weld quality. In this study, the equation for the distribution of heat flux density of the ring beam was derived, and a combined volumetric heat source model for simulating ARMLBW was proposed. The evolution of the weld pool and keyhole during the welding of AZ31B magnesium alloy using ARMLBW and conventional single laser beam welding (SLBW) laser was compared and analyzed. The results showed that the maximum error in size between the simulated and experimental weld cross-sections did not exceed 10%. Compared to SLBW, the duration of "bubbles" in the weld pool decreased by at least 75% in ARMLBW. The oscillation amplitude of the weld pool and keyhole volume increased by 100%, and the oscillation period was also extended by at least 64%. This is consistent with the experimental reduction in the number of porosity defects (65%), validating the effectiveness of the simulation results. Reducing the oscillation frequency of the weld pool and keyhole, as well as increasing the oscillation amplitude, can result in high-quality welds with fewer porosity defects. Additionally, long and narrow keyholes are more prone to porosity defects in smaller weld pool. The impact of the flowing liquid metal in the weld pool on the keyhole surface is an important factor in the formation of "bubbles," not all of which ultimately become porosity defects. Generally, porosity defects are formed by "bubbles" that have not completely merged with the keyhole and "bubbles" that have not overflowed from the weld pool. The combined action of positive and negative pressure is more likely to cause the formation of porosity defects in smaller weld pool. The formation of humps and depressions on the inner surface of the keyhole is related to the static pressure of the weld pool. [ABSTRACT FROM AUTHOR]

The laser cladding of nickel-based fusion alloys makes them prone to cracks and defects that affect the overall performance of the coating. In this study, Ni60A coatings were prepared at different preheating temperatures (25 °C, 200 °C, 400 °C and 600 °C). The effect of the preheating temperature of the substrate on the corrosion resistance of the coating as well as on the development of defects were investigated by electrochemical tests and immersion experiments in a 65 wt% H2SO4 solution at 60 °C. The results indicate that preheating the substrate to 200 °C can completely eliminate cracks in the coating and reduce porosity. Preheating leads to a decrease in the corrosion resistance of the coating. The size of the porosity defects is related to the law of longitudinal development of the defects. Porosity defects with diameters smaller than 100 μm have a more pronounced tendency to expand vertically than those with diameters larger than 100 μm. [ABSTRACT FROM AUTHOR]

This paper investigates the distribution pattern of welding porosity defects in annular weld seams of superalloy during laser welding. The results reveal that the initial portion of the annular weld seam exhibits a higher density and larger size of porosity defects compared to the ending portion. At a laser power of P = 360 W, the numerical simulation combination showed that the A1 region's average cooling rate is 8.767 × 103 °C/s and the A2 region is 8.151 × 103 °C/s that the thermal conduction effect at the initial welding position leads to an elevation in the base metal temperature in the subsequent unwelded region, thereby reducing the solidification rate of the molten pool in the latter half of the weld seam and effectively decreasing the porosity rate at the weld seam. Furthermore, the study demonstrates that elevating the base metal temperature can effectively modify the cooling rate of the molten pool, thereby influencing the formation of porosity defects. At a laser power of P = 360 W, increasing the base metal temperature to 600 °C reduces the cooling rate of the molten pool from 8.767 × 103 °C/s at a base metal temperature of T = 20 °C to 7.451 × 103 °C/s, leading to a decrease in the porosity rate from 3.357% to 0.022%. At a laser power of P = 672 W, increasing the base metal temperature to 600 °C reduces the cooling rate of the molten pool from 6.781 × 103 °C/s at a base metal temperature of T = 20 °C to 5.056 × 103 °C/s, leading to a decrease in the porosity rate from 8.214% to 0.002%. The increase in laser power brings more heat input, so the solidification rate of the molten pool decreases, but the porosity increases significantly. However, increasing the base temperature can effectively suppress the porosity defects under different laser powers. The research further reveals the relationship between welding porosity rate and cooling conditions, providing a control strategy for achieving low porosity rate welds.

In this study, the melt pool size, precisely its width and depth, are numerically investigated for a wide range of values for both laser power and beam speed. A thermal model, developed on Ansys Additive Science, simulates the SLM of a single bead. A parametric study is achieved aiming at understanding the melt pool evolution and the defects appearing while varying these two parameters. The discussed porosity defects, namely the LOF and keyhole, are determined using the calculated melt pool dimensions and through mathematical correlations from the literature. Moreover, these numerical results are validated with experimental results for the reliability of the study. This investigation reveals a proportional relationship between the melt pool size and the laser power and an inversely proportional relationship with the scan speed. The optimal combination of these two parameters has to be well studied to avoid LOF and keyhole, which is afforded by this paper. At lower laser power levels, such as 100 W, it is advisable to choose a slower scan speed ranging from 400 to 500 mm/s. As the laser power increases, so does the optimal scan speed. For instance, with 150 W, the ideal speed falls between 600 to 900 mm/s. Similarly, for 200 W, the recommended scan speed range extends from 900 to 1200 mm/s, and for 250 W, the optimal speed range lies between 1100 and 1400 mm/s. [ABSTRACT FROM AUTHOR]

To explore the size, morphology, and distribution patterns of internal pore defects in WE43 magnesium alloy formed by laser powder bed fusion (LPBF), as well as their impact on its mechanical properties, computer tomography (CT), metallographic microscopy, and scanning electron microscopy were used to observe the material’s microstructure and the morphology of tensile test fractures. The study revealed that a large number of randomly distributed non-circular pore defects exist internally in the LPBF-formed WE43 magnesium alloy, with a defect volume fraction of 0.16%. Approximately 80% of the defects had equivalent diameters concentrated in the range of 10∼40 μm, and 56.2% of the defects had sphericity values between 0.65∼0.7 μm, with the maximum defect equivalent diameter being 122 μm. There were a few spherical pores around 20 μm in diameter in the specimens, and unfused powder particles were found in pore defects near the edges of the parts. Under the test conditions, the fusion pool structure of LPBF-formed WE43 magnesium alloy resembled a semi-elliptical shape with a height of around 66 μm, capable of fusion three layers of powder material in a single pass. Columnar grains formed at the edge of individual fusion pools, while the central area exhibited equiaxed grains. The “scale-like pattern” formed by overlapping fusion pool structures resulted in the microstructure of LPBF-formed WE43 magnesium alloy mainly consisting of fine equiaxed grains with a size of 2.5 μm and columnar grains distributed in a band-like manner.

Ultrasonic-assisted laser-MIG hybrid welding (LMHW), the ultrasonic vibration applying respectively on the filler wire (LMHW-UF) and workpiece (LMHW-UW), was carried out on the 6 mm thick 6082-T6 aluminum alloy plates. The influences of the ultrasonic vibration modes on the weld formation, porosity defects, microstructure, microhardness, and tensile properties were investigated, and compared with those of the traditional laser-Mig hybrid-welded joints. The results indicated that the arc zone depths increased as the ultrasonic amplitudes increased, and which of the LMHW-UF and LMHW-UW joints were deeper than that of the LMHW joint. The porosity defect contents of the LMHW-UF and LMHW-UW weld seams were both smaller than that of the LMHW weld seams, especially of the LMHW-UF weld seams. The cellular dendrite size in the LMHW weld center was smaller than that in the LMHW-UF and LMHW-UW welds, and the parallel dendrite zone width of the LMHW weld was wider than that of the LMHW-UF and LMHW-UW welds, especially of the LMHW-UF weld. The lowest microhardness existed in the partially melted zone of the LMHW and LMHW-UW joints; however, the lowest microhardness existed in a certain distance from the fusion line of the LMHW-UF joints. The average tensile strengths of the LMHW, LMHW-UF, and LMHW-UW joints were 212.6 MPa, 237.5 MPa, and 228.2 MPa, the fractures occurred in the weld seam, near the fusion line and at the heat-affected zone, respectively. The LMHW and LMHW-UW joints both showed typical mixed ductile and brittle fracture feature, and the LMHW-UF joints showed representative ductile fracture feature. [ABSTRACT FROM AUTHOR]

The location of the defect in the CL65 wheel rims was determined by ultrasonic detection, and workblank samples containing the defect were taken out and processed into fatigue samples. Through the fatigue test carried out on Shimadzu electro-hydraulic servo fatigue testing machine, the fatigue life of samples with defects was obtained.After the test, the shape, size and chemical composition of defects were analyzed. The results show that the main component of shrinkage defects is MnS and the main component of inclusion defects is hard oxides. The fatigue life of samples with shrinkage defects is longer than that of samples with inclusion defects. To analyze the difference between the influence of the two defects on the fatigue properties of wheel materials, the finite element models were established based on the tessellated stress theory. Compared with inclusion defects, the tessellated stress at the interface between shrinkage defects and the matrix is significantly reduced and the extent of stress concentration around defects is small. Therefore, the damage of shrinkage defects containing MnS on the fatigue properties of wheel materials is significantly less than that of oxide inclusions.

The base plate temperature ranks among the crucial building parameters whose effect on melt pool dimensions and porosity defects generation has not been sufficiently discussed in literature. In the current study, with the aim to explore the dependence between melt pool dimensions, porosity defects and base plate preheating, a 3-dimensional thermal finite element model is carried out to create IN718 single beads, at various base plate temperatures. The dimensions of the melt pool behave favourably with the base plate preheating. Indeed, the melt pool depth, width and length increase continuously with the heat of the base plate, from 20 °C to 500 °C. The melt pool width is more responsive to the base plate temperature than the melt pool depth. Numerical results also indicate that the melt dimensions become more responsive to the temperature of the base plate at a slower scan speed. The degree of porosity is predicted under multiple values of base plate temperature and the results show that porosity tends to disappear with further preheating of the base plate. A satisfying accordance between the numerical finding and the experimental results from literature is identified.

Copyright of Journal of Mechanical Strength / Jixie Qiangdu is the property of Zhengzhou Research Institute of Mechanical Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A novel wire arc additive manufacturing process is proposed to produce porous metal (PM). The innovation is to convert harmful welding pore defects into a beneficial structure of PM, and then the PM parts can be additive manufactured layer by layer. Air was transferred to the molten pool as pore promoters to maximize the formation of the welding pore defects. The triple-wire indirect arc process was used for fabricating multi-layer parts to reduce interlayer filling. The primary advantage is that the process can directly produce macro-pore structures rather than depositing pore walls along micro paths in laser additive manufacturing. The uniform diameter of the pore ranges from 500 to 2700 μm, and the typical porosity is 64%, 49%, and 87%. The process could achieve a high deposition rate of 11.5 kg/h. Due to martensite formation, the pore walls can achieve high microhardness of over 200 HV.

Generally, sand casted Al–Li alloy castings have very serious problems in pore defect and interfacial reaction, remarkably weakening the Al–Li alloy castings quality. How to solve these problems is always a challenge. In this paper, a novel inorganic binder coating containing lithium silicate solution binder and silicon carbide powder on the surface of the sand mold was developed to eliminate pore defect and prevent interfacial reaction of the sand casted Al–Li alloy castings. The effective mechanism of the inorganic binder coating on the pore defect and interfacial reaction was discussed. The obtained results showed that the inorganic binder coating formed a thin shell on the surface of the sand mold after pouring, which can effectively prevent the metal-mold interface reaction of the sand casting Al–Li alloy and significantly reduce the porosity defects. Among them, the Al–Li alloy casting prepared using the inorganic binder coating consisting of the lithium silicate binder and silicon carbide powder basically had no pore defects, and the porosity was only 3.73%, which was 66.7% lower than that of the casting prepared by the sand mold without any treatment. Moreover, the interface reaction of the Al–Li alloy and sand mold was fully eliminated, which was attributed to the low overall atomic diffusion rate of the silicon carbide based on molecular dynamics simulation results.

Hot-rolled Al-Si alloy sheets are used to be formed Al alloy welding additions and cladding materials that are widely used in the aviation, automotive, and air conditioning industries. However, if parameters of hot-rolling process are not properly selected, porosity defects during casting will lead to cracks or fractures in Al-Si alloy sheets. This study based on meso-damage theory and established a damage equation describing porosity defects in the hot rolling process. The parameters of GTN (Gurson-Tvergaard-Needleman) void defect evolution, including the volume fraction of voids in different periods, the plastic equivalent variation of void nucleation, the standard variance of equivalent variation of void nucleation, and the correction coefficient of the material, are determined in this study. A finite element model of the porosity defects is developed based on the determined parameters. The finite element method is used to analyze the effect of different process parameters on the porosity defects and cracks in Al-Si alloy sheets during hot rolling, then we performed the hot rolling test to validate the results. The results show that the void stress and diameter are reduced and the quality of porosity defects is improved by decreasing rolling reductions and increasing the temperature. The optimized process parameters were a 20% depression and a temperature of 550°C. [ABSTRACT FROM AUTHOR]

A fully transient discrete-source 3D Additive Manufacturing (AM) process model was coupled with a 3D stochastic solidification structure model to simulate the grain structure evolution quickly and efficiently in metallic alloys processed through Electron Beam Powder Bed Fusion (EBPBF) and Laser Powder Bed Fusion (LPBF) processes. The stochastic model was adapted to rapid solidification conditions of multicomponent alloys processed via multi-layer multi-track AM processes. The capabilities of the coupled model include studying the effects of process parameters (power input, speed, beam shape) and part geometry on solidification conditions and their impact on the resulting solidification structure and on the formation of inter layer/track voids. The multi-scale model assumes that the complex combination of the crystallographic requirements, isomorphism, epitaxy, changing direction of the melt pool motion and thermal gradient direction will produce the observed texture and grain morphology. Thus, grain size, morphology, and crystallographic orientation can be assessed, and the model can assist in achieving better control of the solidification microstructures and to establish trends in the solidification behavior in AM components. The coupled model was previously validated against single-layer laser remelting IN625 experiments performed and analyzed at National Institute of Standards and Technology (NIST) using LPBF systems. In this study, the model was applied to predict the solidification structure and inter layer/track voids formation in IN718 alloys processed by LPBF processes. This 3D modeling approach can also be used to predict the solidification structure of Ti-based alloys processes by EBPBF. [ABSTRACT FROM AUTHOR]

[Display omitted] Metal 3D printing (3DP), a state-of-the-art manufacturing technology that brings the potential to fabricate complex structures at low cost and reduced energy consumption, has been extensively adopted in various industries. However, the porosity defects inherited from the printing process can significantly impede the mechanical properties and weaken the performance of as-printed components, potentially challenging this approach's reliability and reproducibility. The advancement of detection techniques currently opens up a more intuitive and deeper study of porosity defects. Given that, this review systematically states the 'restriction role' of porosity defects in metal 3DP by generalizing the detailed information on porosity defects, including their characterizations, formation and migration mechanisms, and their impacts on the performance of printed parts. Furthermore, feasible porosity mitigation measures are discussed to inspire more advanced methodologies for the next generation of metal 3DP. [ABSTRACT FROM AUTHOR]

Titanium alloys have many advanced applications where they are subject to fatigue. Here, we compare the fatigue crack growth resistance and microstructures of TC4-DT alloys fabricated by wire-arc additive manufacturing (WAAM; sample S1) and in-situ rolled WAAM (sample S2) followed by quasi-β heat treatment. Sample S2 had superior fatigue crack growth resistance, which is mainly attributed to its finer defects and finer α phase. The critical size of the defects related to fatigue performance was calculated. The majority of defects in sample S2 were smaller than the critical size, while several coarse defects (∼1.91 mm) were observed in sample S1. Multiple secondary cracks could be detected near the fracture surface in sample S2, leading to long and tortuous fatigue crack propagation paths. Therefore, in-situ rolling can improve the fatigue properties of titanium alloys fabricated by additive manufacturing.

• A method involving the laser shock modulation of molten pool (LSMMP) was proposed. • In situ LSMMP was implemented in laser additive manufacturing for single-layer and multi-layer thin-walled samples of dissimilar metals. • The modulation mechanism of LSMMP was elucidated. • A maximum densification of over 99.73 % and an ultimate tensile strength of 140.25 MPa were achieved with a pulsed laser shock energy of 2 J. • LSMMP resulted in reduced porosity, improved microhardness, and enhanced connection performance. A method was proposed to address the porosity defects and intermetallic compound production during laser additive manufacturing in dissimilar metals by employing in situ laser shock modulation of molten pool in a hybrid additive manufacturing process. Using wire-feed laser additive manufacturing directed energy deposition technology, single-layer and multi-layer thin-walled samples were prepared. The modulation of molten pool and porosity defects in single-layer molten tracks was analyzed under different pulsed laser shock energies. The mechanism of in situ laser shock modulation of molten pool was investigated through experiment and numerical simulations. The findings indicate that the shock force is the main factor causing oscillating convection in the molten pool, thereby accelerating Marangoni convection, bubble overflow, and pore closure. A pulsed laser shock energy of 2 J resulted in the optimal interface between the aluminum alloy and stainless steel. This interface exhibited a desirable molten track feature size, achieved a maximum densification of 99.73 % and an ultimate tensile strength of 140.25 MPa. Furthermore, in situ laser shock modulation of molten pool has the potential to reduce porosity defects and enhance microhardness, the number of porosity defects decreased by approximately 53.51 % and the total volume of porosity defects decreased by approximately 50.09 %, and microhardness approximately enhanced by 7.86 % at 2 J. Consequently, this method is anticipated to be implemented in the joining of critical thin-walled components to reduce defects and enhance tensile performance. [Display omitted] [ABSTRACT FROM AUTHOR]

This study systematically investigates the gas adsorption characteristics of silicon carbide (SiC) as a potential contributor to defects, including shrinkage and porosity, in SiC particle reinforced Aluminum (SiCp/Al) composites. Key factors affecting these composite materials defects such as SiC particle size, gas atmosphere, and gas pressure, are comprehensively explored. The research begins by elucidating the mechanism through which the melt absorbs gas during the processing flow. Additionally, mathematical models correlating [O] growth with temperature and [N] peak contentwith gas pressure are established. The paper concludes by proposing effective strategies to diminish defects and enhance the purity of composite materials. These strategies include reducing melting time, decreasing melting temperature, increasing the vacuum during melting, and implementing high-temperature degassing. • The fundamental factor of porosity defects in aluminum matrix composites is pointed out, that is the characteristic of SiC adsorbing gas. • Key factors affecting these composite materials defects such as SiC particle size, gas atmosphere, and gas pressure, are comprehensively explored. • The inspiratory mechanism of aluminum melt in three stages of the preparation process is described, and some suggestions for reducing inspiratory and degassing at high temperature are given. [ABSTRACT FROM AUTHOR]

Copyright of Transactions of Nanjing University of Aeronautics & Astronautics is the property of Editorial Department of Journal of Nanjing University of Aeronautics & Astronautics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The base plate temperature ranks among the crucial building parameters whose effect on melt pool dimensions and porosity defects generation has not been sufficiently discussed in literature. In the current study, with the aim to explore the dependence between melt pool dimensions, porosity defects and base plate preheating, a 3-dimensional thermal finite element model is carried out to create IN718 single beads, at various base plate temperatures. The dimensions of the melt pool behave favourably with the base plate preheating. Indeed, the melt pool depth, width and length increase continuously with the heat of the base plate, from 20 °C to 500 °C. The melt pool width is more responsive to the base plate temperature than the melt pool depth. Numerical results also indicate that the melt dimensions become more responsive to the temperature of the base plate at a slower scan speed. The degree of porosity is predicted under multiple values of base plate temperature and the results show that porosity tends to disappear with further preheating of the base plate. A satisfying accordance between the numerical finding and the experimental results from literature is identified. [ABSTRACT FROM AUTHOR]

• Seeded defects did not influence static strengths values but reduced the ductility. • Process inherent defects were critical and led to significant fatigue life reduction. • For the same size of spherical gas pores, defect location is critical. • Murakami's equation is a good parameter to correlate the fatigue life and defects. This study is focused on the individual effect of gas porosity size and location on fatigue strength of a laser powder bed fusion Ti6Al4V alloy. Tensile and fatigue samples were manufactured by artificially seeding gas pores either at the centre or close to surface. Seeded porosity reduced the ductility by 20% and fatigue strength by 33% but had little influence on the static strength. Despite seeded pores, 50% fatigue samples had crack initiation from process inherent defects that also caused fatigue data scatter. A modified Kitagawa-Takahashi diagram was established to predict the fatigue strength under the influence of porosity defects. [ABSTRACT FROM AUTHOR]

[Display omitted] • A fatigue life prediction approach considering crack growth environment is developed. • A crack length criterion for environmental transition is determined. • Marker load method retrieves crack growth data from the fracture surfaces. • Small crack growth rate in pseudo-vacuum is estimated by inverse method. Porosity defects are the main cause of the fatigue failures of directed energy deposited Ti-6Al-4V. Porosity defect-induced cracks initiate in pseudo-vacuum and switch to air upon reaching the specimen surface. In view of this failure characteristic, this study establishes a fatigue life prediction approach based on the fracture mechanics framework considering crack growth environment. Firstly, high cycle fatigue tests at room temperature under atmospheric conditions and fracture surface observation were carried out to obtain the fatigue lives, key defect parameters and crack initiation types. Then, long and small crack growth tests in air were conducted, and crack growth rate curve in air was fitted by the Hartman-Schijve equation variant. Next, the crack growth data in the fracture surfaces were retrieved by the marker load method, which were compared with the crack growth data in vacuum in the existing literature to determine the long crack growth data in pseudo-vacuum. Small crack growth data in pseudo-vacuum were determined by inverse method. And thus, the crack growth rate curve in pseudo-vacuum was established. Finally, a crack length criterion for environmental transition was proposed to segmentally predict the fatigue lives which were compared with the experimental lives to verify the effectiveness of the approach. [ABSTRACT FROM AUTHOR]

The internal porosity defects detected in four castings produced in two foundries were carefully observed and analyzed by using FE-EPMA, and then, the root cause of the defects was investigated. Based on the generally authorized theory that the internal porosities might be formed at late stage of the solidification, the experimental results are verified. The formation mechanism of dual layer and its characteristic folds or creases on inner surface of defects shown in the experimental results could not be explained by the authorized theory, but the features seem to be possible to explain when the basic films might originate on the surface of the liquid metal and might be entrained in turbulent flow. The reductive reaction of carbon and FeO in the solid bifilms formed in the liquid metal is considered to contribute the pore nucleation in ductile iron. Therefore, in addition to the control of the process variables already developed by many researchers, the internal porosity defects in ductile cast irons using resin-bonded sand and cores might be prevented by avoiding the entrainment of carbon and oxide films. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Highlights • Fatigue strength was reduced by a factor of 1.5 owing to gas pore defects. • Fatigue life can be correlated with the stress intensity factor range of the crack initiating pore. • The fatigue limit vs. pore size relation can be represented by a sigmoidal curve. • Fatigue limit can be predicted by a modified El-Haddad model. Abstract 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. [ABSTRACT FROM AUTHOR]

Electron beam melting (EBM) is one of the powder bed fusion technologies, which utilizes a high-energy electron beam, as a moving heat source, in order to melt (by rapid self-cooling) metal powder and produce parts in a layer-building fashion. Anyway, many technical aspects concerning the quality of EBM-produced components are still industrial open items and studies need to be carried out. In accordance with the industrial needs, in this work researchers have studied the influence of two process parameters, i.e., samples orientation and height in the build chamber. The experiments have consisted in rectangular parallelepiped (50 × 10 × 10 mm) samples Ti6Al4V produced by EBM following a two-factor DOE. A tomographic investigation of all the samples produced by EBM has been carried out in order to get a complete set of data on porosity defects that have been analyzed showing the influence of process parameters on the porosity generation and pointing out typical features of defect distributions. The results obtained from this work have given precious information to designers and EBM technologists in order to: optimize the components' design and the building setup obtaining a manufacturing process with the minimal level of porosity defects (best growth orientation). provide information for mechanical post-processing (metal removal). [ABSTRACT FROM AUTHOR]

Highlights • The boiling points of 5A06 alloy under various ambient pressures were calculated. • Wider and deeper keyhole with less humps was obtained under subatmospheric pressures. • Weakened vortices and larger melt flow velocity was produced under subatmospheric pressures. • The keyhole stability and porosity defects was improved obviously. Abstract 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. [ABSTRACT FROM AUTHOR]

Double-sided laser beam welding—successive and simultaneous—is a promising method for joining the skin-stringer T-joints in the aircraft industry, because higher production efficiency and lower airframe weight can be achieved. In this work, a comparative study on the microstructure, mechanical properties, and porosity defects of the successive and simultaneous double-sided laser beam-welded AA6056-AA6156 aluminum alloy T-joints was conducted. The results indicate that there are no significant differences in the microstructure near the fusion line in the two cases, but the microstructure characteristics are markedly different at the centre of the fusion zone. The micro-hardness of the simultaneous weld fusion zone is lower than that of the successive weld unaffected fusion zone but higher than that of the successive weld reheat region. Lower head tensile strength and higher hoop tensile strength are obtained for the successively welded specimens. For the two T-joints, the failure propagates along the fusion layer upon tensile loading, but their final fracture positions are distinctly different. The radiographs of the optimized successively and simultaneously welded T-joints show that many porosity defects exist in the successively welded T-joints, whereas there are very few and even no porosity defects exist in the simultaneously welded T-joints. [ABSTRACT FROM AUTHOR]

A three dimensional mathematic model is proposed to investigate the correlations of melt flow and keyhole behavior with the porosity defects formation at different welding speed. The effects of welding speed on the keyhole behavior and melt flow have been simulated, and the mechanisms influencing the porosity formation and evolution have been discussed. The results indicate that the porosity defects number not only heavily depends on the convective flow above the keyhole, but also relies on the flow direction in the trail of weld pool. Increasing welding speed is benefit for bubbles to escape from weld pool and reducing the porosity defects number. In addition, the porosity defects mainly exist in the upper and lower weld seam when adopting lower and higher welding speed, respectively. The distribution characteristics of the porosity defects at different welding speed have an excellent agreement with the simulated results under the experimental conditions examined. [ABSTRACT FROM AUTHOR]

To address the surface oxidation and porosity defects in the wire-arc directed energy deposition of aluminum alloys, a laser-arc composite cleaning method is proposed for the irregular surface morphology of the deposited layer during the deposition process. The study analyzed the microscopic morphology of the surfaces after various cleaning methods and the changes in surface oxygen content, exploring the complementary rules and enhancement mechanisms of laser and electric arc cleaning in combined cleaning, as well as the optimal process parameter matching strategy. The effectiveness of the cleaning methods was evaluated by testing the porosity of the secondary deposited specimens after cleaning. The uncleaned specimens exhibited a porosity of 2.81%. Laser cleaning reduced this to 2.11%, while arc cleaning achieved 0.13%. The most significant reduction came from composite cleaning, with a minimum porosity of 0.05%. This indicates that composite cleaning can effectively reduce the porosity defects in additive manufacturing.

The effect of traveling magnetic fields (TMFs) on the grain and micro-pore formation in an Al alloy was studied by scanning electron microscope and X-ray microtomography in this work. The results show that with the increasing magnetic flux density, the three-dimensional morphology of the micro-pores transformed from dendrite to a relatively equiaxed structure. Quantified results show that both the micro-pore volume fraction and the average grain size of the primary phase decreased as the TMF density increased. The analyses show that the forced convection induced by TMF can break the dendrites, refine the grain size, and promote the liquid feeding, leading to the decrease in the volume fraction of the porosity and improved mechanical property. The TMF performed at different stages during solidification reveal that the maximum effect of TMF on reducing the micro-pore formation was found when TMF was applied in the stage of nucleation and the early stage of grain growth during solidification.

Scanning laser-MAG hybrid welding with weaving laser was developed for the bead-on-plate welding of high-strength steel. Weaving laser exerted little effect on the droplet transfer behaviour during the scanning laser-MAG hybrid welding process. In order to achieve the keyhole mode of laser welding, the weaving amplitude should be no more than 2 mm. The weld penetration decreased with the increasing weaving frequency. Compared to laser-MAG hybrid weld, the ratio of arc zone width to laser zone width decreased significantly. Scanning laser-MAG hybrid welding could suppress the porosity defects of weld efficiently. The optimal weaving frequency for the porosity defects suppression was 20 Hz. However, the porosity defects increased sharply with the weaving frequency of laser over 40 Hz. The impact-absorbed energy value of specimen fabricated by scanning laser-MAG hybrid welding was about 46 J, increased by about 31.4% compared to about 35 J of laser-MAG hybrid welding. [ABSTRACT FROM AUTHOR]

• A steady electromagnetic field is designed to suppress defect of laser welded joint. • Develop a 3D model to analyze thermal fluid flow in magnetic-assisted laser welding. • Influencing mechanism of magnetic field on stabilizing keyhole is revealed. • The numerical model is validated by the experimental analysis. A steady electromagnetic field with high magnetic flux density was designed to improve weld appearance and suppress porosity defect of aluminum alloy laser welded joint. A comprehensive numerical model considering the multiple reflection of laser beam, the magnetohydrodynamics (MHD), and the vapor shear stress on free surface of keyhole was established to reveal the effect of electromagnetic field on thermal fluid flow and keyhole dynamic. A sufficient Lorentz force (∼1.255 × 106 N/m3) is induced when magnetic flux density reaches to 500 mT. The fluids flow downward, forward, and upward in the upper, middle, and lower region are inhibited by the Lorentz force, respectively. Therefore, less heat exists in the bottom and more heat accumulates in the middle edge of the weld pool, leading to the smaller weld depth, larger length at half depth, and disappeared necking when an electromagnetic field is used. Moreover, the impact of vortex on the rear keyhole wall is weakened by the Lorentz force, which decreases the oscillation and breakup of keyhole. This work can help improve the understanding concerning influence of the electromagnetic field on thermal fluid flow behavior in laser welding and provide guidelines for the suppression of porosity and splash defects. [ABSTRACT FROM AUTHOR]

Laser bead on plate welding of 10 mm thick aluminium alloy under atmospheric and subatmospheric pressures were comparatively investigated. With the decrease of ambient pressure, the penetration depth increased sharply at first and then gradually levelled off. The largest penetration depth could reach 8·7 mm when welded under the pressures of 101 Pa, while only 4·9 mm under atmospheric pressure. Weld bead without any porosity was produced under ambient pressures of 10−1 Pa. The average tensile strength of joints welded under the pressure of 101 Pa was 300·2 MPa. The tensile strength remained constant as the ambient pressure decreased further. The shielding effect of plasma plume on laser beam was suppressed as the ambient pressure decreased. Therefore, the laser power deposition inside the keyhole was enhanced effectively. Under subatmospheric pressure, the porosity defects were eliminated effectively due to the keyhole stability and the change of liquid flow, i.e. moving upward along the rear wall of keyhole. [ABSTRACT FROM AUTHOR]

• Porosity-induced mid-cycle fatigue (MCF) behavior of directed energy deposited (DED) Ti-6Al-4V are investigated. • Pore size classification method is quantitatively developed by combining microstructure and da/dN-Δ K curve in high vacuum. • A quantitative study of the fine granular area (FGA) shows the Δ K FGA in the MCF regime is not a material constant. • The stress level, pore size and pore-to-surface distance are important factors affecting fatigue behavior and performance. This paper investigates the mid-cycle fatigue (MCF) behavior of as-built DED Ti-6Al-4V. Fatigue tests in groups at three stress levels were completed in a laboratory environment. The results show that internal pore initiation mode is more common for this material. The porosity-induced crack initiation mechanism involves a mixture of cleavage and intergranular fracture. Pore size classification method is quantitatively developed, which in turn elucidate the potential relationships among microstructure, pore size, fatigue behavior, and fatigue crack growth stages in high vacuum. A quantitative study of pore sizes and fine granular area (FGA) shows that the FGA stress intensity factor range Δ K FGA in the MCF regime is not a material constant. In addition, three key factors (i.e., stress level, pore size and pore-to-surface distance) affecting the fatigue behavior and performance of the material are discussed. [ABSTRACT FROM AUTHOR]

Although there are many works on the formation mechanism of porosity defects due to solidification in ductile cast iron, the formation mechanism is still not clear and decreasing the porosity defects is still a main issue in the industry. This paper critically reviews conventional explanations for the porosity formation including estimation methods. Based on the discussion the authors propose a formation theory where it considers gas and oxide entrapment during mould filling, expansion of outer part of casting due to graphite formation and pressure decrease in the inner part, followed by the growth of entrapped small gas bubbles. This mechanism can explain various facts in practice and be usable to estimate the defects. It also gives a good way to design effective risers. Future challenges are also discussed including the effect of inoculation on the fluidity of the mushy region. [ABSTRACT FROM AUTHOR]

• A numerical tool is presented for investigating the microstructure and process-induced defects in Ti6Al4V due to the Additive Manufacturing process. • The numerical tool is able to predict the fatigue crack growth rates in the Ti6Al4V considering the impact of material microstructure and porosity defects. • Results show that the crack propagation behaviour is highly dependent on the levels of the porosity in the samples. • Results proved that the developed columnar microstructure due to the layer-by-layer manufacturing contributes to the anisotropy of the fatigue crack growth performance. Additive Manufacturing (AM) has gained a lot of interest due to the freedom to produce complex metal geometries directly from the designed digital model. Despite the high potential of AM, the process induced imperfections, like pores and the microstructural changes due to the layer-by-layer manufacturing, made a significant impact on the fatigue resistance and crack growth behaviour. Consequently, this work aims to evaluate the effect of microstructure and existence of pores in additively manufactured Ti6Al4V on Fatigue Crack Growth (FCG) utilizing bond-based Peridynamics (PD) fatigue model. Employing the columnar granularity and different levels of porosities indicated a substantial impact on FCG rates. [ABSTRACT FROM AUTHOR]