Porosity Defects Research Articles

Effect of magnetic field orientation on suppressing porosity in steady-magnetic-field-assisted aluminum alloy deep-penetration laser welding

The steady magnetic fields were utilized to decrease the porosity of aluminum alloy laser welded joints, and the effect is related to the magnetic field orientation. The influence of the magnetic field orientation on the suppression of porosity was investigated by establishing a three-dimensional heat flow model coupled with magnetohydrodynamics (MHD) and free surface of a keyhole. The Lorentz force decreased the backflow velocity toward the keyhole wall when the magnetic fields were horizontally parallel to the welding direction (magnetic field X-axis, MFX) and perpendicular to the welding direction (magnetic field Y-axis, MFY), thereby clearly improving the keyhole stability. Additionally, the melt flows in the rear region of the molten pool were suppressed downward and forward under the MFX and MFY, respectively, leading to additional heat accumulation in the rear periphery of the molten pool. As a result, a long molten pool in the longitudinal section and a small penetration depth were observed under the MFX and MFY. Therefore, the porosity defects decreased from 9.4% to 3.5% and 3.2% under the MFX and MFY, respectively. However, many pores were observed when the magnetic field orientation was vertical (magnetic field Z-axis, MFZ), owing to the high penetration depth and poor keyhole stability. This study provides a deeper understanding of the effect of magnetic fields on the laser welding of aluminum alloys and potentially lays a foundation for the adjustment of suitable magnetic field parameters toward obtaining the desired effect.

Evolution of Microporosities in Wide‐Thick Continuous Casting Slab during Heavy Reduction Process

Heavy reduction (HR) is an effective process that can significantly minimize the internal porosity and other internal defects in continuous casting steel. Herein, samples taken from different wide‐thick slab positions are examined by scanning acoustic microscopy and 3D reconstructed. The morphology and distribution of microporosities <1 mm in size in the wide‐thick slab are revealed. The microporosities are mainly distributed in a stripping zone symmetric about the centerline within a 1/3‐slab thickness, and most have a considerably obvious long‐ and short‐axis tendency, which can be approximated as ellipsoids. The microporosity evolution in the wide‐thick slab during HR at the solidification end is numerically investigated with a 3D thermal–mechanical coupled model. The microporosity evolution in an aggregated distribution region during HR is little influenced by the surrounding microporosities. With increasing slab centerline‐microporosity distance, the microporosity minimization effect of HR becomes poor. When the axial ratio (long axis/short axis) is 1.43, the microporosity closure is the largest, and the porosity closure is the highest when the long axis is perpendicular to the reduction direction. When the total segment reduction is constant, the reduction mode with gradually decreasing reduction results in the highest total porosity closure and minimum reduction force.

Microstructure evolution and enhanced mechanical properties of additive manufacturing Al-Zn-Mg-Li alloy via forging and aging treatment

Porosity defects are difficult to overcome in Al-Zn-Mg-Li alloys, despite their high potential for application in the military and aerospace industries. Multidirectional forging and aging treatment were applied to tune the microstructure and mechanical properties of wire arc additive manufacturing Al-Zn-Mg-Li alloy. The grains were elongated along the forging direction attributed to the severe extrusion stress, accompanied by a great density of equiaxed recrystallized grains. As the forging reduction expanded from 30% to 50%, the average grain size was evidently reduced from 23.1 μm to 16.9 μm. A considerable number of porosities were heat-welded during the forging deformation process. The Al2CuMg, Al7Cu2Fe, and Al5CuLi3 phases were severely fragmented and agglomerated in the form of condensed clusters. Furthermore, numerous needle-shaped Al3(Li, Mg) phases were stochastically distributed. As the forging reduction increased from 30% to 50% and the aging temperature/time increased (from 140 °C/5 h to 170 °C/10 h), the rod like η′-MgZn2 phase was obviously coarsened, which considerably deteriorated the properties. The ultimate tensile strength was dramatically increased from 386 MPa to 473 MPa when comparing the specimens with 30% and 50% forging reduction. Both the yield strength and hardness exhibited a similar trend. The elongation was greatly enhanced with a higher forging reduction. Ultimately, the aging behavior was thoroughly investigated and the relationship between the strength η′-phase and yield strength was theoretically discussed.

Study of mode transition in three-dimensional laser beam oscillating welding of aluminum alloy

Although 2-d (two-dimensional) beam oscillation can reduce the porosity defects during laser welding of aluminum alloys, it distributes the heat input, resulting in a higher power required to form a capillary. Therefore, this research draws attention to an alternative approach, which is considered to be beneficial to laser absorption in oscillation laser beam welding, 3-d (three-dimensional) oscillation of the laser beam. (3-d oscillation means that the laser spot moves on the two-dimensional plane and the defocus change simultaneously, causing the heat input to change) An experiment was designed where the variation of oscillation frequency leads to significant changes in capillary behavior. The result shows that the increase of vertical oscillation can promote the formation of a capillary. When the oscillation frequency reaches some special values, the capillary appears and disappears periodically, such as the vertical oscillating frequency is 5 Hz, rotation oscillating frequency is 200 Hz. Meanwhile, welding pores are generally distributed at the position where the capillary begins to disappear. The reason for promoting capillary formation during 3-d laser beam oscillation, and the change of normalized temperature, which is meaningful for understanding the welding mode transition, is illustrated by a simplified model. Compared with traditional linear welding, the experimental results show that it is possible to obtain lower porosity seam during 3-d laser beam oscillation welding in low feed rate, and the mechanism of stabilizing the molten pool is discussed qualitatively.

The effect of cooling rate and degassing on microstructure and mechanical properties of cast AZ80 magnesium alloy

The effect of casting cooling rate and degassing on the mechanical properties of cast AZ80 alloy is investigated. A water-cooled copper mould with a wedge geometry is chosen for the casting, which provides different cooling rates at different sections. The casting is performed with and without the addition of hexachloroethane degassing agent. Macrohardness, hot compression, quasi-static tensile, and stress-controlled cyclic tests are performed to characterize the mechanical properties of the cast samples. The microstructure of the material is studied using optical microscopy and X-ray computed tomography methods. The connection between the microstructure and mechanical test results shows that the morphology of primary α-Mg and porosity defects affect the mechanical properties of cast AZ80. The casting cooling rate controls both of these microstructural features. Higher cooling rates result in a finer dendritic morphology of α-Mg that improves mechanical properties and creates a finer and more discrete morphology of β intermetallic particles. The fine dendritic morphology results in smaller interdendritic regions, which in turn prevents pore coalescence and negates the detrimental effect of porosity on the quasi-static tensile and cyclic properties. In lower cooling rates, where the coarse grain structure of the cast material cannot prevent pore coalescence, degassing benefits the mechanical properties, especially fatigue resistance, by reducing the volume fraction of porosity defects. Moreover, the hot deformation behavior of cast samples shows that the addition of the degassing agent and casting with a higher cooling rate improves the range of deformation conditions where cracking will not occur.

Melt pool feature analysis using a high-speed coaxial monitoring system for laser powder bed fusion of Ti-6Al-4 V grade 23

In laser powder bed fusion (LPBF), defects such as pores or cracks can seriously affect the final part quality and lifetime. Keyhole porosity, being one type of porosity defects in LPBF, results from excessive energy density which may be due to changes in process parameters (namely, laser power and scan speed) and/or result from the part’s geometry and/or hatching strategies. To study the possible occurrence of keyhole pores, experimental work and simulations were carried out for optimum and high volumetric energy density conditions in Ti-6Al-4 V grade 23. By decreasing the scanning speed from 1000 to 500 mm/s for a fixed laser power of 170 W, keyhole porosities are formed and later observed by X-ray computed tomography. Melt pool images are recorded in real-time during the LPBF process by using a high-speed coaxial Near-Infrared (NIR) camera monitoring system. The recorded images are then pre-processed using a set of image processing steps to generate binary images. From the binary images, geometrical features of the melt pool and features that characterize the spatter particle formation and ejection from the melt pool are calculated. The experimental data clearly show spatter patterns in case of keyhole porosity formation at low scan speed. A correlation between number of pores and amount of spatter is observed. Besides the experimental work, a previously developed high fidelity finite volume numerical model was used to simulate the melt pool dynamics with similar process parameters as used during the experiments. Simulation results illustrate and confirm the keyhole porosity formation by decreasing laser scan speed.

Industry 4.0 Foundry Data Management and Supervised Machine Learning in Low-Pressure Die Casting Quality Improvement

Low-pressure die cast (LPDC) is widely used in high performance, precision aluminum alloy automobile wheel castings, where defects such as porosity voids are not permitted. The quality of LPDC parts is highly influenced by the casting process conditions. A need exists to optimize the process variables to improve the part quality against difficult defects such as gas and shrinkage porosity. To do this, process variable measurements need to be studied against occurrence rates of defects. In this paper, industry 4.0 cloud-based systems are used to extract data. With these data, supervised machine learning classification models are proposed to identify conditions that predict defectives in a real foundry Aluminum LPDC process. The root cause analysis is difficult, because the rate of defectives in this process occurred in small percentages and against many potential process measurement variables. A model based on the XGBoost classification algorithm was used to map the complex relationship between process conditions and the creation of defective wheel rims. Data were collected from a particular LPDC machine and die mold over three shifts and six continuous days. Porosity defect occurrence rates could be predicted using 36 features from 13 process variables collected from a considerably small sample (1077 wheels) which was highly skewed (62 defectives) with 87% accuracy for good parts and 74% accuracy for parts with porosity defects. This work was helpful in assisting process parameter tuning on new product pre-series production to lower defectives.

Numerical simulation on evolution process of molten pool and solidification characteristics of melt track in selective laser melting of ceramic powder

A three-dimensional high-fidelity physical model for selective laser melting (SLM) of ceramic powder was created based on computational fluid dynamics (CFD) to examine the physical mechanism of molten pool and solidified tracks at mesoscopic scale. The discrete element method (DEM) was used to generate a randomly packed powder bed, and the volume-of-fluid method (VOF) was applied to dynamically monitor the free surface of the molten pool. The formation mechanism and evolution characteristics of the molten pool were found and analyzed, and the effects of laser power on the typical characteristics of solidified ceramic tracks of SLM were investigated. The molten pool was eventually solidified into a concave geometric shape track by surface tension. The laser power played a significant impact on the shaping quality of solidified ceramic track. When the laser power was too low, the melt track suffers from severe porosity and distortion defects, which can be effectively solved with increasing laser power. The simulation results were validated via single track selective laser melting of TiC ceramic powder.

The effect of porosity defects on the mid-cycle fatigue behavior of directed energy deposited Ti-6Al-4V

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 ΔKFGA 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.

Microstructure related analysis of tensile and fatigue properties for sand casting aluminum alloy cylinder head

The microstructure and mechanical properties of top plate, force wall and bottom plate in low-pressure sand-casting aluminum alloy cylinder head were evaluated at room temperature. The results of the comparison study showed that with different positions of the cylinder head, the microstructure and mechanical properties were considerably different. For the convenience of analysis and discussion, the quality index (Q) was used to characterize tensile test data, which equals ultimate tensile strength plus 150 log elongation. The top plate has the best tensile performance with Q of 443.5 MPa, while the Q of the force wall (292 MPa) and bottom plate (355.5 MPa) is only 66% and 80% of that on the top plate. The tensile properties of the cylinder head are mainly controlled by dendrite arm spacing (DAS), including primary dendrite arm spacing (PDAS) and second dendrite arm spacing (SDAS). The influence intensity varies with the value of DAS. In general, the tensile properties decreased with the increase of DAS. When the aluminum alloy specimen has small DAS (PDAS ≤ 46.49 µm, SDAS ≤ 33.64 µm), Q increases rapidly with the decrease of DAS. However, when PDAS ≥ 104.33 µm, SDAS ≥ 49.82 µm, the effect intensity of DAS on tensile properties is partially reduced. When the number of cycles is 2.5 × 107, the fatigue stress limits of the top plate (69.2 MPa) and bottom plate (70.0 MPa) are highly comparable, which is mainly due to the similar SDAS and eutectic Si average area. Porosity defects also have a significant impact on fatigue performance. The presence of porosity defects increases the scatter of fatigue properties. In addition, more than 90% of the fractures are caused by porosity defects, which indicates that porosity defects are the main factor causing fatigue cracking of low-pressure sand-casting aluminum alloy cylinder head. Therefore, the casting process should be optimized to avoid porosity defects as much as possible in order to improve fatigue performance.

Overlap laser welding of 5052-H36 aluminum alloy: experimental investigation of process parameters and mechanical designs

In this study, the laser welding process is used to join 1.6-mm-thick AA5052-H36 sheets in an overlap joint configuration. Both pulse and oscillation laser beam welding were investigated for the first laser pass. Oscillation beam laser welding in continuous-wave mode show more stable and sound weld with no porosity defects compare to pulse wave (PW) mode. The adopted welding power, speed, frequency, and defocus are 8 kW, 6.5 m/min, 150 Hz, and + 8 mm, respectively. The obtained stitch welds are defects free (blowholes, micro-cracks, or porosities). A circular oscillation ramp-up/ramp-down PW mode is adopted for a second laser surface re-melting (LSR) pass. The corresponding welding power, speed, frequency, and defocus are 5 kW, 2.5 m/min, 500 Hz, and + 15 mm, respectively. Shear tests are then performed to evaluate the mechanical properties of single lap joints (SLJ) for different stitch weld shapes, 2 gap tolerances (0 and 0.5 mm), as well as with/without LSR pass. The best tests’ reproducibility and highest dissipated energies (~ + 42% when compared to the perpendicular direction) are obtained when the stitch weld direction corresponds to the loading direction. The second LSR pass provides more aesthetic joints with higher shear resistance (~ + 1% to + 3%) due to a decrease in the weld surface underfill and undercut imperfections of the stitch weld. The part-to-part gap leads to higher shear resistance (~ + 20%) owing to 2 main reasons: larger welding surfaces at the joint interface and higher hardness of the fusion zone. These findings are of great value for including laser welding technology in the automotive and surface transportation industries.Graphic abstract

Design of the integrated laser welding head with a continuously adjustable focus rotation radius

In this paper, the integrated laser welding head with a continuously adjustable focus rotation radius was achieved by using the output shaft (hollow shaft) of the high-frequency motor as the laser transmission tube for laser transmission, laser deflection and laser focus rotation. The continuous adjustment of the focus rotation radius was achieved by changing the relative phase angle between the two wedge prisms and using a focusing lens. And the function relationship between the focal rotation radius and the relative phase angle of two wedge prisms is verified and modified based on the measured results. The energy loss rate of the 5 kW laser through the inner diameter of the hollow shaft is less than 5% and it has little effect on weld depth. The temperature of the hollow shaft can rise to 57.4 °C only at the entrance of the hollow shaft under the long action time (120 s) of high-power (6 kW) laser. Both the weld forming and porosity defects of 5083 aluminiumalloy can be effectively regulated and controlled by using the integrated laser welding head.

Measurement of the Anisotropic Dynamic Elastic Constants of Additive Manufactured and Wrought Ti6Al4V Alloys.

Additively manufactured (AM) materials and hot rolled materials are typically orthotropic, and exhibit anisotropic elastic properties. This paper elucidates the anisotropic elastic properties (Young’s modulus, shear modulus, and Poisson’s ratio) of Ti6Al4V alloy in four different conditions: three AM (by selective laser melting, SLM, electron beam melting, EBM, and directed energy deposition, DED, processes) and one wrought alloy (for comparison). A specially designed polygon sample allowed measurement of 12 sound wave velocities (SWVs), employing the dynamic pulse-echo ultrasonic technique. In conjunction with the measured density values, these SWVs enabled deriving of the tensor of elastic constants (Cij) and the three-dimensional (3D) Young’s moduli maps. Electron backscatter diffraction (EBSD) and micro-computed tomography (μCT) were employed to characterize the grain size and orientation as well as porosity and other defects which could explain the difference in the measured elastic constants of the four materials. All three types of AM materials showed only minor anisotropy. The wrought (hot rolled) alloy exhibited the highest density, virtually pore-free μCT images, and the highest ultrasonic anisotropy and polarity behavior. EBSD analysis revealed that a thin β-phase layer that formed along the elongated grain boundaries caused the ultrasonic polarity behavior. The finding that the elastic properties depend on the manufacturing process and on the angle relative to either the rolling direction or the AM build direction should be taken into account in the design of products. The data reported herein is valuable for materials selection and finite element analyses in mechanical design. The pulse-echo measurement procedure employed in this study may be further adapted and used for quality control of AM materials and parts.

The Effect of Salinized Nano ZrO2 Particles on the Microstructure, Hardness, and Wear Behavior of Acrylic Denture Tooth Nanocomposite.

The wear of acrylic denture teeth is a serious problem that can change the vertical dimensions of dentures. This study evaluates the effect of adding salinized nano ZrO2 particles on the microstructure, hardness, and wear resistance of acrylic denture teeth. Heat polymerizing polymethyl methacrylate resin was mixed with salinized ZrO2 at concentrations of 5 wt.% and 10 wt.%. Acrylic resin specimens without filler addition were used as a control group. SEM/EDS analyses were performed and the Vickers’ hardness was evaluated. Two-body wear testing was performed using a chewing simulator with a human enamel antagonist. After subjecting the samples to 37,500 cycles, both height loss and weight loss were used to evaluate the wear behavior. The microstructural investigation of the reinforced-denture teeth indicates sound nanocomposite preparation using the applied regime without porosity or macro defects. The addition of zirconium oxide nanofillers to PMMA at both 5% and 10% increased the microhardness, with values of up to 49.7 HV. The wear mechanism in the acrylic base material without nanoparticle addition was found to be fatigue wear; a high density of microcracks were found. The addition of 5 wt.% ZrO2 improved the wear resistance. Increasing the nanoparticles to 10 wt.% ZrO2 further improved the wear resistance, with no microcracks found.

Effect of Runner Distance on Microstructure and Properties of Semi-Solid Squeeze Casting CuSn10P1 Alloy

It is easy to form reverse segregation and shrinkage porosity defects during the solidification of CuSn10P1 alloy, which leads to the poor properties and limits its application in high strength and toughness parts. In this paper, semi-solid CuSn10P1 alloy slurry was prepared by enclosed cooling slope channel (for short ECSC). The effect of runner distance on microstructure and properties by liquid squeeze casting and semi-solid squeeze casting was studied. The results showed that the microstructure of semi-solid squeeze casting is finer than that of liquid squeeze casting, and the shrinkage defects are improved. The solid fraction with 65 mm runner is lower than that without runner in liquid squeeze casting and semi-solid squeeze casting due to the retention effect of solid phase in semi-solid slurry flow, but the properties with 65 mm runner is better than that without runner. The ultimate tensile strength, yield strength and elongation of semi-solid squeeze casting CuSn10P1 alloy with 65 mm runner distance reached 466.5 MPa, 273.6 MPa and 13.4%, which were improved by 26%, 19% and 97%, respectively, as compared to that of liquid squeeze casting.

Porosity and surface defects characterization of hot isostatically pressed Inconel 718 alloy turbine blades printed by 3D laser metal fusion technology

Porosity and surface defects characterization of hot isostatically pressed Inconel 718 alloy turbine blades printed by 3D laser metal fusion technology

Influence assessment of artificial defects on the fatigue behavior of additively manufactured stainless steel 316LVM

The laser powder bed fusion of metals (PBF-LB/M) is one of the most promising techniques to realize lightweight optimized parts and structures. Possible design elements are internal cooling channels or topology optimized geometries. However, not only does the process suffer of instabilities causing pores and lack-of-fusion defects but also a low surface quality. A further knowledge about these defects and their influence on the mechanical behavior are needed to use additively manufactured parts in structural relevant applications. In this work, the austenitic stainless steel 316LVM (X2CrNiMo18-15-3) has been processed by PBF-LB/M. In total, four different batches were manufactured with either no intended porosity or specific cubic defects ranging between 0.3 and 1.5 mm edge length. The fatigue behavior was evaluated at stress ratio R = -1 up to 1E7 cycles. The fracture surface was analyzed by scanning electron microscopy and the relationship between artificial defect size and fatigue strength was investigated by Kitagawa-Takahashi (KT) diagram and its modification by El Haddad’s intrinsic crack length. The results show that the KT-diagram underestimates the fatigue strength of the investigated steel indicating a high defect tolerance and possible hardening mechanisms during cyclic loading such as possible nano-twinning. An influence of the entrapped process gas could also play a role. As long as this is unclear, the models can only be used conservatively as the full potential of the PBF-LB/M steel cannot be fully exploited.

Mechanism of Porosity Development and Defect Engineering in Chemically Activated Woven Carbon Fibres

Mechanism of Porosity Development and Defect Engineering in Chemically Activated Woven Carbon Fibres

Effect of MIG Welding Speed Butt-Joint on Physical and Mechanical Properties of Materials AA 5083

Aluminium is a non-ferrous metal known as a light metal that has good corrosion resistance properties. Aluminium materials can be classified according to their alloying elements. Aluminium alloys are widely applied in the manufacture of marine structure, especially 5083 series aluminium. Parameters in a welding process, especially aluminium, are very important things to consider. The high and low welding speed is a parameter that can affect the mechanical and physical properties of the welded material. The purpose of this study was to determine the mechanical and physical properties of the 5083-aluminium connection. The welding process was carried out on the Butt-Joint connection using 5083 aluminium alloy materials, with plate dimensions of 300 mm × 150 mm × 3 mm. The speed variations used were 10 mm/s, 12 mm/s, and 14 mm/s with a voltage of 20 V, a filler diameter of 0.8 mm, a protective gas flow of 25 L/minute and a torch distance of 5 mm. The results showed that the welding speed variation of 10 mm/s had the largest distortion with a value of 6.89 mm and the highest tensile strength of 194.83 MPa. There were incomplete penetration and linear porosity defects in every variation of welding speed. Micro-porosity observations showed an increase in the welding speed results in more porosity.

Compression of Pulsed Infrared Thermography Data With Unsupervised Learning for Nondestructive Evaluation of Additively Manufactured Metals

Additive manufacturing (AM) of high-strength metals, which is typically based on laser powder bed fusion (LPBF), can introduce microscopic pores in the AM metal. Pulsed Infrared Thermography (PIT) offers several advantages for nondestructive imaging of subsurface defects in AM structures because the method is one-sided, non-contact and scalable to structures of arbitrary size. However, high-resolution PIT imaging results in the generation of a large volume of thermography data (~TB), which creates challenges for the storage and transmission of data. Compression of thermography data requires an approach that achieves high data compression ratio while preserving weak thermal features corresponding to microscopic material defects. We investigate thermography data compression using several unsupervised learning (UL) algorithms, which include Principal Component Analysis (PCA), Independent Component Analysis (ICA), Exploratory Factor Analysis (EFA), Sparse Dictionary Learning (SDL), and a novel lightweight Thermography Compressive Sparse Autoencoder (TCSA). Algorithms are benchmarked using PIT experimental data obtained from imaging of a stainless steel plate with calibrated porosity defects imprinted with AM process. For all algorithms, we obtain compression ratio >30 (highest compression of 46 is achieved with TCSA), and peak signal-to-noise ratio for reconstruction accuracy >73dB. Compared to existing methods, advantages of UL algorithms include achieving high compression ratio while preserving weak features to allow extraction of microscopic material defects from images. UL-based methods have general applicability because they are adaptable to compression of different data types, and allow for memory-efficient training and rapid on-line augmentation of the model.