Lack Of Fusion Defects Research Articles

Deepening the scientific understanding of different phenomenology in laser powder bed fusion by an integrated framework

Additive manufacturing technology has greatly improved the design flexibility and accelerated the optimization verification of structure that cannot be easily and economically produced by traditional subtractive manufacturing processes. However, common defects such as surface roughness and porosity, affect the quality and reliability of the components, hindering their wide application. In this study, an integrated framework incorporating high-fidelity powder-scale mechanistic model and physics-informed machine learning is developed to predict the built quality of aluminum and to determine the hierarchy importance of mechanistic variables for different printing qualities in a multi-classification problem in the processing space. The influence of different processing parameters on the built quality is explored by the mechanistic model. A decision tree is constructed and the quality prediction index (QPI) connecting five variables and the printing quality is established. The hierarchy importance of the mechanistic variables is determined by the QPI and three machine learning inductions. The most important factor for balling, good printing quality, keyhole and lack of fusion defects are Fo, TP, fr and Tp, respectively. As the mechanistic variable values are the comprehensive results of multiple processing parameters, this hierarchy ranking not only deepens the scientific understanding of different phenomenology, but also provides new insights and strategies for the process optimization.

Rotating bending fatigue mechanisms of L-PBF manufactured Ti-6Al-4V alloys using in situ X-ray tomography

Lightweight titanium alloys are being increasingly used in high value structures, but in-depth knowledge of the rotating bending fatigue (RBF) mechanism remains insufficient, particularly for additively manufactured components. In response, an in situ RBF test rig was developed for the first time, using time-lapse synchrotron radiation X-ray microtomography (SR-μCT) to identify the failure behavior of laser powder bed fused Ti-6Al-4V alloys. The RBF damage evolution was characterized by combining the non-destructive SR-μCT with post-mortem scanning electron microscopy, and electron backscatter diffraction. Compared with RBF life data and fatigue long crack growth from standard specimens, in situ RBF tests proved to be reliable, in revealing the fatigue mechanism. However, fatigue short cracks in the representative mini RBF testing campaign were found to initiate from microstructural features of outer surfaces, instead of gas pores or lack of fusion defects. It is demonstrated that neither the standard nor the improved Kitagawa-Takahashi diagrams are unable to correlate the fatigue strength to manufacturing defects. The equivalent defect should include the effect of surface microstructural defects, in addition to internal defects, when studying additively manufactured alloys.

Influence of heat treatment, microstructure evolution, and damage mechanism on high cycle fatigue behaviour of additively manufactured Ti-modified Al 2024 alloy

The high cycle fatigue behaviour of titanium modified Al 2024 alloy produced by laser powder bed fusion (LPBF) is investigated to elucidate the effect of the microstructural features, residual stress, and defects (such as porosity and lack of fusion) on dynamic behaviour. The titanium acts as a grain refining agent; it can reduce the solidification cracking and produce the ultrafine-grained (UFG) microstructure in aluminium alloys. The 2% titanium addition resulted in the UFG microstructure with an average size of 0.52 μm in as-built samples. The microstructural features were examined through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron back-scattered diffraction (EBSD) analysis. The influence of the microstructural features, such as grain size, dislocation density, precipitate-dislocation interaction, etc., on the fatigue life of as built and T6 heat treated samples was analysed in detail. The effect of varying stress amplitude at a stress ratio of 0.1 and a frequency of 30 Hz was investigated for the as-built and heat-treated sample. The T6 heat-treated sample showed 21.9% higher fatigue strength compared to the as-built samples (87.8 MPa in the heat-treated sample compared to 72 MPa in the as-built sample). The corresponding damage mechanisms are thoroughly explored through scanning electron fractography, TEM, and EBSD. The defects like porosity and lack of fusion defects in the surface or sub-surface level were observed to be critical regions as crack initiation sites.

Detection and effects of lack of fusion defects in Hastelloy X manufactured by laser powder bed fusion

Laser Powder Bed Fusion (L-PBF) is increasingly used for the manufacturing of complex metal parts. A major hurdle for L-PBF to manufacture critical parts is the lack of knowledge about the effects of inner material defects on the material properties. This paper presents a new method to manufacture specimens with inner lack of fusion (LOF) defects and to detect and localize them simultaneously by the in-process monitoring tool Optical Tomography (OT). The presence of defects at locations indicated by OT was verified through Computed Tomography analyses. Correlative metallographic preparation of the processed specimens showed LOF defects with their main extent within the build plane and sharp edges. Moreover, microstructural investigations revealed a fine globular grain structure and coarse dendritic segregations in the area, influenced by the defect. Correlations between the LOF defects analyzed in the microstructure and corresponding OT indications were established. Tensile tests and high cycle fatigue tests were performed on defective and non-defective material to evaluate the effects of LOF defects on the material properties of Hastelloy® X manufactured by L-PBF. While a minor influence of the LOF defects on static material properties was identified, the fatigue life of the defective specimens was significantly reduced.

Classification of weld defects using machine vision using convolutional neural network

Welding is an important aspect in commercial use of almost every industry. Because weld flaws can cause irregularities or inconsistencies during welding process, welding quality control is a critical step in ensuring the product’s quality and overall longevity. This study focuses on recognizing contamination defects, lack of fusion defects, or if the weld belongs to the good weld category among the defects that occur during the welding process. This category categorization is carried out for the Convolutional Neural Network (CNN) algorithm and the accuracy metric is obtained to evaluate the efficiency of the algorithm for the 3 – class dataset. According to this research, the pure CNN approach gave an accuracy result of 96.1%.

3D numerical modeling for thermo-mechanical behavior of additively manufactured titanium alloy parts with process-induced defects

The defects due to the rapid heating and cooling in powder bed fusion affect significantly the mechanical properties of additively manufactured as-built parts. However, the effect of process-induced defects on the thermo-mechanical behaviors of as-built parts is still unclear. In this paper, considering the process-induced interfacial lack of fusion (LoF) and pores, we developed a coupled thermo-mechanical model of titanium alloy parts in the powder bed fusion process. The equivalent temperature-dependent thermal properties of the parts with defects are calculated based on mixture theory. The three-dimensional thermal, mechanical, and displacement fields are predicted based on the differential quadrature method (DQM). The accuracy of the proposed model is verified by the comparison with the predictions and experimental data in the references. The effect of crucial parameters on the thermo-mechanical responses of as-built parts is analyzed, including energy densities, scan patterns, preheating temperature, LoF dimension, porosity, and powder layer thickness. The predicted results show that the process and geometry parameters play a vital role in the temperature distribution, residual stress buildup, and deformation of the printed parts. In addition, the LoF defect triggers the stress jump at the interface of the defect/powder bed and powder bed/substrate. The predicted results can be beneficial to provide a guideline for the design and additive manufacturing of titanium alloys.

Evaluation of the role of hatch-spacing variation in a lack-of-fusion defect prediction criterion for laser-based powder bed fusion processes

Lack of fusion (LOF) defects impact adversely on the mechanical properties of additively manufactured components produced via laser-based powder bed fusion. Following a stress-relieving heat treatment, the tensile properties and hardness of Ti6Al4V components were found to be negatively impacted by the presence of LOF defects. This work considers a geometrical-based inequality for the prediction of LOF defects. We critically evaluate an LOF criterion using both the experimentally and analytically obtained melt pool geometries. Experimentally, we determined melt pool dimensions by analysing a single-layer, multi-track deposition with oversized hatch spacing in order to establish depth and width from non-overlapping melt pools. Analytically, Rosenthal-based predictions of melt pool size (width and depth) are applied. To investigate LOF defects, we used hatch spacing as the main parameter variation to investigate defects while keeping all other controllable parameters unchanged. An original LOF criterion from the literature was found to be an adequate predictor of LOF defects when experimentally obtained melt pool geometry was used. Critically, however, the analytical expressions for melt pool geometry were found to be in error and this caused the LOF criterion to fail in predicting LOF defects in all cases where defects were observed experimentally. However, an adaptation to the LOF prediction criterion is proposed whereby it is recommended that a correction factor {R}_{c}^{2}=0.7 (or {R}_{c}=0.83) is used with the analytically derived melt pool geometry. Furthermore, this correction is extended into the laser power versus scanning speed operating space to give minimum (corrected) line energy for LOF avoidance in Ti6Al4V components.

A new data-driven framework for prediction of molten pool evolution and lack of fusion defects in multi-track multi-layer laser powder bed fusion processes

This study proposes a new data-driven approach to predict the area fraction of lack of fusion and morphology of 3D-printed parts produced using the laser powder bed fusion process. Different processing parameters were considered to develop this methodology, such as laser power, laser speed, hatch spacing, and layer thickness. Melt pool geometrical parameters were numerically predicted using a mathematical model optimized using the Nelder-Mead optimization method and a machine learning model. The Gaussian process was later employed to further modify the layerwise deposition. The accuracy of the proposed methodology was then extended to predictions of the area fraction of lack of fusion and the morphological aspects of the melt pools in 3D-printed parts by comparing them with the experimental observations. The results revealed that this data-driven approach, which is independent of the thermophysical properties of the material, is capable of reasonably predicting the area fraction of lack-of-fusion formed during material deposition using the laser powder bed fusion process.

Effect of Hot Isostatic Pressing and heat treatments on porosity of Wire Arc Additive Manufactured Al 2319

Porosity and lack of fusion defects that can occur during additive manufacturing are often managed with Hot Isostatic Pressing (HIP). However, the thermal conditions experienced during the HIP cycle may create undesirable phases in some alloys and post HIP heat treatments including solution treatment + ageing may be necessary to achieve the desired final properties. It is currently unknown whether post-HIP heat treatments at standard atmospheric pressures promote the re-emergence of previously closed porosity in aluminium alloys produced by Wire Arc Additive Manufacturing (WAAM). Using micro 3D computed tomography, this work quantitatively investigates the extent of pore regrowth during post-HIP solution treatment and ageing of Al 2319 produced by WAAM. It is found that large porosity (diameters > 35 µm) that is closed during HIP mostly remain closed during subsequent heat treatment but almost a 3-fold increase in the frequency of fine gas porosity occurs during post HIP heat treatment. However, the overall porosity distribution after post HIP heat treatment remains up to 95 % lower than in the original as-built condition, indicating that HIP + heat treatments remain a viable pathway for defect reduction in industrial products.

Effects of Scanning Strategy on the Densification and Microhardness of Selective Laser Melting Mg–Y–Sm–Zn–Zr Alloy

Herein, Mg–Y–Sm–Zn–Zr alloy samples are prepared using selective laser melting (SLM). The effect of the scanning strategy on pore defects and microhardness is investigated. Pore defects are classified into lack of fusion (LOF) defects, gas pore (GP) and keyhole (KH) defects, and irregular pore (IRP) defects according to lengths and roundness. The LOF defect is caused by powder accumulation, spheroidization, droplet solidification preferentially, and pore defects accumulation. The recoil pressure causes the GP and KH defect, and different remelting zones affect its release. The filling behavior of LOF defects at different layer rotation angles and the connection of GPs and KH defects will form IRP defects. The microhardness test shows that the average microhardness is inversely proportional to porosity. The island size of 4 × 4 mm eases the accumulation of powder at the island boundaries, prevents spheroidization, and facilitates adequate spreading of droplets. The layer rotation angle of 67° inhibits the accumulation of pore defects and facilitates the filling of pore defects. The sample with the maximum densification (98.53%) and the highest average microhardness value (95.64 HV) is obtained under this scanning strategy.

Keyhole pores reduction in laser powder bed fusion additive manufacturing of nickel alloy 625

Keyhole pores are common in additively manufactured parts and can badly deteriorate the part's performance. In this study, we demonstrated that the keyhole pores formation in the laser powder bed fusion additive manufacturing process can be significantly reduced by the constant laser power density scan strategy. The constant laser power density is implemented on a custom-built testbed by continuously varying the laser power with the laser scan speed through the time-stepped digital commands developed. Two cubic nickel alloy 625 parts of identical geometry were built, one with the constant laser power density scan strategy, and another with the conventional constant laser power scan strategy. The X-ray computed tomography (XCT) measurement shows a 67% porosity reduction in the part built with constant laser power density. However, the mechanisms for defect formation are not easily distinguishable in XCT, which gives a ‘total’ count of pores. To further investigate the effect of scan strategies on pore formation, two digital twins of process monitoring (DTPM), meltpool intensity volume (MPIV) and melt pool area volume (MAV), were created. The DTPM not only helps to distinguish the keyhole pores from the lack of fusion defects but also provides a foundation for the future development of machine learning models.

Defect formation mechanism of laser additive manufacturing joint for large-thickness Ti6Al4V titanium alloy with Y2O3 nanoparticles

Laser additive manufacturing is a method, through which parts are joined by laser melting deposition (LMD) layer by layer as opposed to the conventional welding processes. It presents superior advantages in joining large-size structural parts in aerospace due to its flexibility in technology and equipment. Owing to the large size of the welded joint and the complex welding process, the defect is easily produced in joining large structural parts. Given its detrimental influence on mechanical properties, defects are a major problem for the connection of large thickness components. This study aims to explore the formation mechanism of defects in joining 80 mm thick Ti6Al4V titanium alloy using laser additive manufacturing. The microstructure and element distribution detection of the welded joint is analyzed to verify the defect formation mechanism. It is indicated that defects during laser melting deposition (LMD) of large thickness Ti6Al4V titanium alloy can be classified into two types, porosity and lack of fusion (LOF). It is revealed that the irregular-shaped pores with a size of 1–5 μm can be observed in the joint area without nanoparticles due to larger dendrite spacing. Additionally, nanoparticles can reduce the porosity, but excessive content of nanoparticles will lead to metallurgical porosity, which decreases the performance of the weld joint. The LOF defects are caused by insufficient energy, which mostly appears between the layers or near the fusion line and has sharp edges.

Importance of Build Design Parameters to the Fatigue Strength of Ti6Al4V in Electron Beam Melting Additive Manufacturing.

The fatigue properties of metals resulting from Powder Bed Fusion (PBF) is critically important for safety-critical applications. Here, the fatigue life of Grade 5 Ti6Al4V from Electron Beam PBF was investigated with respect to several build and component design parameters using a design of experiments (DOE). Part size (i.e., diameter), part proximity, and part location within the build envelope were considered. Overall, metal in the as-built condition (i.e., no post-process machining) exhibited a significantly lower fatigue life than the machined surface condition. In both conditions, the fatigue life decreased significantly with the decreasing part diameter and increasing radial distance; height was not a significant effect in the machined condition. Whereas the surface topography served as the origin of failure for the as-built condition, the internal lack of fusion (LOF) defects, exposed surface LOF defects, and rogue defects served as the origins for the machined condition. Porosity parameters including size, location, and morphology were determined by X-ray micro-computed tomography (XCT) and introduced within regression models for fatigue life prediction. The greatest resistance to fatigue failure is obtained when parts are placed near the center of the build plane to minimize the detrimental porosity. Machining can improve the fatigue life, but only if performed to a depth that minimizes the underlying porosity.

Effect of defects in laser selective melting of Ti-6Al-4V alloy on microstructure and mechanical properties after heat treatment

In this paper, the selective laser melting (SLM) Ti-6Al-4V sample with defects is heat treated (HT). The heat treatment temperatures are 750 °C, 850 °C, and 950 °C. The three temperatures are kept for 2 h, and then air cooling (AC) is used for treatment. Observe the changes in microstructure, phase composition, hardness, and tensile properties. The results show that the existence of defects will not affect the overall change trend of the microstructure, hardness and phase composition of the Ti-6Al-4V sample after heat treatment. However, there are obvious changes in the surrounding of two kinds of defects, which are the surface defects with large area and the lack of fusion (LOF) defects that are communicated with the outside world and extend to the inside of the sample. These two types of defects water, oil and air are easy to enter, and the defects are not easy to remove, heating will produce a large amount of hydrogen. Hydrogen enters the sample through defects, resulting in a large number of β phase around the two types of defects, and white around the defects. The hardness near the defect decreased significantly, and the closer the distance, the more the hardness decreased. A large amount of hydrogen entered into the Ti-6Al-4V sample, resulting in the appearance of new phase δ phase in the phase composition, resulting in the decrease of tensile strength（TS）and yield strength (YS) of the tensile properties after heat treatment, and the obvious decrease of elongation (EL). Moreover, among the various defects, the most important influence on the phase composition and tensile properties is the LOF defect that communicates with the outside world and extends to the inside of the sample.

Joining 30 mm Thick Shipbuilding Steel Plates EH36 Using a Process Combination of Hybrid Laser Arc Welding and Submerged Arc Welding

This article presents a cost-effective and reliable method for welding 30 mm thick sheets of shipbuilding steel EH36. The method proposes to perform butt welding in a two-run technique using hybrid laser arc welding (HLAW) and submerged arc welding (SAW). The HLAW is performed as a partial penetration weld with a penetration depth of approximately 25 mm. The SAW is carried out as a second run on the opposite side. With a SAW penetration depth of 8 mm, the weld cross-section is closed with the reliable intersection of both passes. The advantages of the proposed welding method are: no need for forming of the HLAW root; the SAW pass can effectively eliminate pores in the HLAW root; the high stability of the welding process regarding the preparation quality of the weld edges. Plasma cut edges can be welded without lack of fusion defects. The weld quality achieved is confirmed by destructive tests.

Analysis of element loss, densification, and defects in laser-based powder-bed fusion of magnesium alloy WE43

It is well known that laser-based powder-bed fusion (L-PBF) additive manufacturing of magnesium (Mg) and its alloys is associated with high Mg loss due to vaporization (Mg Loss ) and high incidence of many types of defects in the manufactured parts/samples. Despite this, Mg Loss , densification, and defect characteristics have not been holistically considered in the determination of the optimal values of L-PBF processing parameters for Mg and its alloys. This study presents a combined modeling and experimental approach applied for a widely used Mg alloy (WE43) to address this shortcoming in the literature. First, an experimentally calibrated model is proposed to determine Mg Loss as a function of the L-PBF processing parameters. The model couples the temperature profile using a double ellipsoidal heat source with a Langmuir vaporization model and is calibrated using the width of the single-track L-PBF process and the measured Mg loss using inductively coupled plasma mass spectrometry (ICP-MS). Second, the densification of the samples is determined using a modification of the Archimedes method that considers the amount of Mg Loss in the calculation of the relative density. Third, a comprehensive and quantitative study is conducted on the relationships between the characteristics of porosity defects and the L-PBF processing parameters. Finally, the optimized L-PBF processing parameters are determined by considering the Mg Loss , densification, and the characteristics of defects. The present study yields 0.23 wt.% Mg Loss compared to 2 wt.% Mg Loss that was reported in the previous studies. Furthermore, more than 99.5% densification is achieved while only ∼2% and ∼0.5% of the total defects are characterized as keyhole and lack of fusion defects, respectively.

High cycle fatigue behavior of 316L steel fabricated by laser powder bed fusion: Effects of surface defect and loading mode

The mechanical performances of additive manufactured (AM) material are highly dependent on the fabrication process which inevitably results in surface imperfection as well as porosity. In the present study, the high cycle fatigue (HCF) behavior of an AM stainless steel 316L is experimentally investigated to characterize and evaluate the effect of the inherent surface defects. Profilometry and Computed Tomography are used. A series of fatigue experiments is carried out under different loading modes including tension, bending, and torsion fatigue tests. For each loading condition, different surface preparations are used to investigate the effect of surface state. Fatigue tests reveal that surface treatment can improve fatigue performances, the improvements observed being higher under tension/bending loading than under torsion loading. The fractographic analysis is performed for all the available tested specimens to reveal the mechanism of fatigue crack initiation. Lack-of-fusion (LoF) defects play the predominant role in the fatigue performance of SS 316L fabricated by laser powder bed fusion (LPBF). The presence of multiple LoF defects at the surface or subsurface is detrimental to the endurance under cyclic loading. By using Murakami approach modeling the relationship between fatigue strength and defect size, it is found that the multiple clustering defects act synergistically as one large virtual crack to initiate the fatigue crack.

PBF-EB of Fe-Cr-V Alloy for Wear Applications.

Due to the small variety of materials, the areas of application of additive manufacturing in the toolmaking industry are currently still limited. In order to overcome these material restrictions, AM material development for high carbon-containing iron-based materials, which are characterized by high strength, hardness, and wear resistance, must be intensified. However, these materials are often susceptible to crack formation or lack of fusion defects during processing. Therefore, these materials are preferentially suited for electron beam powder bed fusion (PBF-EB). In this paper, an Fe-Cr-V alloy with 10% vanadium is presented. Investigations were carried out on the PBF-EB system Arcam A2X. Specimens and demonstrators are characterized by a three-phase microstructure with an Fe-rich matrix and VC and M7C3 reinforcements. The resulting microstructures were characterized by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Furthermore, mechanical and physical properties were measured. A final field test was conducted to evaluate durability in use.

Microstructural Development in Inconel 718 Nickel-Based Superalloy Additively Manufactured by Laser Powder Bed Fusion

Excellent weldability and high temperature stability make Inconel 718 (IN718) one of the most popular alloys to be produced by additive manufacturing. In this study, we investigated the effects of laser powder bed fusion (LPBF) parameters on the microstructure and relative density of IN718. The samples were fabricated with independently varied laser power (125–350 W), laser scan speed (200–2200 mm/s), and laser scan rotation (0°–90°). Archimedes’ method, optical microscopy, and scanning electron microscopy were employed to assess the influence of LPBF parameters on the relative density and microstructure. Optimal processing windows were identified for a wide range of processing parameters, and relative density greater than 99.5% was achieved using volumetric energy density between 50 and 100 J/mm3. Microstructural features including melt pool geometry, lack of fusion defect, keyhole porosity, and sub-grain cellular microstructure were examined and quantified to correlate to LPBF parameters. A simple empirical model was postulated to relate relative sample density and LPBF volumetric energy density. Melt pool dimensions were quantitatively measured and compared to estimations based on Rosenthal solution, which yielded a good agreement with the width, but underestimated the depth, particularly at high energy input, due to lack of consideration for keyhole mode. In addition, the sub-grain cellular-dendritic microstructure in the as-built samples was observed to decrease with increasing laser scan speed. Quantification of the sub-micron cellular-dendritic microstructure yielded estimated cooling rate in the order of 105–107 K/s.

Exploiting lack of fusion defects for microstructural engineering in additive manufacturing

Rapid cooling rates and stochastic interactions between the heat source and feedstock in additive manufacturing (AM) result in strong anisotropy and process-induced defects deteriorating the tensile ductility and fatigue resistance of printed parts. We show that by deliberately introducing a high density of lack of fusion (LoF) defects, a processing regime that has been avoided so far, followed by hot isostatic pressing (HIP), we can print Ti-6Al-4V with reduced texture and combinations of strength (TS=1.0 ± 3E-2 GPa) and ductility (ε failure =20 ± 1%) surpassing that of wrought, cast, forged, annealed, solution-treated and aged counterparts. Such improvement is achieved through the formation of low aspect ratio α-grains around LoF defects upon healing, surrounded by α-laths. This occurrence is attributed to surface energy reduction and recrystallization events taking place during the healing of LoF defects via HIP post-processing. Our approach to design duplex microstructures is applicable to a wide range of AM processes and alloys, and can be used in the design of damage tolerant microstructures.