Balling Phenomenon Research Articles

Investigation of the Softening Resistance Behavior and Its Mechanism in Cu-Ni-Si Alloys with Discontinuous Precipitates

In this study, isothermal annealing experiments were conducted on copper-nickel-silicon alloys containing continuous precipitates (CP) and discontinuous precipitates (DP) to investigate the effects of different types of precipitate phases on the microstructural evolution and softening temperature during annealing, as well as to analyze the differences in softening mechanisms. The experimental results revealed that the softening temperature of the CP alloy, subjected to 75% cold deformation, was 505 °C. In contrast, the DP alloy achieved softening temperatures of 575 °C and 515 °C after 75% and 97.5% cold deformation, respectively. This indicates that the DP alloy exhibits significantly superior softening resistance compared to the CP alloy, attributed to the distinct softening mechanisms of the two alloys. In the CP alloy, softening is primarily influenced by factors such as the coarsening of the precipitate phase, the occurrence of recrystallization, and the reduction in dislocation density. In the DP alloy, the balling phenomenon of the DP phase is more pronounced, and its unique microstructure exerts a stronger hindrance to dislocation and grain boundary motion. This hindrance effect reduces the extent of recrystallization and results in a smaller decrease in dislocation density. In summary, the DP alloy, due to its unique microstructure and softening mechanisms, demonstrates better softening resistance, providing higher durability and stability for high-temperature applications.

Balling behavior and diamond graphitization of metal-matrix diamond composites by selective laser melting: A quantitative investigation

Selective laser melting (SLM) is an effective and potential technology to prepare metal-matrix diamond composites (MDC). In this study, balling phenomenon and graphitization of diamond of CuSn10-diamond composites by SLM were investigated. By univariate analysis of laser power and scanning speed on the morphology of single track, the balling phenomenon is directly affected by the flowability and wettability of the molten pool. By introducing the concept of laser energy density, balling and diamond graphitization are quantitatively evaluated by the standard deviation σ of surface balling height and IG: ID value, respectively. Balling degree increases with laser energy density, but diamond-graphitization turns out to be the opposite. The CuSn10-diamond composites with low balling and no diamond-graphitization can be obtained at moderate laser energy density of 0.17 J/mm. In terms of performances, bending strength is mainly affected the defects caused by balling, and wear properties is determined by the graphitization of diamond abrasives. Graphitization of diamond results in the transformation of wear mechanism from abrasive to adhesive wear. The optimal properties of 185.36 MPa in bending strength, 0.41 in coefficient of friction and minimum wear loss is obtained in the composite sample prepared at 0.17 J/mm.

Empirical calibration method for the thermal simulation of Cu47Ti34Zr11Ni8 single tracks in laser powder bed fusion

Bulk metallic glasses (BMGs) are materials that, due to their amorphous microstructure, offer a unique combination of high strength, hardness, and elasticity, making them attractive for various applications. Using laser powder bed fusion (PBF-LB/M) enables overcoming the current limitations of BMGs in size and shape imposed by traditional manufacturing methods such as casting. Despite its potential, challenges such as porosity, (nano-) crystallization, and impurities affect the mechanical performance of additively manufactured BMGs. This study focuses on the Cu–Ti-based alloy Vit101, known for its higher strength and improved cost-effectiveness compared to Zr-based BMGs. In-situ high-speed pyrometry and thermal simulations of single tracks are employed to enhance the understanding of processing and controlling the thermal cycling of Vit101. The proposed experimental calibration is performed through an off-axis integration of the pyrometer, allowing for in-situ temperature measurements. The acquired data show sufficient congruence with the simulated cooling profiles. Minimal cooling rates in the range of 104 K/s were measured and simulated above the glass transition temperature, indicating a large leeway for further development of glass-forming alloys. Scan track widths are evaluated for validation, resulting in minor deviations between 0.47% and 3.17%. However, challenges emerge at high scanning speeds, leading to higher deviations attributed to balling phenomena, which are not considered in the numerical model.

Towards Selective Laser Melting of High-Density Tungsten

Selective laser melting (SLM) of tungsten (W) is challenging due to its high melting point and brittleness, resulting in defects including balling phenomenon, porosity and cracks. In this work, high-density crack-free SLM W was fabricated by employing cost-effective powders modified through air jet milling. The influence of the SLM processing parameters on microstructure, density, crack formation and the resulting mechanical properties of SLM W was investigated. Laser energy density and hatch distance were found to be the most important parameters in controlling porosity and crack formation of SLM W. The check-like microstructure in horizontal plane was induced by the difference in thermal gradients, which were caused by the movement of the heat source between overlapping regions and central regions of the molten pool. Combined efforts including powder modification through air jet milling, a 67° rotation scanning strategy, a hatch distance of 0.08 mm and a laser speed of 450 mm/s result in dense crack-free SLM W with relative density of 99.3%, microhardness of 403 HV50, and bending strength of 154 MPa. Additionally, the microstructure changed upon annealing at 1200 °C, accompanied by the reduced anisotropy of mechanical properties on both horizontal and vertical plane.

Particle-scale computational fluid dynamics study on surface morphology of GH4169 superalloy during multi-laser powder bed fusion with low energy density

Multi-laser powder bed fusion (ML_PBF) can improve the efficiency of traditional single-laser process, but the study on the potential defects causing by the introduction of multiple lasers is still open. In this paper, the particle-scale computational fluid dynamics (CFD) method is used to simulate the molten pool behavior of nickel-based superalloy in triple-laser PBF (TL_PBF) process with low energy density. The processing parameters on the flow and temperature fields and balling phenomenon in the TL_PBF process are mainly studied. The comparison between the simulation and the experimental results shows that the proposed model is reliable and can reproduce the balling phenomenon. The simulation results show that in the competition of three melt flows, the molten track of the main laser is randomly deviated to one of the molten tracks of the auxiliary lasers due to the uneven distribution of the powder bed. In the TL_PBF process, the larger the vertical offset is, the more serious the balling phenomenon is. When the parallel offset is larger than the sum of the radii of the two adjacent lasers, the larger the parallel offset is, the more the pits generated on the surface of the molten tracks are, leading to the extreme degradation of surface quality. It can be concluded that the synchronous TL_PBF with low energy density can obtain better surface quality when the parallel offset is about the sum of the radii of two adjacent lasers. This work can be used to guide multi-laser additive manufacturing to improve products with better surface quality.

Single-track study of A20X aluminum alloy fabricated by laser powder bed fusion: Modeling and experiments

Optimum process parameter window for recently developed metal powders used for laser powder bed fusion (LPBF) is strongly correlated with characteristics of each single track and single layer. In the present research, the influence of LPBF process parameters on geometrical and microstructural characteristics of A20X aluminum single tracks was studied theoretically and experimentally. Increasing the laser scan speed led to formation of non-homogenous, irregular single tracks. Moreover, enlarging the powder layer thickness to 120 µm induced balling phenomenon owing to a poor wetting of substrate by melt pool. Experimental measurements indicated that width and depth of melt pools decreased up to 53 % and 68 %, respectively by increasing the scan speed from 500 to 1700 mm/s and layer thickness from 40 to 120 µm. These findings were in agreement with predictions of an exponentially decaying heat input model used in this study. The model took into account thermo-physical properties of the alloy at different states. Bead height was observed to increase with the powder layer thickness, but remain unchanged with the scan speed. The process parameter window (energy density range) resulting in the conduction melting mode for the A20X alloy was found to be more than two times wider compared to conventional LPBF aluminum alloys. For all process parameters, a fine equiaxed grain structure was observed in the vertical section of the single tracks. Enhancing the scan speed and reducing the powder layer thickness resulted in a substantial grain refinement. This was well explained by the simulated cooling rates. In contrast to cast Al-Cu alloys, shape, size, and volume fraction of the second phase precipitates were determined to be independence of cooling rate over the investigated cooling rate range (1.4 × 106 to 7.5 × 106 °C/s).

Dual-phase synergistic deformation characteristics and strengthening mechanism of AlCoCrFeNi2.1 eutectic high entropy alloy fabricated by laser powder bed fusion

Eutectic high entropy alloy (EHEA) possesses promising prospects for industrial application due to its controllable and near-equilibrium dual-phase structure. Due to the advantages of high material utilization, efficient production, and design freedom, the laser powder bed fusion (LPBF) technique provides a new path to prepare EHEA components with complex structure and excellent performance. In this study, near fully dense AlCoCrFeNi2.1 samples were obtained by adjusting the process parameters of LPBF. Considering the balling phenomenon and powder splashing during the LPBF process, laser remelting was selected as an optimized scanning strategy to further improve the forming quality of AlCoCrFeNi2.1. The microstructure of remelted AlCoCrFeNi2.1 sample exhibited regular eutectic lamellae consisting of nano-scale face-centered-cubic (FCC) and B2 phases, in which the FCC phase accounted for a higher proportion. By investigating the tensile behavior and deformation mechanism, it was revealed that the ultrafine eutectic lamellae could induce a strong dual-phase synergistic strengthening, thereby significantly improving the strength of the sample. Compared with the vacuum induction melted (VIM) sample, the remelted sample showed a 54% increase in ultimate tensile strength (UTS -1518 MPa) and a 130% increase in yield strength (YS -1235 MPa) with reasonable plasticity. This study indicates that by combining the design and manufacturing freedom of LPBF with the EHEA, it is expected to fabricate high-property 3D EHEA parts, expanding the application field of EHEA.

Melt pool geometry dependent microstructural evolution induced by inoculants in IN718 processed by selective laser melting

This preliminary study aims to clarify how Co, TaCr2 and TiB2 grain refinement inducing inoculants influence the melt pool physics and resulting grain structure in superalloy IN718 processed by laser powder bed fusion (L-PBF). It was demonstrated that the presence of solid inoculant particles could affect the melt pool convection leading to a noticeable reduction in the melt pool width and penetration depth as well as the balling phenomenon. Interestingly, it was found that the penetration depth of the melt pool in samples containing TiB2 appeared to be unchanged due to the presence of nano-particles. Characterization of the resulting grain structures of the various samples revealed that the difference in melt pool geometries contributed to a predominant columnar-grained structure with< 101 > texture in samples containing Co or TaCr2 inoculants while an equiaxed-grained structure with< 001 > texture in samples containing TiB2 inoculants. The effectiveness of inoculants was found to be closely related to their impact on the melt pool physics, in addition to the degree of lattice disregistry with the IN718 matrix.

Toward the Understanding of CoAl2O4 Additions on the Formation of Microstructure in Alloy 718 Processed by Laser Powder Bed Fusion

This investigation rationalizes the impact of oxide particles on the melt pool and subsequent formation of microstructure in superalloy Inconel 718 (Alloy 718) processed by laser powder bed fusion (L-PBF). A mixture of Alloy 718 powder with 0.2 wt pct of CoAl2O4 was processed without scan rotation. It was found that the physical presence of oxides appeared to increase the degree of melt pool penetration by inhibiting the heat transfer. Without favorable thermal conditions to trigger heterogeneous nucleation, a more noticeable tendency to form columnar grains was characterized in samples with CoAl2O4. It is believed that the presence of oxide particles in the melt pool contributed to decreasing the cooling rates during solidification and retarded the columnar-to-equiaxed transition. A comparatively deeper degree of melt pool penetration also enhanced the relative density of samples with CoAl2O4. Despite the occurrence of the balling phenomenon, the reduced Marangoni convection also contributed to less micro-humping and better continuity of the melt tracks that decreased the surface roughness by at least 16.7 pct. The addition of CoAl2O4 to the powder feedstock significantly impacted the processing window of L-PBF processed Alloy 718 and may potentially be used as a means to engineer the microstructure.

Achieving crack-free CuCrZr/AlSi7Mg interface by infrared-blue hybrid laser cladding with low power infrared laser

A coaxial infrared-blue hybrid laser with low power infrared laser was used to clad high-reflectivity CuCrZr alloy on the AlSi7Mg substrate, and the formability and solidification behaviour in cladding with the high crack sensitivity materials under the infrared, blue and hybrid laser were compared. We demonstrated that the combination of a small power (100–200 W) infrared laser with 960 W blue laser can effectively eliminate the cracking and balling phenomenon existed in the cladded samples under the action of both infrared and blue laser. Compared with the coarser Al-dendrite (1.37–2.50 µm) and Al2Cu phase in the cladded tracks formed under the infrared laser (2600 W) and the hybrid laser with high infrared power (1000–2600 W), finer Cu-dendrite (0.45–0.89 µm) is the main precipitation phase in the cladded tracks under the combination of 960 W blue laser and low power infrared laser (100–200 W). The improvement of the formability and solidification behaviour should be related to the stable and high absorption rate (∼65%) of blue laser, as well as the concentrated power density of low power infrared laser. This work validated the potential of the infrared-blue hybrid laser with low power infrared laser in cladding with high-reflectivity and high crack-sensitivity alloys.

Effect of spattering on formation mechanisms of metal matrix composites in laser powder bed fusion

Metal matrix composites (MMCs) with specially designed structures can be fabricated by laser powder bed fusion (LPBF) time-efficiently and cost-effectively, but reinforcements pose enormous challenges to the technique. In this work, the effect of reinforcement on the formation of MMCs in LPBF is studied by taking diamond grinding wheels (GWs) as an example. Based on the balling phenomenon observed in the LPBF process, the formation characteristics of GWs such as surface morphology, porosity, and flexural strength are investigated. The balling size is increased with an increase in the linear energy density of the laser beam due to more intense diamond spattering. The acting forces applied by the plume and ambient gas flow generated around the melt pool on a diamond grain are analyzed to investigate the generation of diamond spatter. For MMCs with coarse reinforcements, spattering is the dominant cause of the balling phenomenon due to the induced discontinuous melt tracks and uneven powder layers. Furthermore, the performance of a cup-type diamond GW fabricated by LPBF in the electrical discharge grinding (EDG) process of reaction-bond silicon carbide (RB-SiC) is evaluated by the surface and subsurface quality. The grinding process removes the resolidified layer of RB-SiC induced during the electrical discharging process and suppresses crack generation in the subsurface. The results presented in this study reveal the influence of diamond grains on the formation mechanism of GWs, which is also suitable for MMCs with coarse reinforcements.

Deep learning-based X-ray computed tomography image reconstruction and prediction of compression behavior of 3D printed lattice structures

Complex thermal processing during laser powder bed fusion ( L -PBF) inevitably induces heterogenous and anisotropic defects, which can affect the macromechanical response of components. The initial morphological features of L -PBF printed samples can be reconstructed with X-ray micro-computed tomography (μCT) scans and the effects of defects on the mechanical properties of the sample can be predicted using image-based finite element modeling. However, due to balling phenomena, several powder particles adhere to the surface of the lattice structure and the mechanical properties of these particles differ from that of the parent material. The predicted modulus and strength of the direct reconstructed models are much higher than the experimental values because of overestimated particle effect. Therefore, these particles should be removed from the CT images before high-fidelity numerical simulation. This paper proposes a new image segmentation method based on a U-Net convolutional neural network to remove adhered particles on the L -PBF manufactured lattice structure from μCT slices. High-fidelity image-based finite element models were constructed with and without the deep learning-based image preprocessing. The results were compared with those obtained from simulations using an ideal CAD model and experimental results. The proposed deep learning-based preprocessing method enables high-precision reconstruction and efficient finite element simulation prediction of additive manufactured lattice structures.

Effects of microbial strains on the mechanical and durability properties of lightweight concrete reinforced with polypropylene fiber

This research investigated the impact of calcium carbonate precipitation induced by the activity of four microbial strains known as Sporosarcina pasteurii, Bacillus megaterium, Sporosarcina ureae, and Bacillus licheniformis at cell concentrations of 107 cells/ml on lightweight fiber concrete (LWFC). Three mixed designs with different fiber percentages (0%, 0.5%, and 1%) were considered for evaluation of the effects of these bacteria and polypropylene fiber on the mechanical properties and durability of LWC, which contained Leca aggregate. The experimental results indicated that the simultaneous addition of bacteria and fiber was effective and improved compressive, tensile, and flexural strength. It also reduced concrete water absorption and permeability in the specimens. In specimens with B. licheniformis and 1% fiber, a maximum increase of approximately 32.65% was observed in tensile strength and a 28.39% decrease was observed in water absorption with respect to the control specimens. A comparison of the bacterial specimens revealed that B. licheniformis and B. megateterium performed better in the induction of calcium carbonate precipitation and distribution of fiber in the concrete matrix than S. pasteurii and S. ureae to reduce the balling phenomenon. In addition, SEM-EDX and XRD analyses were conducted to verify the formation of calcium carbonate in the concrete matrix and characterize the crystalline phases of the calcite.

Wetting behavior of selective laser melted (SLM) bio-medical grade stainless steel 316L

Improvement in the surface wettability of the additively manufactured 316L stainless steel (316L SS) is still a challenging factor for the permanent implant's materials. The present study utilizes a laser-based additive manufacturing method, the selective laser melting (SLM) technique, to fabricate the biomedical grade stainless steel 316L (316L SS) sample. The microstructural, mechanical, and wetting behavior of the SLM sample are studied and compared with the conventionally casted 316L SS. sample. The SEM and 3D images present the top surface morphology of the fabricated parts at different magnifications. SEM micrographs show that the sample is formed bi-directionally in a 67° rot scan with the balling effects. The mechanical property of the selective laser melted and conventionally casted 316L SS is evaluated using the Vickers micro-hardness measurement. The average value of micro-hardness is 291.0147 and 186.3013HV for fabricated and casted 316L SS samples, respectively. Furthermore, the effects of balling phenomena on the surface wettability are analyzed by the sessile drop method using deionized (DI) water and Hank's balanced salt solution (HBSS), which simulates the human body fluid.Selective laser melted samples and casted 316L stainless steel sample present the contact angle values in the range of 59.3–86.9°. Selective laser melted samples show more hydrophilic wetting behavior and present a high affinity for the HBSS. Obtained results indicate that the SLM technique is a promising approach to fabricate 316L SS implants with enhanced mechanical and wetting properties.

Formation characteristics of nickel-based diamond abrasive segment by selective laser melting

Selective Laser Melting (SLM) technique provides an alternative to fabricating a metal bond grinding wheel with a high porosity ratio and controllably complex geometries. In this paper, SLM is applied to making nickel-based diamond abrasive segments based on single-tracks SLM tests. The characteristics of the SLMed wheel segments are investigated through analyses on morphology, porosity, cracks, and diamond grains. Balling phenomenon of the Ni-Cr alloys, which leads to the rough surface and porosities, is observed through the surface and cross-sectional observations. The porosity ratio of the as-built abrasive segment ranges from 22.6% to 27.3%. Two types of diamond spatters, entrainment-driven and vapor-driven diamond spatters, are formed in the SLM process based on the observation by a high-speed video camera. Diamond spatters are considered the main cause of the balling phenomenon. Based on microstructure and hardness measurements, the eutectic structure and granular crystallites result in high hardness and increase the cracking susceptibility of the Ni-Cr alloys. This paper reveals the formation characteristics of the SLMed nickel-based wheel segments and the mechanism of porosity formation in the SLM process.

Investigation into the effect of energy density on densification, surface roughness and loss of alloying elements of 7075 aluminium alloy processed by laser powder bed fusion

In this study, the effects of volume energy density (VED) on densification behaviour, surface morphology and loss of alloying elements of 7075 aluminium alloy processed by laser powder bed fusion at a broad range of processing parameters were investigated. The results showed that the overall relative density of 99.4% was obtained by averting the irregular pores and keyhole pores owing to the optimisation of processing parameters. However, the hot cracks cannot be fully removed, which was mainly attributed to the substantial columnar grains and lack of liquid backfill during the last stage of solidification. Severe balling phenomenon and large gaps between tracks were recognized as the principal causes that may worsen the surface quality. There was a good linear relationship between the VED and the loss in weight contents of low-melting-point elements Zn and Mg, despite the fact that the VED had no influence on the loss of Cu and Cr. The thermodynamic calculation and experiment showed that the weight ratio of Zn to Mg, a critical element ratio in 7075 alloy, varied inversely with VED.

Balling phenomenon in metallic laser based 3D printing process

A comprehensive model is developed coupling major physical phenomena inherent to the Selective Laser Melting process together with a 3D numerical model based on the discrete element method to study the effect of the process parameters on the generation of balling droplets in the laser melting process. Effects of several process parameters as well as material properties on the balling phenomenon are investigated. Simulation results compare well to the experimental findings in the literature.

Single-track investigation of IN738LC superalloy fabricated by laser powder bed fusion: Track morphology, bead characteristics and part quality

• Single-track laser powder bed fusion experiments were performed on IN738LC. • Bead dimension varied significantly with respect to used processing parameters. • Balling occurred when solidification time was less than spread time of droplets. • Conduction mode transferred to keyhole as normalized enthalpy was larger than ∼ 50. • High porosity appeared in bulk samples at both low and high energy input densities. Single-track experiments were performed to investigate how the scan speed and the laser power affected the behavior of the single tracks during laser powder bed fusion, including the track stability, the melt pool dimension and the bead mode, using the nickel-based superalloy IN738LC. The processing parameters had an evident effect on the track morphology and the bead features in the experimental conditions investigated. The balling phenomenon was studied by a competitive model between the spread and the solidification of droplets. The analysis and experimental results clearly demonstrated that the track lost its stability when the droplets solidified before they spread out on the substrate with a contact angle greater than 90°. The keyhole mode was investigated by establishing the relationship between the normalized enthalpy and the normalized bead depth. The conduction mode would convert to the keyhole mode as normalized enthalpy was greater than ∼ 50. Finally, bulk samples were built with the same parameters as the single-track testing. It can be seen that the parts with high porosity appeared at both low and high energy input densities due to the un-melted powders and the keyhole pores, respectively.

Balling Behavior of Selective Laser Melting (SLM) Magnesium Alloy.

Macroscopic surface morphology and balling mechanism of AZ61 magnesium alloy prepared by Selective laser melting (SLM) have been investigated. This article studied and analyzed the surface morphology and balling phenomenon of Mg in the laser processing from the aspects of Mg inherent metal properties and laser processing. In terms of laser processing, the results show that, in the direction of increasing scanning speed, the energy density decreases, and the phenomenon of balling and porosity on the surface of the magnesium alloy is serious. When the energy density is 133.9–187.5 J/mm3, balling particles are significantly reduced. It can be seen from the low-magnification SEM image that, even at a scanning speed of 250 mm/s (Ev is 187.5 J/mm3), there are still a few small-sized balling particles on the surface. Therefore, in terms of inherent metal properties, the wettability, capillary instability, thermodynamic, and kinetic analysis of the balling behavior of Mg and other metal (Al, Fe, Cu, Ni, Ti) droplets in the SLM process has been carried out, and the dynamic model of magnesium droplet spreading/solidification was established basic on the result of experiment and metal inherent properties. The results show that SLMed magnesium alloy is a competitive process of melt diffusion and solidification. The final result depends on the intrinsic properties of the magnesium alloy and the applied laser processing parameters. The spreading process of Mg melt is very fast. Although the solidification time of Mg melts changes slowly with the increase of metal droplet temperature, the spreading speed is still very fast due to the low melt density, so the balling phenomenon of SLMed Mg can be controlled to a certain extent. Theoretically calculated, the solidification time of Mg melt droplet is longer than the wetting time at 1173 K (900 °C), so the spreading process is dominant, which can minimize the balling and realize the densification of SLMed Mg. The dynamic spreading of molten pool, the analysis of wetting and solidification process, and the establishment of SLM balling model can provide reference for the design of the SLM forming parameters of Mg and other different metals.

Effect of laser parameters on microstructure and phase evolution of Ti-Si-C composites prepared by selective laser melting

In this paper, blended Ti/SiC powder was processed by selective laser melting (SLM) method to prepare a novel kind of Ti-Si-C composite. Microstructure, elements distribution and phase evolution of Ti-Si-C composites fabricated by SLM were studied. SEM, EDS, TEM and XRD were performed to evaluate the microstructure and phase evolution during the SLM process. The results showed that processing parameters significantly affected the microstructure and element distribution of the samples. It was found that higher laser energy density could promote the densification process and hence suppress balling phenomenon of Ti-rich phases. At the micro-level, the microstructure could be classified into the dense area and the porous area, which was closely related to the redistribution of elements. The phases obtained by the SLM process were composed by TiCx, TiSi2, Ti5Si3, and Ti5Si4, which appeared in different microstructures. The results indicated that composites with different phases and microstructure could be obtained by merely modifying laser processing parameters, which could give experimental guidance for the fabricating of gradient Ti-Si-C composites by selective laser melting without additional apparatus on powder feeding.