Keyhole-induced Porosity Research Articles

A mechanism-based optimization strategy with adaptive laser power for laser powder bed fusion

The presence of keyhole-induced porosity significantly limits the application of laser powder bed fusion (L-PBF) in the additive manufacturing industry. Typically, a trial-and-error approach is used to select the best printing strategies for material type, powder bed, and laser beam to optimize the manufacturing process. This paper proposes an optimization strategy that uses adaptive laser power based on key understanding of the physical mechanisms of keyhole fluctuations. We identify and analyze five stages of the keyhole pore formation process, including the "J"-like keyhole formation, keyhole closure, keyhole pore collapse, keyhole pore splitting, and keyhole pore motion. Our simulation results suggest that reducing the laser power at the onset of "J"-like keyhole formation is an appropriate optimization approach. We propose adaptive indices to quantify keyhole fluctuations and predict the onset of "J"-like keyhole formation, which facilitates the use of adaptive laser power for optimization. Two parametric studies are conducted to investigate the impact of the optimization criterion and laser power adjustment on keyhole instability. The results demonstrates that our optimization strategy can effectively stabilize the keyhole and reduce the occurrence of keyhole porosity.

Towards implementation of alloy-specific thermo-fluid modelling for laser powder-bed fusion of Mg alloys

Multi-physics thermo-fluid modeling has been extensively used as an approach to understand melt pool dynamics and defect formation as well as optimizing the process-related parameters of laser powder-bed fusion (L-PBF). However, its capabilities for being implemented as a reliable tool for material design, where minor changes in material-related parameters must be accurately captured, is still in question. In the present research, first, a thermo-fluid computational fluid dynamics (CFD) model is developed and validated against experimental data. Considering the predicted material properties of the pure Mg and commercial ZK60 and WE43 Mg alloys, parametric studies are done attempting to elucidate how the difference in some of the material properties, i.e., saturated vapor pressure, viscosity, and solidification range, can influence the melt pool dynamics. It is found that a higher saturated vapor pressure, associated with the ZK60 alloy, leads to a deeper unstable keyhole, increasing the keyhole-induced porosity and evaporation mass loss. Higher viscosity and wider solidification range can increase the non-uniformity of temperature and velocity distribution on the keyhole walls, resulting in increased keyhole instability and formation of defects. Finally, the WE43 alloy showed the best behavior in terms of defect formation and evaporation mass loss, providing theoretical support to the extensive use of this alloy in L-PBF. In summary, this study suggests an approach to investigate the effect of materials-related parameters on L-PBF melting and solidification, which can be extremely helpful for future design of new alloys suitable for L-PBF.

Interfacial microstructure, element diffusion, mechanical properties and metallurgical bonding mechanism of 316L-AlSi10Mg multi-material parts fabricated by laser powder bed fusion

This work explored the interfacial microstructure, element diffusion, mechanical properties and metallurgical bonding mechanism of 316L-AlSi10Mg multi-material parts fabricated by laser powder bed fusion (LPBF). Experimental results revealed that insufficient volumetric energy density (VED) of the laser caused lack-fusion porosity in the 316L-AlSi10Mg transition zone, while too high VED produced keyhole-induced porosity defects. Using the optimal process parameters, multi-material parts can be produced with a good interface metallurgical bonding without significant defects. The partial Fe-FCC phase in 316L stainless steel changed into the Fe-BCC structure, and this shift has also changed the preferred orientation of the grains. The intermetallic compound Al5Fe2 and AlFe phases were found in the transition zone. In addition, Al–Fe icosahedral quasicrystals with five-fold symmetry were found at the boundary of the molten pool, which was caused by an extremely high cooling rate. The tensile strength of 316L-AlSi10Mg specimens is higher than that of AlSi10Mg but lower than that of 316L. In contrast to the 316L and AlSi10Mg regions, the fracture mechanism of multi-material fusion zone exhibits a quasi-cleavage fracture mode. The Vickers microhardness of the Al–Fe interface zone was higher than that of 316L with an average value of 235.57 HV0.2 and AlSi10Mg with 124.59 HV0.2, and the interfacial maximum hardness reached 526.68 HV0.2, which was caused by the very hard intermetallic compound Al5Fe2 and AlFe. The metallurgical bonding mechanism of multi-materials was that the dissimilar metals were mixed and in-situ alloyed in the molten pool by the Marangoni convection-induced strong circular flow during LPBF processing.

Resilience of laser powder bed fusion additive manufacturing to programmatically induced laser power anomalies

Spatter interactions, varying power or scanner parameters, and uneven powder spreading in laser powder bed fusion (LPBF) can trigger the formation of lack-of-fusion or keyhole pores. In this paper, a strategy to mimic natural process anomalies is developed by varying the programmed laser power in a predefined region over sequential layers in order to understand the physics of pore formation and enable the systematic study of the sensitivity of LPBF processed Ti-6Al-4V to process anomalies. Results indicate that lack-of-fusion pores, caused by a decreased laser power input, and located at a depth equal to or less than the subsequent melt pool depth, are partially or fully healed by subsequent, nominally processed layers. Under conditions tested here, lack-of-fusion pores as deep as two layers (∼120 μm) below the surface are healed on subsequent layers. Conversely, local increases in laser power cause persistent keyhole pores, owing to the depth at which keyhole pores become entrapped into the melt—in this work, up to eight layers or 420 μm deep. These results show that while keyhole-induced porosity remains unaffected by the processing of subsequent layers, LPBF is resilient to a set of process anomalies, which would result in lack-of-fusion if not for subsequent remelting on layers above.

The influence of shielding gases on keyhole-induced porosity and nitrogen absorption in SS 304 stainless steel fiber laser welds

Regarding porosity formation and gas content, choosing the appropriate shielding gas for laser welding is essential for achieving high-quality joints. Keyhole-induced porosity formation tendency and nitrogen content in SS 304 stainless steel welds were investigated based on the nitrogen content in shielding gases during fiber laser welding. Beads-on-plate autogenous welds were made at 5 kW continuous wave (CW) fiber laser in N2 and Ar mixtures. Optical metallography, micro-focused X-ray, X-ray radiography, and high-speed images of the molten pool were used to investigate the porosity formation. In addition, a gas analyzer was used to study the weld metal nitrogen content. The results show that nitrogen significantly impacts the reduction of porosity in the melting zone and increases the dissolved nitrogen in the solidified weld metal, as using pure nitrogen leads to an increase in dissolved nitrogen by 36% higher than the nitrogen content in the base metal. In contrast, it has almost no significant effect on the keyhole mode.

Research on keyhole-induced porosity and melt flow kinetic behavior of 2219 alloy T-joint manufactured by dual laser-beam bilateral synchronous welding

Research on keyhole-induced porosity and melt flow kinetic behavior of 2219 alloy T-joint manufactured by dual laser-beam bilateral synchronous welding

Laser-Arc Hybrid Cladding of Al-Mg Alloy Coating on AZ80 Mg Alloy: Effect of Laser Beam Oscillations Amplitude.

The effect of beam oscillating amplitude on the microstructure and performance of AZ80 Mg alloy cladded with Al-Mg alloy coating by laser–arc hybrid welding was studied. The penetration depth decreases significantly while welds are widened because of the increase in the oscillating area of a laser beam. Alloy segregation and keyhole-induced porosity can be suppressed by the laser beam oscillation. With the increase in the oscillating amplitude, the Al distribution becomes uniform in the weld seam because of the rapid and fierce stirring by the oscillating laser. However, the diluting of the cladded Al alloy restrains the formation of the brittle Mg17Al12 phase, and then causes the weakening of hardness and wear resistance of the cladded layer. Considered comprehensively, the optimized oscillating amplitude was 1 mm, which can produce the weld seam with good appearance, fewer segregation and porosity defects, and acceptable hardness and wear resistance.

Remote laser welding simulation for aluminium alloy manufacturing using computational fluid dynamics model

The process of remote laser welding is simulated in this study to identify the keyhole-induced porosity generation mechanisms and keyhole. Three processes are simulated and discussed: laser power levels, laser-beam shaping configurations, and laser keyhole process. The simulation finding reveals that pore development is caused by strong melt flow behind the keyhole. As verification, the equivalent experimental test is also carried out. According to the findings, a welding speed with a high level helps to keep the keyholes released and prevents the flow of strong melt; a big advanced leaning-angle also provides inactive molten pool flow, making it difficult for bubbles to float to the backside of the molten pool. The conclusions of this study offer crucial insight into the method of porosity of aluminum (Al) alloys laser welding, as well as advice on how to avoid keyhole-induced porosity. It is also obtained that a smaller laser beam with constant power raises the velocity, welding pool depth, and liquid metal temperature.

Keyhole-induced Porosity in Laser Manufacturing Processes: Formation Mechanism and Dependence on Scan Speed

Laser welding and laser-based powder-bed fusion additive manufacturing in the deep penetration (keyhole) mode are promising technologies for the synthesis of metal components. The significant potential of these technologies remains latent because of structural defects (porosity), which significantly degrade the structural integrity and performance of the end products. Practical strategies for reducing those defects are addressed through fundamental understanding of their formation. In this study, pore formation of hydrodynamic origin is investigated, including the dynamics and mechanisms of the formation based on the above mentioned technologies. The pore volume and frequency of pore appearance, depending on the amplitude and frequency of capillary vibrations, are considered. Physical analysis is performed to obtain the scanning velocity values for the maximum and zero amplitudes and the frequency of capillary waves. A comparison between calculated curves and experimental data confirms both the capillary origin of the pores and the estimated scanning speeds at which the parameters of the pores exhibit their maximum values or vanish. The results obtained may facilitate in the selection of the optimal scanning speed when designing a pore-free technology.

A study of the magnetohydrodynamic effect on keyhole dynamics and defect mitigation in laser beam welding

In this paper, the highly transient keyhole dynamics, e.g., laser absorption, keyhole geometry, and fluctuation, etc., under a magnetic field are investigated using an experimental approach and multi-physical modeling. The model provides accurate predictions to the variation of penetration depth and weld pool profiles caused by the MHD effect, which is validated by the measurements of optical micrographs and in-situ metal/glass observation. The micro-X-ray computed tomography shows a remarkable reduction of keyhole-induced porosity with the magnetic field. The correlation between the porosity mitigation and the weld pool dynamics influenced by the magnetic field is built comprehensively. It is found that the magnetic field gives a direct impact on the laser energy absorption at the keyhole front wall by changing the protrusion movement. The porosity mitigation comes from multiple physical aspects, including keyhole stabilization, widening of the bubble floating channel, and the electromagnetic expulsive force. Their contributions vary according to the bubble size. The findings provide a deeper insight into the relationship between electromagnetic parameters, keyhole dynamics, and suppression of keyhole-relevant defects.

Thermal flow characteristics and the evolution of laser absorption in laser powder bed fusion of Cu-Cr-Zr alloy

The ultra-high transience and complexity of the LPBF process brings critical challenges to understanding the underlying physics behind the defect formation, and mathematical modeling of the melt pool dynamics in LPBF is being a good method to overcome this issue. Since the laser heating is central to the LPBF process, it is vital to pay more attention to how to define the laser heat source in simulation. This study focuses on the “top free surface” heat source model that is widely used in existing literatures, of which two aspects of deficiencies are modified. On the one hand, the loading pattern of heat source at the interface is optimized by a proposed scattered heat source (SHS) model. The results show that the SHS model has an apparent advantage of less generation of artificial high temperature and better energy convergence. On the other hand, a calorimetric measurement is adopted to obtain the practical effective laser absorptivity under different process parameters. With implantation of measured absorptivity, the simulation fidelity compared to experiments is significantly enhanced in terms the track width and depth, and the absorptivity uncertainty derived from existing literatures on modeling copper alloy is well solved. Next, the physics behind the variation of effective absorptivity with scan speed is studied. Then, the characteristics of Cu-Cr-Zr alloy in LPBF induced by the huge thermal conductivity is identified by comparing with the case of commonly LPBF-used material (stainless steel 316 L). Finally, the influence of scan speed on the resultant track morphology and keyhole-induced porosity evolution is discussed.

Numerical study of keyhole-induced porosity suppression mechanism in laser welding with beam oscillation

The suppression mechanism of keyhole-induced porosity of oscillating laser beam welding (OLBW) was investigated via numerical simulation with keyhole dynamics and melt flow. The established model was verified by the corresponding experimental results. Results show that the weld porosity of 304 stainless steel can be controlled within 0.6% in OLBW and the large keyhole and well-balanced energy of the melt pool are contributing to porosity reduction. Also, two kinds of porosity suppression mechanisms in OLBW were clarified: first, the stirring effect of the oscillating beam keeps the bubble migration speed is superior in the competition with the solidification rate; second, bubbles can be absorbed timely by the dynamic keyhole when the moving speed of the keyhole reaches 504.6 mm s−1.

Laser-based powder bed fusion additive manufacturing of pure copper

In this article, the laser-based powder bed fusion (L-PBF) processing behavior of pure copper powder is evaluated by employing a conventional infrared fiber laser with a wavelength of 1080 nm, a small focal spot diameter of 37.5 µm, and power levels up to 500 W. It is shown that bulk solid copper parts with near full density (ρ Archimedes = 99.3 ± 0.2%, ρ Optical = 99.8 ± 0.1%) can be produced using a laser power of 500 W for the chosen combination of powder particle size, L-PBF settings, and pure copper baseplate. Moreover, at 500 W, parts with a relative density exceeding 99% are manufactured within a volumetric energy density window of 230–310 J/mm3, while laser power levels below 500 W did not produce parts with a relative density above 99%. An analytical model is used to elucidate the L-PBF processing behavior, wherein both conduction and keyhole regimes corresponding to the employed L-PBF settings are identified. The analytical model-based results predict that the bulk solid copper parts with near full density are produced in a keyhole regime prior to the onset of keyhole-induced porosity, which is in accordance with the porosity types observed in the parts. The L-PBF fabricated copper parts exhibit an electrical conductivity of 94 ± 1% compared to the international annealed copper standard (IACS) and demonstrate a tensile strength of 211 ± 4 MPa, a yield strength of 122 ± 1 MPa, and an elongation at break of 43 ± 3% in the as-built condition.

Effect of the welding position on weld quality when laser welding Inconel 617 Ni-based superalloy

Inconel 617 Ni-based superalloy sheets were joined using a laser-welding process. The welding experiments were conducted on four different welding positions (i.e., flat, horizontal, vertical-up, and vertical-down), using two sets of welding parameters with different heat inputs. Then, the geometry, internal defects, mechanical properties, and molten-pool dynamics of the laser joints were evaluated. The results showed that, when using high-heat input parameters, the welding position has an obvious effect on the laser weld quality. The top surface of the flat weld and the root surface of the vertical-down weld are both slightly concave. Moreover, the horizontal weld and vertical-down weld have serious keyhole-induced porosity defects, which lead to relatively worse mechanical properties. The analysis shows that the stability of keyhole differs for different welding positions; keyhole-induced porosity is easily generated in the horizontal and vertical-down positions. Moreover, solid metal blocks escape routes of the bubbles, resulting in more serious porosity for these welding positions. However, when using low-heat input parameters, the welding position has less influence on the laser weld quality because there is less molten metal, which makes the keyhole more stable in different welding positions.

Effect of plate thickness on weld pool dynamics and keyhole-induced porosity formation in laser welding of Al alloy

A three-dimensional (3D) transient numerical model is developed to simulate the keyhole dynamics, melt flow, and keyhole-induced porosity formation. The influence of the plate thickness on keyhole-induced porosity size and number is thoroughly investigated. The simulation results show that the average keyhole depth in plate thickness of 3, 6, and 10 mm is 2.07 mm, 1.89 mm, and 1.77 mm, respectively, and the value of keyhole depth reaching the quasi-steady state is 34 ms, 28 ms, and 14 ms, respectively. Compared to the thinner thickness plate, it is not easy to see the phenomenon of bubble amalgamation in the thickness of 10 mm under the parameter of low laser power and high welding speed. The less porosity in thicker plate laser welding is attributed to lower frequency of keyhole collapse, higher ratio of keyhole open after keyhole collapse to total keyhole collapse number, lower ratio of bubble number to total keyhole collapse number, higher ratio of bubble escape to the total bubbles number, and lower ratio of bubble amalgamation to the total.

Modeling and numerical study of keyhole-induced porosity formation in laser beam oscillating welding of 5A06 aluminum alloy

This paper presents a numerical framework of keyhole-induced porosity formation and methods to suppress porosity in laser beam oscillating welding. Circular and infinity oscillating paths with amplitude of 2 mm and frequencies of 100 Hz and 200 Hz were used. A numerical model for multiple phases, including solid metal, liquid metal and shielding gas is presented using the commercial software FLUENT. An adaptive rotated Gaussian volumetric heat source was developed for analysis of the heat input and temperature distribution during laser oscillating welding. The mechanism of porosity formation caused by keyhole collapse is studied by means of numerical analysis and experiments, and compared to conventional laser welding without oscillation. The numerical simulations were in good agreement with the experimental results. It can be concluded that upon the use of oscillation during welding, porosity decreased and was fully inhibited when using infinity-oscillating path with a frequency of 200 Hz. The developed multi-physics model aids in understanding the dynamics characteristics and keyhole-induced porosity formation during laser beam oscillating welding of 5A06 aluminum alloy.

Modeling of Keyhole-Induced Pore Formation in Laser-Arc Hybrid Welding of Aluminum Alloy with a Horizontal Fillet Joint

A three-dimensional transient model is proposed to investigate the weld pool dynamic behavior in laser + metal inert gas (MIG) hybrid fillet welding of aluminum alloy in the horizontal position, which allows for the joint configuration and coupling of the keyhole, droplet and weld pool as well as the heat and mass exchange between gas and liquid phases and is able to simulate the temperature distribution, fluid flow and formation process of a keyhole-induced pore in hybrid welding with a horizontal fillet joint. The weld porosity is also measured using x-ray nondestructive testing technology. Keyhole behavior and the formation mechanism of keyhole-induced porosity were analyzed. The calculated results are in generally good agreement with the experimental ones. A clockwise vortex always exists at the middle part of the weld pool. The formation and growth of the molten metal bulge on the keyhole wall are responsible for the occurrence of a gas bubble, which has a variation in size and shape and can be split during welding. The keyhole collapses easily at its middle or upper part in horizontal fillet welding, and the capture of the bubble by the upper molten pool boundary enhances the possibility of porosity formation to some degree. The keyhole-induced pore is mainly formed at the regions near the keyhole bottom and the upper fusion line of the weld pool.

On the study of keyhole-mode melting in selective laser melting process

A physics-based computational fluid dynamics (CFD) model was developed to simulate selective laser melting (SLM) process. The heat source model imitates the multiple reflections of the laser beam by using the Fresnel absorption function. The model is able to simulate the fluid flow and heat transfer of keyhole-mode laser melting process, which is validated by single track experiments. In addition, the simulation results show that the melt pool dynamics of the well-deep keyhole is unsteady as compared to the medium-deep keyhole. Different modes of fluid flow, such as downward flow, bottom backward flow, clockwise flow and top forward flow are noticed in the well-deep keyhole melt pool. On the other hand, the melt pool dynamics of the medium-deep keyhole is more stable with two main flows of downward flow and backward flow. Furthermore, the model brings the benefit of predicting the keyhole-induced porosity within the solidified track.

Influence of Arc Power on Keyhole-Induced Porosity in Laser + GMAW Hybrid Welding of Aluminum Alloy: Numerical and Experimental Studies.

A three-dimensional numerical model is used to simulate heat transfer and fluid flow phenomena in fiber laser + gas metal arc welding (GMAW) hybrid welding of an aluminum alloy, which incorporates three-phase coupling and is able to depict the keyhole dynamic behavior and formation process of the keyhole-induced porosity. The temperature profiles and fluid flow fields for different arc powers are calculated and the percent porosities of weld beads were also examined under different conditions by X-ray non-destructive testing (NDT). The results showed that the computed results were in agreement with the experimental data. For hybrid welding, with raising arc power, the keyhole-induced porosity was reduced. Besides the solidification rate of the molten pool, the melt flow was also closely related to weld porosity. A relatively steady anti-clockwise vortex caused by arc forces tended to force the bubble to float upwards at the high temperature region close to the welding heat source, which benefits the escape of the gas bubble from the melt pool. When increasing the arc power, the anti-clockwise region was strengthened and the risk of the gas bubble for capture by the liquid/solid interface underneath the keyhole tip was diminished, which resulted in the lower weld percent porosity.

Role of welding speed on keyhole-induced porosity formation based on experimental and numerical study in fiber laser welding of Al alloy

The keyhole behaviors were observed directly by laser welding experiment with quartz glass. Based on Fresnel absorption of laser beam and multi-reflection combined with volume of fluid (VOF) method, a three-dimensional mathematical model was established to study the role of welding speed on keyhole behaviors and keyhole-induced porosity formation in laser welding of aluminum alloy. The keyhole behaviors and weld pool fluid flow were discussed, and the result shows that, although the welding speed varies, the mechanism of keyhole collapse was similar. However, the keyhole stability at higher welding speed was improved due to reduced weld depth fluctuations, spatter number, keyhole depth to width ratio, and keyhole collapse frequency. Furthermore, the improved keyhole stability impeded the keyhole collapse, and the lower weld depth and solidification rate facilitated bubble escaping from the weld pool easily, which are the main factors for suppression of porosity at higher welding speed.