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Laser powder bed fusion (LPBF) is a 3D printing technology that can print metal parts with complex geometries without the design constraints of traditional manufacturing routes. However, the parts printed by LPBF normally contain many more pores than those made by conventional methods, which severely deteriorates their properties. Here, by combining in-situ high-speed high-resolution synchrotron x-ray imaging experiments and multi-physics modeling, we unveil the dynamics and mechanisms of pore motion and elimination in the LPBF process. We find that the high thermocapillary force, induced by the high temperature gradient in the laser interaction region, can rapidly eliminate pores from the melt pool during the LPBF process. The thermocapillary force driven pore elimination mechanism revealed here may guide the development of 3D printing approaches to achieve pore-free 3D printing of metals., 3D printing pore-free complex metal parts remains a challenge. Here, the authors combine in-situ imaging and simulations to show thermocapillary force can eliminate pores from the melt pool during a laser powder bed fusion process.

The quality of the powder bed is known to be one of the main factors that influence the quality of the part being manufactured by powder-bed-based additive manufacturing processes [1]. For example,...

In additive manufacturing (AM) processes, materials are selectively added in layer-wise fashion to build three-dimensional objects. This approach provides several advantages over conventional manuf...

An Al–Cu alloy micro-alloyed with Mn and Zr (ACMZ) was examined to understand the thermal stability and strengthening mechanism of metastable θ'-Al2Cu precipitates with interfacial segregation after prolonged thermal exposure. The microstructure was characterized at multiple scales with techniques including synchrotron x-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy, and atom probe tomography. The θ' precipitates did not exhibit measurable coarsening after thermal exposure at 300°C for 5000 h. Kinetic effects of Mn and Zr interfacial segregation, which dominate over thermodynamic effects under these conditions, were necessary to understand the complete inhibition of precipitate coarsening. The θ' phase fraction was stable during the 5000 h exposure. This stable phase fraction was regarded as the metastable equilibrium value and was smaller than that predicted by the θ' solvus line of the ACMZ alloy. As expected from the observed phase stability, the alloy hardness also remained stable during the 5000 h exposure. An Orowan mechanism alone was inadequate to explain θ' precipitate strengthening. Additional strengthening mechanisms by θ' precipitates specifically related to the transformation strain may explain the observed hardness values.

Powder spattering is a major cause of defect formation and quality uncertainty in the laser powder bed fusion (LPBF) additive manufacturing (AM) process. It is very difficult to investigate this with either conventional characterization tools or modeling and simulation. The detailed dynamics of powder spattering in the LPBF process is still not fully understood. Here, we report insights into the transient dynamics of powder spattering in the LPBF process that was observed with in-situ high-speed high-energy x-ray imaging. Powder motion dynamics, as a function of time, environment pressure, and location, is presented. The moving speed, acceleration, and driving force of powder motion that are induced by metal vapor jet/plume and argon gas flow are quantified. A schematic map showing the dynamics and mechanisms of powder motion during the LPBF process as functions of time and pressure is constructed. Potential ways to mitigate powder spattering during the LPBF process are discussed and proposed, based on the revealed powder motion dynamics and mechanisms.

Temperature-dependent atomistic structure evolution of liquid gallium (Ga) has been investigated by using in situ high energy X-ray diffraction experiment and ab initio molecular dynamics simulation. Both experimental and theoretical results reveal the existence of a liquid structural change around 1000 K in liquid Ga. Below and above this temperature the liquid exhibits differences in activation energy for self-diffusion, temperature-dependent heat capacity, coordination numbers, density, viscosity, electric resistivity and thermoelectric power, which are reflected from structural changes of the bond-orientational order parameter Q6, fraction of covalent dimers, averaged string length and local atomic packing. This finding will trigger more studies on the liquid-to-liquid crossover in metallic melts.

The structural evolution and dynamical properties of Al2Ag and Al2Cu liquids have been investigated in the temperature range of 943–1153 K by synchrotron X-ray diffraction and ab initio molecular dynamics simulations. It is demonstrated that the local atomic packing of Al2Ag liquid has lower ordering than the Al2Cu liquid, in which Al and Ag atoms are more randomly distributed while Al and Cu atoms tend to form pairs. With decreasing temperature, the bond-angle distributions of Ag Ag Ag and Ag Al Ag triplets shift to 60°, indicating that the local ordering transforms from the icosahedron-like to the hexagonal close-packed structure, while the Cu Cu Cu and Cu Al Cu triplets have the similar angles to those in the crystalline Al2Cu compound. These are consistent with the major Voronoi polyhedrons in both liquids. The dynamic results reveal that Al and Ag atoms diffuse much easier in the Al2Ag liquid than Al and Cu in the Al2Cu liquid, making Al and Ag atoms more randomly distributed and thus reducing the local ordering in the Al2Ag liquid.

Laser powder bed fusion (LPBF) is a 3D printing technology that can print metal parts with complex geometries without the design constraints of traditional manufacturing routes. However, the parts printed by LPBF normally contain many more pores than those made by conventional methods, which severely deteriorates their properties. Here, by combining in-situ high-speed high-resolution synchrotron x-ray imaging experiments and multi-physics modeling, we unveil the dynamics and mechanisms of pore motion and elimination in the LPBF process. We find that the high thermocapillary force, induced by the high temperature gradient in the laser interaction region, can rapidly eliminate pores from the melt pool during the LPBF process. The thermocapillary force driven pore elimination mechanism revealed here may guide the development of 3D printing approaches to achieve pore-free 3D printing of metals.

The effective weight reduction in the automotive industry by the wide adoption of lightweight magnesium (Mg) alloys demands high-quality joint between magnesium alloys and massively-used steels in order to wring the excess weight with strength and safety assurance. However, Mg-steel joint is difficult to achieve because there is no mutual solubility between magnesium and steel and huge disparity in physical properties. An impact-based welding method recently showed successful Mg-steel joining. In this work, the characteristics of Mg-steel interface joined by the impact welding method were investigated. Synchrotron high-energy X-ray computed tomography and diffraction were applied to characterize the microstructure across Mg-steel interface. Results revealed a deposit layer formed at the joint interface where Fe-rich particles spread deep into the Mg matrix. High-resolution 3D morphology of Mg-steel interface demonstrated the trapped pores and cracks inside the deposit layer. The formation of the deposit layer and the void/cracking evolution were analyzed by using finite element models. These findings provide insights into the immiscible Mg-steel joining process.

Powder spreading is a key step in the powder-bed-based additive manufacturing process, which determines the quality of the powder bed and, consequently, affects the quality of the manufactured part. However, powder spreading behavior under additive manufacturing condition is still not clear, largely because of the lack of particle-scale experimental study. Here, we studied particle-scale powder dynamics during the powder spreading process by using in-situ high-speed high-energy x-ray imaging. Evolution of the repose angle, slope surface speed, slope surface roughness, and the dynamics of powder clusters at the powder front were revealed and quantified. Interactions of the individual metal powders, with boundaries (substrate and container wall), were characterized, and coefficients of friction between the powders and boundaries were calculated. The effects of particle size on powder flow dynamics were revealed. The particle-scale powder spreading dynamics, reported here, are important for a thorough understanding of powder spreading behavior in the powder-bed-based additive manufacturing process, and are critical to the development and validation of models that can more accurately predict powder spreading behavior.

The atomic dynamics and its structure dependence in a glass are fundamental issues but still little understood for decades. Through state-of-the-art experiment and simulation, we reveal that the annihilation of excess free volume in a β-relaxation pronounced La–Al–Ni metallic glass slows down the atomic motion by one order of magnitude upon annealing, which is mainly caused by the change of subatomic cavities and enhanced network structure resulting from the presence of Ni atoms, showing significant difference from a β-relaxation unpronounced La–Al–Cu glass. Our findings provide insights into the dynamics and atomic structure of La-based metallic glasses and will be helpful in understanding the microscopic dynamic behavior of metallic glasses in general.

The structures and dynamics of liquid Ag74Ge26 alloy at different temperatures were investigated by high-energy X-ray diffraction and ab initio molecular dynamics simulation. The calculated structure factors agree well with the experimental data. Local atomic structure orders have been characterized by angular distribution functions, Honeycutt-Andersen index, Voronoi tessellation and the atomic cluster alignment. We reveal that (i) , , , and polyhedral dominate in the undercooled Ag74Ge26 liquid. (ii) The icosahedral-like short-range order is enhanced as the temperature decreases while , , , and polyhedral become the major short-range orders at low temperature. (iii) The medium-range order which was formed for amorphous phase at ambient temperature persists in the undercooled liquid. In addition, it is found that the diffusion constant of Ag is lower than that of Ge over the studied temperature range. Our findings elucidate that highly dominated short-range order associated with nearest-neighbors shell could empower the development of medium-range order and the formation of metallic glasses. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Melt flow plays a critical role in laser metal additive manufacturing, yet the melt flow behavior within the melt pool has never been explicitly presented. Here, we report in-situ characterization of melt-flow dynamics in every location of the entire melt pool in laser metal additive manufacturing by populous and uniformly dispersed micro-tracers through in-situ high-resolution synchrotron x-ray imaging. The location-specific flow patterns in different regions of the melt pool are revealed and quantified under both conduction-mode and depression-mode melting. The physical processes at different locations in the melt pool are identified. The full-field melt-flow mapping approach reported here opens the way to study the detailed melt-flow dynamics under real additive manufacturing conditions. The results obtained provide crucial insights into laser additive manufacturing processes and are critical for developing reliable high-fidelity computational models.

In situ high-pressure X-ray diffraction experiments and reverse Monte Carlo simulations on Pd81Si19 glassy alloy were performed up to similar to 35 GPa. High-quality structure factor data with a q range up to 13.5 angstrom(-1) were obtained. The pressure dependence of the relative volume of Pd81Si19 glassy alloy can be well described by a third-order Birch Murnaghan equation of state with bulk modulus of B-0 = 229 GPa and a pressure derivative of B-0(') = 2.7. The continuous and monotonic change of relative volume as a function of pressure does not indicate any first-order phase transitions in Pd81Si19 glassy alloy in the studied pressure range. The pressure dependence of the atomic structures was systematically investigated by means of bond angle distribution, the Honeycutt and Andersen index, Voronoi tessellation, total and partial coordination numbers, and local chemical ordering analyses. It is found that the topological and chemical atomic structures of the Pd81Si19 glassy alloy do not change much with pressure, indicating the stability of this material under pressure. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The local atomic structure evolution in Al2Au alloy during solidification from 2000 K to 400 K was studied by ab initio molecular dynamics simulations and analyzed using the structure factor, pair correlation functions, bond angle distributions, the Honeycutt–Anderson (HA) index and Voronoi tessellation methods. It was found that the icosahedral-like clusters are negligible in the Al2Au stable liquid and supercooled liquid states, and the most abundant clusters are those having HA indices of 131 and 120 or Voronoi indices of 〈0, 4, 4, 0〉, 〈0, 3, 6, 0〉 and 〈0, 4, 4, 2〉 with coordination numbers of 8, 9 and 10, respectively. These clusters are similar to the local atomic structures in the CaF2-type Al2Au crystal, revealing the existence of structure heredity between liquid and crystalline phase in Al2Au alloy.

The dynamic tensile properties of additively manufactured (AM) and cast Al-10Si-Mg alloy were investigated using high-speed synchrotron X-ray imaging coupled with a modified Kolsky bar apparatus. A controlled tensile loading (strain rate ≈ 750 s−1) was applied using the Kolsky bar apparatus and the deformation and fracture behavior were recorded using the high-speed X-ray imaging setup. The synchrotron X-ray computed tomography (CT) and high-speed imaging results worked together to identify the location of the critical flaw and to capture the dynamics of crack propagation. In all experiments, the critical flaw was located on the surface of each specimen. The AM specimens showed significantly higher crack propagation speed, yield strength, ultimate tensile strength, strain hardening coefficient, and yet lower ductility compared to the cast specimens under dynamic tension. Although the strength values were higher for the AM specimens, the critical mode I stress intensity factors were comparable for both specimens. The microstructures of the samples were characterized by CT and scanning electron microcopy. The correlation between the dynamic fracture behavior of the samples and the microstructure of the samples is analyzed and discussed.

The composition- and temperature-dependent liquid structures in eight Alrich-Cu binary alloys (from hypoeutectic Al93Cu7 to hypereutectic Al70Cu30) have been experimentally and computationally studied by x-ray diffraction (XRD) experiments and ab initio molecular dynamics (AIMD) simulations. The remarkable agreements of structure factors for all liquid Alrich-Cu alloys obtained from high-temperature high-energy XRD measurements and AIMD simulations have been achieved, which consolidates the analyses of structural evolutions in Alrich-Cu liquids during the cooling processing by AIMD simulations. The heat capacity of liquid Alrich-Cu alloys continuously increases and presents no abnormal peak when reducing the temperature, which differs from the reported prediction for 55-atom Alrich-Cu nanoliquids. The diffusivities of Al and Cu undergo an increasing deviation from Arrhenius behavior by tuning Cu concentration from 7 to 30 atomic percentages, correlated to the local ordering in these liquids by means of coordination number, bond-angle distribution, Honeycutt-Andersen index, bond-orientational order and Voronoi tessellation analyses. Upon cooling, the microstructure of the liquid Alrich-Cu alloys inclines to form Al2Cu crystal-like local atomic ordering, especially in the hypereutectic liquids. The favorable short-range ordering between Cu and Al atoms could cause the non-Arrhenius diffusion behavior.

X-ray diffraction and electrostatic levitation measurements, together with the ab initio molecular dynamics simulation of liquid Al(75)Cu(25) alloy have been performed from 800 to 1600 K. Experimental and ab initio molecular dynamics simulation results match well with each other. No abnormal changes were experimentally detected in the specific heat capacity over total hemispheric emissivity and density curves in the studied temperature range for a bulk liquid Al(75)Cu(25) alloy measured by the electrostatic levitation technique. The structure factors gained by the ab initio molecular dynamics simulation precisely coincide with the experimental data. The atomic structure analyzed by the Honeycutt-Andersen index and Voronoi tessellation methods shows that icosahedral-like atomic clusters prevail in the liquid Al(75)Cu(25) alloy and the atomic clusters evolve continuously. All results obtained here suggest that no liquid-liquid transition appears in the bulk liquid Al(75)Cu(25) alloy in the studied temperature range.