Direct Laser Sintering Of Cu-Based Metal Powder: Key Processes And Basic Mechanisms

As an important branch of rapid prototyping (RP) techniques, direct metal laser sintering (DMLS) enables the quick fabrication of complex shaped three-dimensional (3D) components by layerwise fusing loose powder with a scanning laser beam, without the use of fixturing or tooling. However, DMLS is still in its early stage of development. Significant further research and understanding are required in the aspects of materials preparation and characterization, process control and optimization, and theories of physical and chemical metallurgy involved in DMLS. In the present thesis, the direct laser sintering of Cu-based metal powder is carried out, with the main research work listed as follows.In the first part of this thesis, key processes and basic mechanisms involved in direct laser sintering of multi-component Cu-based alloy powder (i.e., Cu-Cu10Sn-Cu8.4P) are studied, with the main conclusions drawn as follows.(1) Design, preparation, and laser sintering mechanism of multi-component Cu-based metal powder, obtaining materials properties, preparation technique, and characterization method of metal powder for the use of DMLSDue to the special forming fashion (i.e., line-by-line sintering followed by layer-by-layer bonding) and sintering process (i.e., transient metallurgical process induced by high-energy laser scanning) of DMLS, the material formability is of special concern, i.e., the chemical compositions and physical properties of powder materials should be feasible for powder deposition and laser sintering. In the present work, a multi-component Cu-based metal powder, which consists of a mixture of Cu, Cu-10Sn, and Cu-8.4P, is developed for DMLS. Laser sintering of this powder system is through the mechanism of liquid phase sintering with partial melting of the powder. The Cu acts as a structural metal, the Cu-8.4P acts as a binder, while the Cu-8.4P is taken as a deoxidizing agent or a fluxing agent. Such a semi-solid sintering mechanism can well alleviate the"balling"effect and decrease the curling deformation induced by thermal stress. The component ratio, particle morphology, particle size distribution, and loose packing density of the powder system are optimized. It shows that a proper increase in the amount of the Cu-10Sn leads to a denser microstructure; however, at a high content larger than 50 wt.%, the"balling"phenomena occur. A suitable weight fraction of the Cu-8.4P in this powder system is 15 wt.%. The additive phosphorus element can act as a deoxidizing agent to prevent the sintering system from oxidation. The phosphorus can also act as a fluxing agent to decrease the surface tension of molten materials, thereby improving the liquid-solid wettability and the resultant sintered densification. On the other hand, using a bimodal mixture with a broad size distribution (≥70μm) produced by mixing coarse Cu powder with a mean particle size of 54μm and fine Cu-10Sn powder with a mean particle size of 28μm, and using spherical or near-spherical fine powders can generally lead to an increase in the loose packing density of the powder and thus the densification of the laser sintered powder. A novel"ball mixing"process is primarily developed to prepare the powder system. With a reasonable setting of the weight ratio of balls to powders (5: 1), the rotation speed of mixer (100 rpm), and the mixing time (90 min), the multi-component system is homogeneously dispersed without destroying the initial characteristics of the starting powder, thereby giving a material basis for improving the sintered density and microstructural homogeneity.(2) Optimization of processing conditions of multi-component Cu-based metal powder using DMLS, realizing the precise fabrication of complex shaped Cu-based metal parts with high densityWith consideration of the complex physical and chemical metallurgical processes during DMLS, the author studies the basic operating mechanism of DMLS and the influence of laser processing conditions and powder depositing parameters, so as to obtain the generic principles for increasing fabricating precision by alleviating"balling"effect and for improving sintered densification by controlling sintering mechanism. At different combinations of laser power and scan speed, the mechanisms of powder melting are divided into slight melting, partial melting, excessive melting, balling, and complete melting, among which the partial melting proves to be a feasible one. With a feasible sintering mechanism permitted, setting a suitable spot size (0.3 mm), increasing laser power (>300 W), decreasing scan speed (<0.06 m/s), narrowing scan line spacing (≤0.15 mm), or lowering layer thickness (≤0.30 mm) generally lead to a higher densification and a more homogeneous microstructure. Three kinds of balling mechanisms under different processing conditions are proposed, i.e.,"first line scan balling","shrinkage-induced balling", and"self-balling". Setting a suitable preheating temperature of powder bed or controlling laser power and scan speed can well alleviate the balling phenomena. In order to provide a precise and steady control of laser sintering process, an"energy density by volume"is defined. Using the"energy density by volume"of 0.23 kJ/mm3 is able to produce best quality Cu-based metal parts with complex configurations. The maximum dimensions are 210 mm×70 mm×9 mm, the one-step sintered density is large than 90% theoretical density, the maximum dimension error is less than 2%, and the tensile strength is larger than 140 MPa.In the second part of this thesis, processing conditions, metallurgical mechanisms, and basic theories involved in direct laser sintering of submicron WC-Co particulate reinforced Cu matrix bulk composite materials are studied, with the main conclusions drawn as follows.(1) Material design and process control in direct laser sintering of submicron WC-10Co particulate reinforced Cu matrix composites, obtaining key materials and processes for laser rapid manufacturing of high quality particulate reinforced metal matrix composites (MMCs)DMLS, due to its flexibility in feedstock and shapes, exhibits a great potential for developing novel materials and components. Meanwhile, the highly non-equilibrium state induced by a scanning laser beam might lead to the formation of some special structures and properties. As to MMCs reinforced with ceramics particulates, the ceramics phase can be firstly coated with a metal to form a metalloceramics before mixing with the matrix metal, so as to form metal-metal interfaces during laser sintering. In the present work, laser sintering of a composite system consisting of the submicron Co-coated WC powder (i.e., the reinforcement) and the Cu powder (i.e., the matrix) is performed; and the WC-10Co particulate reinforced Cu matrix composites in bulk form are successfully prepared. It is found that with a rapid action of a high-energy laser beam, the WC reinforcing particulates are either well remained the submicron characteristics or further refined to nanometer scales. The processing conditions including laser parameters and layer thickness are optimized. It shows that increasing the laser power to 700 W, increasing the scan speed above 0.04 m/s, and decreasing the powder layer thickness below 0.30 mm generally lead to a higher densification with a uniform particulate dispersion and a coherent particulate/matrix bonding ability. With regard to the interfacial design of such a composite system, the WC reinforcing phase is added in the form of WC-10Co composite powder, so as to change the WC/Cu (ceramics/metal) interface into the WC/Co/Cu (ceramics/interlayer metal/metal) interface. In other words, the Co can act as a wetting intermedium to improve the wetting characteristics between the Cu matrix and the WC particulates, leading to a sound interfacial bonding coherence after sintering. On the other hand, the weight fraction of the WC-10Co reinforcement in the powder system is optimized. It shows that using a low reinforcement content of 20 wt.% results in severe balling phenomena, due to a high average composite coefficient of thermal expansion (CTE) and a superheating of the melt. A severe particulate aggregation occurs at a high reinforcement content of 40 wt.%, because of a limited liquid formation and the resultant high liquid viscosity. An optimal content of WC-10Co reinforcement is found to be 30 wt.%.(2) Theoretical studies on mechanism of laser-induced pushing/trapping of reinforcing particulates by matrix metal, providing process and material measures to improve particulate/interface trapping effect, homogenize particulate dispersion, and enhance particulate/matrix bonding coherenceThe interaction between the reinforcing particulates and the advancing solidification front under the rapid action of a mobile laser beam is another important factor in determining the laser formability of particulate reinforced MMCs. In most studies concerning the particle-interface interaction, the problem is simplified by accounting a single spherical particle or a small amount of well separated particles ahead of a steady state planar solidification front, which is unlikely to be realized in the laser-induced solidification process of a solid-liquid composite system. This is because (i) a local destabilization of an initially planar solid-liquid interface caused by the different thermal conductivities and specific heats of the solidifying matrix and the particulate material is inherent to the solidification process of a composite system; (ii) a significant turbulence in the laser-generated melt pool will easily destroy such a planar solidification interface. In the present work, a theoretical model, which introduces a dendritic solidification front, is developed for describing the behavior of the reinforcing particulates at an advancing solid-liquid interface during the laser-induced rapid solidification process. The substantial governing parameter for particulate engulfment is found to be the laser-induced critical undercooling. In order to favor the particulate trapping effect and improve the particulate dispersion homogeneity under condition of high weight fraction of reinforcement, a proper increase in scan speed and laser power is regarded as feasible, so as to enhance the undercooling degrees of melts. Moreover, adding a suitable amount of rare earth element in the powder system can (i) decrease the surface tension of the melt and improve the liquid-solid wettability, (ii) pin the grain and/or phase boundaries and resist the grain and/or particulate coarsening, and (iii) increase the heterogeneous nucleation rate and refine the sintered structure. The experimental results show that using an optimal La₂O₃ content of 1.0 wt.% in the WC-10Co/Cu composite system possessing a high weight fraction of reinforcement of 50.0 wt.% can effectively enhance the particulate trapping effect, homogenize the dispersion of reinforcing particulates, and improve the particulate/matrix bonding coherence, leading to a higher sintered densification.