Donghua Dai

Densification behavior, microstructure evolution, and wear property of TiC nanoparticle reinforced AlSi10Mg bulk-form nanocomposites prepared by selective laser melting

Selective laser melting (SLM), due to its unique additive manufacturing processing philosophy, demonstrates a high potential in producing bulk-form nanocomposites with novel nanostructures and enhanced properties. In this study, the nanoscale TiC particle reinforced AlSi10Mg nanocomposite parts were produced by SLM process. The influence of “laser energy per unit length” (LEPUL) on densification behavior, microstructural evolution, and wear property of SLM-processed nanocomposites was studied. It showed that using an insufficient LEPUL of 250u2009J/m lowered the SLM densification due to the balling effect and the formation of residual pores. The highest densification level (>98% theoretical density) was achieved for SLM-processed parts processed at the LEPUL of 700u2009J/m. The TiC reinforcement in SLM-processed parts experienced a structural change from the standard nanoscale particle morphology (the average size 75–92u2009nm) to the relatively coarsened submicron structure (the mean particle size 161u2009nm) as the app...

On the role of processing parameters in thermal behavior, surface morphology and accuracy during laser 3D printing of aluminum alloy

In this paper, a 3D mesoscopic model was established using the finite volume method (FVM) to investigate the process of powder-to-solid transition under different processing parameters during the selective laser melting (SLM) of AlSi10Mg powder, taking into account the initial feeding powder material and the attendant significant complexity of the physical interaction between the powder and laser beam. The effects of different processing parameters on the temperature field, velocity field, surface morphology and Z-direction shrinkage were also studied. It was found that both high and low laser power resulted in a rough surface quality and there were optimal processing parameters (P = 250 W, v = 400 mm s−1, d = 50 μm) for AlSi10Mg powder to get a relatively smooth surface and reasonable shrinkage value. That is because low laser power could not melt the powder completely and high laser power caused excessive liquid formation which developed into so-called self-balling. Meanwhile, it was interesting to find that the temperature rebounding phenomenon was produced, relieving the formation of large residual stress due to the self-heat-treatment. In order to further study self-balling under a long irradiation time, the melt process of particles at a static laser beam is simulated, demonstrating that the volume energy density played a key role in self-balling. Experimental findings were compared with simulation results and they showed good agreement.

Selective Laser Melting Additive Manufacturing of Hard-to-Process Tungsten-Based Alloy Parts With Novel Crystalline Growth Morphology and Enhanced Performance

Selective laser melting (SLM) additive manufacturing (AM) of hard-to-process W-based parts with the addition of 2.5 wt.% TiC was performed using a new metallurgical processing mechanism with the complete melting of the high-melting-point powder. The influence of SLM processing parameters, especially laser scan speed and attendant laser fluence (LF), on densification behavior, microstructural development, and hardness/wear performance of SLM-processed W-based alloy parts was disclosed. The densification response of SLM-processed W-based parts decreased both at a low LF of 10.7 J/mm2, caused by the limited SLM working temperature and wetting characteristics of the melt, and at an excessively high LF of 64 J/mm2, caused by the significant melt instability and resultant balling effect and microcracks formation. The laser-induced complete melting/solidification mechanism contributed to the solid solution alloying of Ti and C in W matrix and the development of unique microstructures of SLM-processed W-based alloy parts. As the applied LF increased by lowering laser scan speed, the morphologies of W-based crystals in SLM-processed alloy parts experienced a successive change from the cellular crystal to the cellular dendritic crystal and, finally, to the equiaxed dendritic crystal, due to an elevated constitutional undercooling and a decreased thermal undercooling. The optimally prepared W-based alloy parts by SLM had a nearly full densification rate of 97.8% theoretical density (TD), a considerably high microhardness of 809.9 HV0.3, and a superior wear/tribological performance with a decreased coefficient of friction (COF) of 0.41 and a low wear rate of 5.73 × 10−7 m3/(N m), due to the combined effects of the sufficiently high densification and novel crystal microstructures of SLM-processed W-based alloy parts.

High-temperature oxidation performance and its mechanism of TiC/Inconel 625 composites prepared by laser metal deposition additive manufacturing

The laser metal deposition (LMD) additive manufacturing process was applied to produce TiC/Inconel 625 composite parts. The high-temperature oxidation performance of the LMD-processed parts and the underlying physical/chemical mechanisms were systematically studied. The incorporation of the TiC reinforcement in the Inconel 625 improved the oxidation resistance of the LMD-processed parts, and the improvement function became more significant with increasing the TiC addition from 2.5u2009wt. % to 5.0u2009wt. %. The mass gain after 100u2009h oxidation at 800u2009°C decreased from 1.4130u2009mg/cm2 for the LMD-processed Inconel 625 to 0.3233u2009mg/cm2 for the LMD-processed Inconel 625/5.0u2009wt. % TiC composites. The oxidized surface of the LMD-processed Inconel 625 parts was mainly consisted of Cr2O3. For the LMD-processed TiC/Inconel 625 composites, the oxidized surface was composed of Cr2O3 and TiO2. The incorporation of the TiC reinforcing particles favored the inherent grain refinement in the LMD-processed composites and, therefor...

Combined strengthening of multi-phase and graded interface in laser additive manufactured TiC/Inconel 718 composites

Laser metal deposition (LMD) additive manufacturing of TiC particle reinforced Inconel 718 composite parts was performed. The influence of laser energy density (LED) on densification, microstructures and wear behaviour of LMD-processed composites was studied. It showed that using a LED of 280 J mm−3 produced ~5% porosity in LMD-processed composites, caused by the aggregation of reinforcing particles. A further increase in LED above 350 J mm−3 yielded near-full densification. Two categories of reinforcing phases, i.e. the substoichiometric TiCx particles and the in situ (Ti,M)C (M = Mo, Nb and Cr) carbide having 7–10 at% Nb and Mo contents, were formed in the matrix of LMD-processed composites. The TiCx reinforcing particles changed from an irregular poly-angular shape to a smoothened and refined structure as the LED increased. An increase in LED resulted in a larger amount of phase formation and an enhanced degree of crystal growth of the in situ (Ti,M)C reinforcement. The interfacial graded layer with thickness of 0.2–1.2 µm, which was identified as (Ti,M)C (M = Mo, Nb and Cr) carbide with 5–6 at% Mo and Nb contents, was tailored between the TiCx particles and the matrix. At an optimal LED of 420 J mm−3, a considerably low coefficient of friction of 0.38 and resultant low wear rate of 1.8 × 10−4 mm3 N−1 m−1 were obtained in sliding tests, due to the combined strengthening of the interfacial graded layer and the multiple reinforcing phases. The wear resistance decreased at an excessive LED because of the coarsening of reinforcement crystals and the decrease in microstructural uniformity of composites.

A Multiscale Understanding of the Thermodynamic and Kinetic Mechanisms of Laser Additive Manufacturing

Abstract Selective laser melting (SLM) additive manufacturing (AM) technology has become an important option for the precise manufacturing of complex-shaped metallic parts with high performance. The SLM AM process involves complicated physicochemical phenomena, thermodynamic behavior, and phase transformation as a high-energy laser beam melts loose powder particles. This paper provides multiscale modeling and coordinated control for the SLM of metallic materials including an aluminum (Al)-based alloy (AlSi10Mg), a nickel (Ni)-based super-alloy (Inconel 718), and ceramic particle-reinforced Al-based and Ni-based composites. The migration and distribution mechanisms of aluminium nitride (AlN) particles in SLM-processed Al-based nanocomposites and the in situ formation of a gradient interface between the reinforcement and the matrix in SLM-processed tungsten carbide (WC)/Inconel 718 composites were studied in the microscale. The laser absorption and melting/densification behaviors of AlSi10Mg and Inconel 718 alloy powder were disclosed in the mesoscale. Finally, the stress development during line-by-line localized laser scanning and the parameter-dependent control methods for the deformation of SLM-processed composites were proposed in the macroscale. Multiscale numerical simulation and experimental verification methods are beneficial in monitoring the complicated powder-laser interaction, heat and mass transfer behavior, and microstructural and mechanical properties development during the SLM AM process.

Thermodynamic behaviour and formation mechanism of novel titanium carbide dendritic crystals within a molten pool of selective laser melting TiC/Ti–Ni composites

Selective laser melting (SLM) was applied to prepare TiC/Ti–Ni composites by using a mixed powder composed of titanium powder, nickel powder and titanium carbide powder. The result indicated that fine TiC particles were transformed into in situ Ti6C3.75 dendrites based on the complete melting mechanism and coarse TiC particles just partly experienced melting to form epitaxial dendrites along the margin of the remaining TiC particles. Besides, laser scan speed was found to have a significant influence on Ti6C3.75 dendrite growth. To give a better insight into the thermodynamic behaviour of TiC within the mesoscopic molten pool which was difficult to be monitored by experimental methods, a numerical simulation method was used. Due to the existence of differences in thermal conductivity between TiC and the matrix, reverse thermal hysteresis within TiC particles was predicted, influencing the temperature and its gradient on the TiC particles. Furthermore, the melting mechanism of TiC particles and growth processes of Ti6C3.75 dendrites were discussed. Moreover, nanoindentation load–penetration depth curves were also measured, reaching a value of 6.84 GPa at the applied v of 350 mm s−1.

The Role of Reinforcing Particle Size in Tailoring Interfacial Microstructure and Wear Performance of Selective Laser Melting WC/Inconel 718 Composites

Dongdong Gu College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics (NUAA), Yudao Street 29, Nanjing 210016, China; Jiangsu Provincial Engineering Laboratory for Laser Additive Manufacturing of High-Performance Metallic Components, Nanjing University of Aeronautics and Astronautics (NUAA), Yudao Street 29, Nanjing 210016, China e-mail: dongdonggu@nuaa.edu.cn