Fig. 1. Backscatter diagram of powder layer along the spreading direction. (a) Photo of the powder layer; (b) SEM images of the powder layer; (c) powder laying image combination in rectangle of Fig. (a)^[33]

Fig. 2. Test results. (a) Tracking of individual particles of different sizes within the powder clusters during the spreading of 316L stainless-steel powder with an average diameter of 67 µm; (b) (c) dynamic X-ray images showing the motion of particles PI, PII, and PIII within the soft powder cluster; (d) (e) dynamic X-ray images showing the motion of particles PIV, PV, and PVI within the hard powder cluster; (f) speed over time for particles within the soft powder cluster; (g) speed versus time for particles within the hard powder cluster^[34]

Fig. 3. Schematics and scan results. (a) Computer aided design of the PBS platform; (b) cross-section view of the recoater module and the attached CIS unit [recoater module can mount 2 types of recoater blade: (b1) a metal blade that mimic the design of the EOS DMLS; (b2) a rubber blade from the SLM500 (SLM Solutions)]; (c) an example powder layer scan at 4800 DPI (individual powder particles are visible by digital zoom of the raw scan. Since the CIS unit acquires colored scans, oxidized powders appear in orange/red/blue)^[35]

Fig. 4. Cone sampler sample for evaluating powder bed density and placement on forming platform^[36]

Fig. 5. Drawing of powder laying device. (a) Before powder laying; (b) after powder laying with scraper^[37]

Fig. 6. Schematic diagram of particle compact packing. (a) Single-size spherical arrangement model; (b) bimodal of spherical arrangement model; (c) trimodal of spherical arrangement model^[38]

Fig. 7. Principle of three kinds of powder atomization process and particle size distribution of Ti-6Al-4V powder. (a) (d) GA;(b) (e) PREP; (c) (f) PA^[41]

Fig. 8. Relationship between bulk density of coarse and fine powder particles and proportion of coarse and fine powder particles^[36]

Fig. 9. Effect of mixing coarse and fine powders on loose packing density and forming defects. (a) Effect of fine powder addition ratio on vibration density of loose powder; (b) particle size distribution of powder under optimal mixing ratio; (c) relative density of coarse powder and coarse and fine mixed powder under different production efficiencies; (d) maximum pore size of coarse powder and coarse and fine mixed powder under different production efficiencies^[36]

Fig. 10. Microstructure and relative density of 316L LPBF-manufactured samples for single-mode and bimodal feedstock powder. (a) (c) Microscopic defects and melt pool morphology of single-peak mode of powder building; (b) (d) microscopic defects and melt pool morphology of bimodal-peak mode of powder building; (e) relative density at different volumetric energy density^[47]

Fig. 11. Effect of particle shape on the apparent density of powders^[55-56]

Fig. 12. Powder bed densities and printed part density of Ti6Al4V three powder lots for two layer thicknesses (powder 1: GA powder; powder 2: PA powder; powder 3: PA powder)^[59]

Fig. 13. Stainless steel powder morphology and molten pool height prepared by two processes. (a) (c) GA; (b) (d) WA^[60]

Fig. 14. GA and WA were used to prepare powder forming 25Cr7Ni samples. (a) XRD pattern; (b) microscopic structure^[65]

Fig. 15. Powder spreading process. (a) Scraper spreading process; (b) roller spreading process^[71]

Fig. 16. Optimized blade type spreader and powder spreading effect. (a) Powder spreader; (b) volume fractions; (c) surface roughness^[75]

Fig. 17. Effect of roller-spreading parameters on the packing density of powder. (a) (b) Effect of roller's diameters on powder-bed density; (c) effect of roller's rotational speed on powder flow velocity at the front of powder stack^[77]

Fig. 18. Effect of powder spreading speed on surface roughness and density of powder layer. (a) Surface roughness of powder layer changes with powder spreading speed; (b) density of powder spreading varies with the speed of powder spreading; (c) simulation of powder layer morphologies at different spreading speeds^[80]

Fig. 19. Simulation results. (a) Physical model of powder particle spreading; (b) powder spreading mechanism^[82]

Fig. 20. Test results. (a) Optical microscope images of the cross sections of the cubic specimens fabricated from Hastelloy-X alloy powder with different spreading speeds and linear energy densities; (b) phase content of samples from electron backscattered diffraction tests; (c) tensile test results; (d) fatigue test results^[83]

Fig. 21. Effect of substrate roughness on powder spreading at a speed of 0.2 m/s. (a) Powder spreading device; (b) substrate surface roughnes; (c) spreading layer thicknesses; (d) surface texture orientation^[84]