Optimization Of Powder Spreading Process In Metal Powder Bed Additive Manufacturing And Its Effect On Forming Performance

Powder bed additive manufacturing(PBAM)is one of the most widely studied and applied additive manufacturing technologies.Powder spreading is the first and extremely important step in the manufacturing process of this technology.It not only determines the quality of the paved powder bed but also affects the printing process and even the final part performance.At present,the traditional scraper powder spreading and counter-rotating roller powder spreading have problems such as low packing density and low efficiency.In addition,it has been pointed out that the powder spreading process affects the forming process of PBAM,but it is still unclear whether it can improve the performance of the fabricated parts.To address this issue,experimental and numerical simulation methods were used to investigate the influence of powder spreading on the quality of powder bed and to reveal the effect of the powder spreading process on the performance of metal part fabricated by laser powder bed additive manufacturing.The main research work are as follows:(1)The role of inclination angle of the scraper on the quality of powder spreading was studied.The experimental results showed that when the inclination angle was reduced from90° to 5°,the packing density increased from 23.89% to 34.8% and the surface flatness decreased from 19.27 μm to 13.91 μm,where the spreading speed and layer thickness were100 mm/s and 100 μm,respectively.The simulation analysis revealed that reducing the inclination angle increased the compaction of the scraper on the powder bed,thus increasing the degree of contact between particles in the powder flow(i.e.the particle coordination number)and the mass of particles deposited per unit time to form the powder bed(i.e.the particle mass flow rate),which ultimately improved the packing density and flatness of the powder bed.In addition,increasing the spreading speed significantly reduced the packing density and flatness.This was because increasing the spreading speed led to a "dynamic expansion" of the particle flow,making the powder bed loose and porous.Furthermore,increasing the spreading speed also increased the particle velocity,causing the particles to move longer distances before depositing onto the substrate,thus reducing the powder bed quality.(2)The role of roller rotation pattern on the quality of powder spreading was studied.The experimental results showed that at the spreading speed of 100 mm/s,the packing density and flatness of the powder bed paved by the forward-rotating roller were 54.31%and 10.26 μm,respectively,which were significantly higher than the results of the conventional counter-rotating roller.The simulation analysis suggested that the forwardrotating roller produced a stronger compaction on the powder,which increased the coordination number of the particle flow.In addition,the forward-rotating roller also generated a braking friction force opposite to the spreading direction on the particles,which induced more particle deposition and increased the particle mass flow rate,resulting in a higher packing density and flatness.Furthermore,for the forward-rotating roller,there was a positive correlation between the rotational speed and the spreading speed,and the packing density could be increased up to 60% after optimizing both of them.The braking friction force and the roller dragging force were essentially equivalent under the optimized conditions,allowing particles to be deposited faster,thus increasing the packing density.(3)A new process of the two-stage powder spreading by a combination of the scraper and the forward-rotating roller was proposed.It was found that the packing density of the two-stage powder spreading could reach 59.1% at the spreading speed of 100 mm/s,and the pressure on the substrate was only 427.17 Pa,which was 1/5 of that of the forward-rotating roller.The first-stage scraper in the two-stage powder spreading reduced the volume of the powder pile,thus decreasing the pressure generated by the second-stage roller.Moreover,the first-stage scraper layer thickness(i.e.,the pre-layer thickness)was the key factor affecting the two-stage powder spreading,with an optimal value of 250 μm.At the optimal pre-layer thickness,the second-stage roller had a saturated compaction effect,resulting in a particle flow with high coordination number and mass flow rate,thus enabling high-quality and low-pressure powder spreading.(4)The effect of the powder spreading process on the properties of GH3536 and S136 alloys formed by Laser powder bed fusion(L-PBF)was investigated.The experimental results showed that for GH3536,the use of 5° scrapers could improve the forming accuracy in the height direction of the part by 37.9%,but the effect on the relative density was not significant.In addition,when the spreading speed increased from 20 mm/s to 240 mm/s,the powder bed packing density decreased from 26.32% to 5.1%,but the relative density of the part increased from 98.36% to 99.58%,and the low circumferential fatigue life increased by 20%.The layer-by-layer evolution of the powder bed and the part during the forming process was revealed through the staircase experiment.It was found that the volume shrinkage of the powder layer(i.e.,the "shrinkage effect")during laser melting was the fundamental reason for the dimensional deviation of the part.Reducing the scraper inclination angle could significantly suppress such effect and thus improving the precision of the part.In addition,the "shrinkage effect" would also increase the actual layer thickness of the next powder layer,increasing the packing density and hence the fusion ratio,which partially compensated the drawbacks caused by the upper powder spreading(i.e.,the "compensation effect").Under the combined effects of shrinkage and compensation,the part could be formed properly even at high powder spreading speed.Furthermore,increasing the powder spreading speed significantly shortened the cooling time of each melted layer and increased the initial temperature of the next melted layer(i.e.,the "thermal cycling effect"),thereby reducing the interlayer temperature gradient and hence the thermal stress,which could reduce the unfused pores and cracks in the part and improve the fatigue life of the part.For S136 alloy,the influence of the powder spreading process on the dimensional accuracy and porosity of the part was the same as that of GH3536,but the "thermal cycling effect" would cause the change of phase content and hence the tensile properties of the part.