Previous studies have found that the fatigue performance of the parts made by the repeated use of Ti6Al4V under the electron beam powder bed fusion process is 69 times lower than that of the parts made with the unused new powder. The reason is that the particle surface oxidation leads to the internal fusion of the parts. defect. In another study, the plasticity and toughness of nickel-based alloy powders printed by a laser powder bed fusion process for multiple cycles were reduced.
Taking into account the hypothesis that powder properties may affect the quality of parts and the needs of industrial applications, the fatigue performance of 3D printing materials (especially the ultra-high cycle fatigue performance) has become one of the urgent scientific problems in the field of additive manufacturing.
The change of powder characteristics and its influence on the effect of spreading powder:
The powder used in the study is 17-4PH stainless steel, the initial powder size distribution is between 15-45μm, and the initial quantity of 80kg is used for 15 consecutive printing. Each time the manufacturing is completed, the powder is sieved and thoroughly mixed with the unused powder. The effects of the placement of the parts and the repeated use of powder on the tensile strength and fatigue life of the parts are studied respectively.
A comprehensive study of materials such as IN718, Ti6Al4V, AlSi10Mg, etc. by a number of units found that the repeated use of the powder will cause changes in the characteristics of the powder, mainly as follows:
The fine powder content decreases, the particle size distribution narrows, and the average particle size increases, especially for AlSi10Mg; the morphology of the particles will also become more slender (found in some studies); compared with the unused powder, the cycle has been Due to the disappearance of satellite powder and long-term high temperature exposure, the moisture content of the powder is reduced, the phenomenon of powder agglomeration is reduced, and the fluidity is generally increasing.
The results of the particle size test in this study basically verified the above conclusions. The particle size width of the initial powder was 45μm, and the particle size of the 15th batch was 41μm. The particle size of the powder has become narrower; the powder used repeatedly has smaller inter-particle gaps than the initial powder. The powder fluidity and filling state are improved. In general, the repeated use of powder can make spreading more uniform and reduce the gap between particles. This change increases the complexity of understanding how powder specifications affect part performance. At the same time, the change of powder characteristics may bring some changes to the mechanical properties of parts at different positions on the substrate.
The study of fatigue behavior found that the repeated use of powder has almost no direct effect on the parts that have not been machined and have printed surfaces, which is mainly due to the retention of surface roughness; while for the machined samples, Under low-cycle and medium-cycle fatigue conditions, continuous repeated use of powder will not affect the fatigue behavior; however, under high-cycle fatigue conditions, the fatigue life of the 15th batch of samples is significantly higher than that of the first batch. The study on the fatigue life of the left and right samples found that the fatigue life of the right sample is higher than that of the left sample, but this gap will still decrease with the increase in the number of powder usage.
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