激光选区熔化成形低角度无支撑结构的方法与工艺研究(特邀)

Method and Process of Selective Laser Melting Forming Low⁃Angle Support⁃Free Structures(Invited)

为了实现激光选区熔化(SLM)无支撑低角度成形,进而提高样件的成形效率和降低打印成本,笔者提出了一种基于加工层角度自适应下表面工艺区域的划分方法,并对该方法成形的样件的表面质量以及该方法的适用性进行了探究。结果表明:过高或者过低的能量密度都会对悬垂区域的多层成形产生负面影响,采用适熔工艺成形悬垂结构能在保证低孔隙率的同时实现多层打印。将悬垂结构进行上、下、内表面区域划分,基于向下比较层数T进行区域面积调控能够显著影响悬垂样件的可制造性和结构完整性,应用下表面工艺区域的面积越大,悬垂样件的成形性越好。对上下表面的成形机理进行了深入分析,结果显示,下表面的成形质量主要受粘粉以及熔池下陷引起的凸起的影响,而上表面除了受粘粉影响外,还主要受阶梯效应和轮廓边界熔道间隙的影响。最终,成形了不同尺寸的测试样件以及最低角度为15°的叶轮零件,证明了所提低角度成形方法的可行性。

Objective Currently,the recognized optimal forming angle for selective laser melting(SLM)is 45°.When the forming angle of a sample is less than 45°,support structures are required for assisting the formation.Although the addition of support structures can effectively assist in the formation of parts,drawbacks,such as increased printing time,increased material consumption,increased difficulty in post-processing,and increased roughness of the support surfaces,are often observed.In some cases,for parts with internal cavities or complex channels,the inability to remove internal support can render SLM unsuitable for fabrication.Therefore,it is important to explore a forming method for samples with low forming angles in SLM to reduce printing costs and expand the applicability range.Methods First,the treatment process of a horizontal suspension sample is discussed.Different process combinations are used to form single-and double-layer overhanging structures to obtain a stable overhang process.Accordingly,an adaptive method of lower surface process area division based on the machining layer angle is proposed to adjust the size of the lower surface process area applied to the overhanging sample.The feasibility of the low angle forming method was verified by forming a 30°overhanging sample with different downward comparison layer numbers.Samples with different widths and thicknesses were formed using the same forming process,and the applicability of the forming method was verified.Results and Discussions No obvious overheating was observed on the surface of the single-layer overhang formed by the fusible process,and only a few micropores were present.The formation of a double-layer overhang effectively ensured bonding between the layers.When the layer comparison interval was opened,the sample was forced to interrupt printing at a 6 mm overhang length when T=1,10,and 20,whereas the sample without layer comparison could be formed up to a 10 mm overhang length.When the layer interval T was further increased to 30,the effective forming length of the sample reached 12 mm.Finally,when T was greater than 40,stable formation of the designed height of the sample was achieved.When printing overhang samples of different sizes,it was found that increasing the thickness exacerbated the deformation of the sample but did not affect the realization of the designed height.This indicates that this method has a certain applicability in low-angle forming.Conclusions(1)In the horizontal overhang formation process,a high energy density caused serious spatter accumulation on the surface of the overhanging sample and destroyed the interlayer bonding under the action of the scraper.However,an energy density that is very low will lead to an insufficient weld lap or even failure to form.Using a fusible process,the sample can achieve an effective multilayer lap.(2)Under suitable conditions for the overhang process,the division of the forming region on the inner surface was a key factor affecting the formation of low-angle samples.For 30°forming,the overhanging area of the sample can resist the thermal stress deformation caused by the high-energy inner surface laser only when the layer comparison interval T is greater than or equal to 40,thus achieving low-angle printing.(3)The quality of the lower surface was mainly affected by powder bonding and sinking of the molten pool.In addition to powder sticking,the upper surface was mainly influenced by the staircase effect and the gap between the laser contour and the boundary of the melt pool.Additionally,the difference in the contact area with the powder was the main reason for the difference in powder sticking between the upper and lower surfaces.(4)When forming the low-angle samples,as the thickness of the sample increased,the deformation of the sample gradually intensified,whereas an increase in the width had almost no effect on the deformation of the low-angle samples.