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•Microstructure is analysed on different planes of SLM 316L stainless steel.•Scanning strategy and building direction affects microstructural characteristics.•Sample fabricated with ...67.5°-rotation scanning shows the lowest corrosion resistance.•The corrosion resistance of the planes perpendicular to building direction is higher.•The boundary of the molten pool is more susceptible to corrosion.
In-depth understanding of corrosion behaviour is a key aspect regarding the application of additively manufactured parts. In this study, 316L stainless steel was manufactured under different scanning strategies using selective laser melting (SLM). Microstructure characterization and electrochemical tests in NaCl aqueous solution (3.5 wt%), including open circuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS), were conducted to study the influence of scanning strategies on the corrosion behaviour. The microstructure and corrosion on different planes were characterized to reveal the influence of building direction. EBSD analysis shows that the scanning strategy affects the continuity of grain growth through adjacent layers and the growth of grains inside the melt track. Electrochemical tests indicate a clear difference in corrosion resistance perpendicular and parallel to building direction and with different scanning strategies. Pitting corrosion is the main form of corrosion in SLM 316L stainless steel and preferentially initiates on molten pool boundaries.
This paper aims to improve the surface quality of 316L stainless steel parts manufactured by selective laser melting (SLM) using dry mechanical-electrochemical polishing (DMECP). DMECP is an advanced ...surface finishing method combining the advantages of both mechanical and electrochemical polishing techniques in a more environmentally friendly manner. In this paper, the SLM process-related defects causing poor surface quality are analysed first. The material removal mechanism of DMECP is investigated to continuously remove the oxide layers formed during polishing. Surface morphology and roughness evolution under different polishing conditions are characterised. The top surface roughness can be reduced by over 91% from 8.72 μm to 0.75 μm compared to side surface by over 93% from 12.10 to 0.80 μm. The material removal on the top surface is more efficient than that on the side surface under the same polishing condition. The secondary defects formed during polishing can be removed using mechanical polishing mode. The chemical element composition of the polished surface exhibits almost identical content to the initial 316L powders. Compared with the initial dark and rough surfaces, the results validate the capability of DMECP as an effective tool to improve the SLM surface quality and achieve a mirror finish.
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•Dry mechanical-electrochemical polishing (DMECP) is applied to finish selective laser melted (SLMed) surfaces.•The submicron-level surface roughness is achieved on SLMed metal parts.•Defects on SLMed 316L stainless steel are analysed and removed.•The relationship between polishing parameters and surface quality is established.•The material removal cycle and mechanism of DMECP are elucidated.
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ABSTRACTWith the prosperity of laser powder bed fusion (LPBF) technology, post-processing tends to be increasingly attractive in further improving the performance of the LPBFed parts. In this study, ...post-heat treatment and electroless plating were performed on the LPBFed 316L stainless steel to investigate the evolution of surface topography, mechanical property and corrosion behaviour. The results show that the as-built part features melt tracks and fine cellular substructures, which presents higher deformation resistance than the cast and recrystallized counterparts. The uniform amorphous Ni-P coating can significantly increase microhardness of as-built 316L stainless steel from ∼300 to ∼800 HV. In addition, the as-built parts with/without coatings have better corrosion resistance than the corresponding cast ones, which can be further improved through 450°C annealing. Moreover, the coatings can also prevent the localised corrosion effectively. Therefore, this study provides available post-processing methods for further improve the performance of LPBFed 316L stainless steel.
Manufacturing CuCrZr alloy parts with high strength and high surface quality by selective laser melting (SLM) is a challenging task. This study investigates the microstructure, mechanical properties ...and machinability of SLM processed and heat-treated CuCrZr alloy through tensile test, compression test and ultra-precision machining. The microstructure analysis shows that the grain morphology and orientation are highly related to the laser track and building direction. Grain epitaxial growth is found on both horizontal (XY) and vertical (XZ) planes, which bends at the boundary of the molten track/pool. After heat treatment, molten track boundaries disappear and the slender and bent grains on the XY plane are replaced by polygonal square grains, but the large epitaxial grains on the XZ plane still remained. X-ray diffraction results indicate that the as-built sample contains α-Cu, Cr and CuxZry phases and building direction has an influence on the crystallographic orientation distribution. As-built (AB) CuCrZr alloy shows a comparable yield strength (218.0 MPa) with solution + ageing-treated (SAT) (231.3 MPa), which is much higher than that of the solution-treated (ST) counterpart (131.0 MPa). Although the ultimate tensile strength of the AB sample is lower than that of the SAT sample, its elongation at break (46.5%) is much higher than the latter (19.1%). The compressive strength of the AB sample is slightly lower than the SAT sample but higher than the ST sample. At low cutting speed, the ST sample shows a higher cutting force. High cutting speed will lead to the increase of cutting force of the AB and SAT samples but has a much lower effect on the ST sample. The XZ plane of the AB sample shows better machined surface quality. SAT sample has the highest material recovery during cutting and its chip will change from spiral to arc shape with the increase of cutting speed.
Manufacturing multi-material part is one of the native advantages of selective laser melting (SLM) due to its layer-by-layer manufacturing method, which is attracting more and more attention in ...recent years. In an effort to reveal the dual interfacial characterization of dissimilar materials manufactured by SLM with different printing sequences, this paper presents the morphology, microstructure, element distribution, phase composition and microhardness of multi-material interfaces between SLMed 316L steel and C52400 copper alloy. Both interfaces display isolated alloy islands with various shapes and different morphologies. The melt pool at 316L/C52400 interface is deeper and narrower than that at C52400/316L interface. Small 316L and C52400 spheres with a size of 1–5 μm are formed under the surface tension, Marangoni convection in the melt pool and rapid cooling condition, in which many smaller particles with a size of <1 μm appear due to the material supersaturation and convection. Interdiffusion of elements and very fine grains with a size of <6 μm result in excellent metallurgical bonding performance. Cracks tend to originate from the interface and extend to the stainless steel side for both interfaces. No separation of the two materials caused by cracks is found in their contact area. Healing of the cracks by C52400 copper alloy at 316L/C52400 interface is easier to complete. The interface thickness depends on the building sequence of the two materials, which features a much thicker transition when building C52400 on 316L. No intermetallic compound but a trace of CuNi alloy is formed at the interface. The microhardness varies in the transition area due to the existence of isolated 316L/C52400 islands and decreases from 316L to C52400 with the highest value of 283.33 ± 5.51 HV to the lowest value of 181.33 ± 17.62 HV. This research advances the understanding of the different interfacial characterizations of dissimilar materials manufactured by SLM and provides guidance and reference for manufacturing multi-material components with complex interfaces using SLM.
•Multi-material interfaces of 316L stainless steel and C52400 copper alloy are manufactured with selective laser melting.•The interfaces display excellent metallurgical bonding performance with fine grains and interdiffusion of elements.•The 316L/C52400 interface has much wider transition zone than the C52400/316L interface.
The performance of the selective laser melting (SLM) parts was critically affected by the surface quality and internal defects that are closely related to process parameters. An in-depth ...understanding of the relationship between the formation and evolution of surface and internal defects and process parameters is needed to achieve defect-free and high-performance SLM parts. In this study, the influencing mechanism of laser power, scanning speed, hatch spacing and layer thickness on melt pool morphology, surface quality and internal hole defect of SLMed 18Ni300 maraging steel was investigated. The thermal and physical behaviour and instability of the molten pool, as well as the formation and distribution behaviour of internal hole defects, were also analyzed and discussed. Recoil pressure, the insufficient overlap between tracks and remelting between layers, Plateau-Rayleigh instability and material aggregation caused by the Marangoni effect were characterized as the main factors closely related to molten pool morphology and surface quality. Within the selected parameters in this study, the obtained surface roughness and tensile strength range from 9.08–26.40 μm and 544.14–1246.24 MPa, respectively. The internal defect changes from irregular lack-of-fusion at low energy density to the keyhole-included spherical hole at high energy density. In addition, the volumetric energy density (VED) has a certain limitation in predicting surface quality and mechanical properties due to the complex physical characteristics of the molten pool.
High manufacturing freedom of additive manufacturing (AM) technology, such as selective laser melting (SLM), innovates an effective way for the fabrication of multi-material components which is ...highly challenging or even impossible for the conventional manufacturing processes. The bonding performance between dissimilar materials in the AMed multi-material component needs further research prior to application. In this study, maraging steel (MS1) part was manufactured on top of the cast CrMn steel using SLM to obtain a multi-material component. The microstructure of CrMn steel substrate and MS1 steel, as well as the interfacial morphology of the hybrid CrMn-MS1 component, was characterized to study the metallurgical property. The microhardness and tensile property tests were conducted to investigate the mechanical behaviour of the hybrid cast/SLM multi-material component. An interface with a width of about 130 μm was formed between the two dissimilar materials. Marangoni convection caused complex phenomena at the interface, leading to the cross-regional distribution of alloy elements and a variation in the microstructure of the first several layers of SLMed MS1. Good metallurgical bonding of the dissimilar materials manufactured by SLM is achieved without pores, inclusions or cracks in the interfacial region. The microhardness value of the interface region is 309 ± 9 HV0.05 . This mitigates the difference in performance mismatch by smoothening the mechanical-property transition between CrMn steel (277 ± 11 HV0.05) and SLMed MS1 (360 ± 9 HV0.05). The hybrid CrMn-MS1 steel presents a higher ultimate tensile strength of 986 ± 30 MPa than cast CrMn steel and higher elongation of 24.5 ± 1.0% than SLMed MS1. This study proves feasibility to manufacture a reliable multi-martial component with a cast substrate and SLMed part of potentially higher complexity.
Additive manufacturing (AM) provides a higher degree-of-freedom manufacturing process for high-mix low-volume manufacturing. However, the as-built surface quality by AM processes is generally ...inferior to that fabricated by conventional subtractive manufacturing and the machinability investigation for post-processing of additively manufactured (AMed) parts is insufficient. In this paper, milling experiments are undertaken to improve the surface finish of the AMed high-strength maraging steel (18Ni300) manufactured by additive manufacturing with and without heat treatment. The influence of microstructure on the machinability is explored, including microhardness, cutting force, surface roughness, tool wear, and chip formation. Significant variation in machinability is identified between the AMed samples that feature distinct microstructures. Surface microhardness of the as-built and heat-treated samples both increased after milling. Cutting forces and tool wear increased sharply after ageing treatment but only a minor change was observed in the as-built samples and those subjected to solution treatment. Roughness value of the as-built samples was reduced from ∼10 to <0.4 μm after milling. Ageing treatment induced chip adhesion on the tool surface and high degree of chip curling. According to the chip morphology analysis, machining of both as-built and solution-treated samples will produce smaller chip serrations and continuous chip formation comparing to the large serrations and fracture morphology in ageing-treated chips. This paper elucidates the relationship between material microstructure and machinability of AMed maraging steel.
Laser powder bed fusion (LPBF) provides an effective and economical solution for fabricating multi-material components of complex structures as it entails a layer-wise manufacturing process. The ...feasibility and reliability of depositing AlSi10Mg alloy on the wrought AA6061 alloy substrate using the LPBF process were studied. The study includes the analysis of metallurgical quality, microstructure evolution, mechanical properties, and corrosion behaviour of the multi-material parts before and after heat treatment. The interface region, decorated with epitaxial growth, shows excellent metallurgical bonding without apparent defects of pores and cracks. LPBF AlSi10Mg comprises fine equiaxed grains and coarse columnar grains on the boundary and inside the molten pool, respectively. They were replaced by large Si particles after heat treatment without altering the grain morphology and //BD (building direction) texture. The as-built multi-material part exhibits a low ultimate tensile strength of 192.8 ± 3.4 MPa, similar to that of wrought AA6061, and a higher elongation (13.6 ± 0.5%) than the LPBF AlSi10Mg alloy (9.4 ± 0.2%). In addition, the ultimate tensile strength and elongation of the multi-material part were slightly improved after heat treatment. Compression testing showed that, in contrast to single-alloy parts, the multi-material part achieved moderate strength and good compressive capacity under both as-built and heat-treated conditions. Interestingly, the galvanic corrosion effects in the interface region are suppressed for both as-built and heat-treated multi-material parts. Moreover, the as-built multi-material sample has a higher corrosion resistance than the heat-treated one. This study verifies the feasibility of efficiently manufacturing a reliable, excellent, and low-cost multi-material component combining conventional and additive manufacturing processes.
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