Additive manufacturing (AM), also known as 3D-printing (3DP) technology, is an advanced manufacturing technology that has developed rapidly in the past 40 years. However, the ceramic material printing is still challenging because of the issue of cracking. Indirect 3D printing has been designed and drawn attention because of its high manufacturing speed and low cost. Indirect 3D printing separates the one-step forming process of direct 3D printing into binding and material sintering, avoiding the internal stress caused by rapid cooling, making it possible to realize the high-quality ceramic component with complex shape. This paper presents the research progress of leading indirect 3D printing technologies, including binder jetting (BJ), stereolithography (SLA), and fused deposition modeling (FDM). At present, the additive manufacturing of ceramic materials is mainly achieved through indirect 3D printing technology, and these materials include silicon nitride, hydroxyapatite functional ceramics, silicon carbide structural ceramics.
A series of single track clads of Inconel 625 alloy were fabricated by laser solid forming. To achieve the high dimensional accuracy and excellent mechanical properties, the effect of processing parameters on the geometry, the formation of Laves phase and the residual stress was investigated. The results show that laser power and scanning speed had a dramatical influence on the width and height of single-track clads. According to the columnar to equiaxed transition curve of Inconel 625, the grain morphology can be predicted during the LSF process. With the increasing laser power and the decreasing scanning speed, the segregation degree of Si, Nb, Mo, the volume fraction and size of Laves phase increased. Vickers indentation was used to demonstrate that optimizing processing parameter can achieve the minimum residual tensile stress.
The application of mixed powders with different mass fraction on laser additive repairing (LAR) can be an effective way to guarantee the performance and functionality of repaired part in time. A convenient and feasible approach is presented to repair TA15 forgings by employing Ti6Al4V-xTA15 mixed powders in this paper. The performance compatibility of Ti6Al4V-xTA15 powders from the aspects of microhardness, tensile property, heat capacity, thermal expansion coefficient and corrosion resistance with the TA15 forgings was fully investigated. The primary α laths were refined and the volume fraction of the secondary α phase was increased by increasing the mass fraction of TA15 in the mixed Ti6Al4V-xTA15 powders, leading to varied performances. In conclusion, the mixed Ti6Al4V-70%TA15 (x=70%) powders is the most suitable candidate and is recommended as the raw material for LAR of TA15 forgings based on overall consideration of the compatibility calculations of the laser repaired zone with the wrought substrate zone.
NiTi shape memory alloy (SMA) with nominal composition of Ni 50.8 at% and Ti 49.2 at% was additively manufactured (AM) by selective laser melting (SLM) and laser directed energy deposition (DED) for a comparison study, with emphasis on its phase composition, microstructure, mechanical property and deformation mechanism. The results show that the yield strength and ductility obtained by SLM are 100 MPa and 8%, respectively, which are remarkably different from DED result with 700 MPa and 2%. The load path of SLM sample presents shape memory effect, corresponding to martensite phase detected by XRD; while the load path of DED presents pseudo-elasticity with austenite phase. In SLM sample, fine grain and hole provide a uniform deformation during tensile test, resulting in a better elongation. Furthermore, the nonequilibrium solidification was studied by a temperature field simulation to understand the difference of the two 3D printing methods. Both temperature gradient G and growth rate R determine the microstructure and phase in the SLM sample and DED sample, which leads to similar grain morphologies because of similar G/R. While higher G×R of SLM leads to a finer grain size in SLM sample, providing enough driving force for martensite transition and subsequently changing texture compared to DED sample.
The mechanical properties of many materials prepared by additive manufacturing technology have been greatly improved. High strength is attributed to grain refinement, formation of high density dislocation and existence of cellular structures with nanoscale during manufacturing. In addition, the super-saturated solid solution of elements in the matrix and the solid solution segregation along the wall of the cellular structures also promote the improvement of strength by enhancing dislocation pinning. Hence, the existence of cellular structure in grains leads to differences in the prediction of material strength by Hall-Petch relationship, and there is no unified calculation method to determine the d value as grain size or cell size. In this work, representative materials including austenite 316L SS were printed by selective laser melting (SLM), and the strength was predicted. The values of cell size and grain size were substituted into Hall-Petch formula, and the results showed that the calculation error for 316L is increased from 4.1% to 11.9%. Therefore, it is concluded that the strength predicted by grain size is more accurate than that predicted by cell size in additive manufacturing materials. When calculating the yield strength of laser additive manufacturing metal materials through the Hall-Petch formula, the grain size should be used as the basis for calculation.
Al7075 alloy is a typical aviation aluminum with good mechanical properties and anodic oxidation effect. Laser engineered net shaping technology has unique advantages in the integrated forming of high-performance large aircraft structural parts. The manufacturing of 7075 aluminum alloy structural parts by laser engineered net shaping technology has become an important development direction in the future aerospace field. Electrochemical corrosion resistance of aluminum alloys is of vital importance to improve reliability and life-span of lightweight components. A comparative study on microstructure and anti-corrosion performance of Al7075 alloy prepared by laser additive manufacturing and forging technology was conducted. There are hole defects in LENS-fabricated Al7075 alloy with uniformly distributed η phase. No defects are observed in Al7075 forgings. The large S phase particles and small ellipsoidal η phase particles are found in Al matrix. The corrosion mechanisms were revealed according to the analysis of polarization curves and corrosion morphology. It was found that compared with that prepared by forgings, the additive manufactured samples have lower corrosion tendency and higher corrosion rate. Corrosion occurred preferentially at the hole defects. The incomplete passivation film at the defects leads to the formation of a local cell composed of the internal Al, corrosion solution and the surrounding passive film, which further aggravates the corrosion.
Ti-6Al-4V specimens were fabricated by selective laser melting (SLM) to study the effect of thermal treatment on the phase transformation, elemental diffusion, microstructure, and mechanical properties. The results show that vanadium enriches around the boundary of α phases with increasing annealing temperature to 973 K, and α′ phases transform into α+β at 973 K. The typical α′ martensite microstructure transforms to fine-scale equiaxed microstructure at 973 K and the equiaxed microstructure significantly coarsens with increasing annealing temperature to 1273 K. The SLM Ti-6Al-4V alloy annealed at 973 K exhibits a well-balanced combination of strength and ductility ((1305±25) MPa and (37±3) %, respectively).
A direct 3D extrusion printing technique was used to produce Ti-TiB filaments and microlattices. The sintering properties of 3D ink extrusion and sintering of in situ Ti-TiB composite structures made from TiH2+TiB2 ink were investigated. The sintering kinetics of TiH2+TiB2 inks was studied during densification by pressureless sintering at 1050–1200 °C for 4–24 h in Ar. The linear shrinkage, grain size, microhardness, X-ray diffraction (XRD) patterns, and microstructural evolution of the Ti-TiB composite were studied. The sintering temperature had a more pronounced influence than the sintering time on the density of the Ti-TiB composite. There were two kinds of pores, irregular and spherical, caused by the Kirkendall effect and indiffusable gases. The TiB formed by in situ synthesis existed as either separated TiB whiskers (needle-like shapes) or clusters of TiB whiskers. The results of this work could be useful for controlling microporosity through incomplete sintering within filaments, especially for the production of in situ Ti-TiB with high volume fractions of TiB or other composites.
V-5Cr-5Ti alloys have been fabricated using a laser melting deposition (LMD) additive manufacturing process, showing precipitates aggregated near the grain/dendrite boundaries. Since the mechanical properties of vanadium alloys considerably depend on the precipitates, solution and aging treatments have been applied to eliminating the aggregations of the precipitates. The results show that as the solution temperature increases from 800 to 1560 °C, the densities and the lengths of the precipitates are reduced, while the widths of the precipitates are increased. When the solution temperature reaches 1560 °C, most impurity elements diffuse into the matrix and form into a nearly uniform supersaturated solid solution. Aging treatments have been applied to the 1560 °C solution treated samples. It shows that as the aging temperature increases from 800 to 1200 °C, the precipitate length increases, and the shapes of precipitates change from near-spherical to lath-like. Compared to 800 and 1200 °C, aging at 1000 °C results in the highest precipitate density. Compared to the LMD and solution-treated samples, the aged samples have the highest micro-hardness, due to the precipitation strengthening.
Wire arc additive manufacturing (WAAM) is a novel manufacturing technique by which high strength metal components can be fabricated layer by layer using an electric arc as the heat source and metal wire as feedstock, and offers the potential to produce large dimensional structures at much higher build rate and minimum waste of raw material. In the present work, a cold metal transfer (CMT) based additive manufacturing was carried out and the effect of deposition rate on the microstructure and mechanical properties of WAAM Ti-6Al-4V components was investigated. The microstructure of WAAM components showed similar microstructural morphology in all deposition conditions. When the deposition rate increased from 1.63 to 2.23 kg/h, the ultimate tensile strength (UTS) decreased from 984.6 MPa to 899.2 MPa and the micro-hardness showed a scattered but clear decline trend.
The expanding of material library of laser powder bed fusion (L-PBF) is of great significance to the development of material science. In this study, the biomedical Ti-13Nb-13Zr powder was mixed with the tantalum particles (2 wt%–8 wt%) and fabricated by L-PBF. The microstructure consists of a β matrix with partially unmelted pure tantalum distributed along the boundaries of molten pool owing to the Marangoni convention. Because the melting process of Ta absorbs lots of energy, the size of molten pool becomes smaller with the increase of Ta content. The fine microstructure exists in the center of melt pool while coarse microstructure is on the boundaries of melt pool because of the existence of heat-affected zone. The columnar-to-equiaxed transitions (CETs) happen in the zones near the unmelted Ta, and the low lattice mismatch induced by solid Ta phase is responsible for this phenomenon. The recrystallization texture is strengthened while the fiber texture is weakened when the tantalum content is increased. Due to the formation of refined martensite α′ grains during L-PBF, the compressive strengths of L-PBF-processed samples are higher than those fabricated by traditional processing technologies. The present research will provide an important reference for biomedical alloy design via L-PBF process in the future.
The selective laser melting (SLM) processed aluminum alloys have already aroused researchers’ attention in aerospace, rail transport and petrochemical engineering due to the comprehensive advantages of low density, good corrosion resistance and high mechanical performance. In this paper, an Al-14.1Mg-0.47Si-0.31Sc-0.17Zr alloy was fabricated via SLM. The characteristics of single track at different process parameters, and the influence of hatch spacing on densification, microstructural features and tensile properties of block specimens were systematically studied. The hatch spacing has an influence on the overlap ratio of single track, and further affects the internal forming quality of printed specimen. At a laser power of 160 W and scanning speed of 400 mm/s, the densification of block specimen increased first and then decreased with the increase of hatch spacing. The nearly full dense specimen (98.7 %) with a tensile strength of 452 MPa was fabricated at a hatch spacing of 80 µm. Typical characteristics of dimple and cleavage on the tensile fracture of the AlMgSiScZr alloy showed the mixed fracture of ductility and brittleness.
Due to their high hardness and high strength, VC reinforced hard materials such as high vanadium high-speed steel (HVHSS) are not suitable for machining to obtain complex shape with low cost. Therefore, 3D gel printing (3DGP) was employed to print HVHSS parts, using highly loaded slurry with 60% solid content as printing slurry. After printing parameters optimization, the printing sample had good surface quality, and obvious printing lines were observed. The extruded filament was in-situ cured, thus enough to maintain the designed shape. Uniform sintering shrinkage with a shrinkage rate of about 15% was obtained in the as-sintered sample with relative density of 99%. The surface roughness decreased from 6.5 µm to 3.8 µm. Fine carbides (<1 µm) and dense microstructure were achieved. Besides, the as-sintered sample had comprehensive performance of HRC60 in hardness, 3000 MPa in bend strength, and 20–26 J in impact energy. This study proposed one promising method to directly manufacture complex-shaped hard materials without subsequent machining.
Functionally graded material (FGM) can tailor properties of components such as wear resistance, corrosion resistance, and functionality to enhance the overall performance. The selective laser melting (SLM) additive manufacturing highlights the capability in manufacturing FGMs with a high geometrical complexity and manufacture flexibility. In this work, the 316L/CuSn10/18Ni300/CoCr four-type materials FGMs were fabricated using SLM. The microstructure and properties of the FGMs were investigated to reveal the effects of SLM processing parameters on the defects. A large number of microcracks were found at the 316L/CuSn10 interface, which initiated from the fusion boundary of 316L region and extended along the building direction. The elastic modulus and nano-hardness in the 18Ni300/CoCr fusion zone decreased significantly, less than those in the 18Ni300 region or the CoCr region. The iron and copper elements were well diffused in the 316L/CuSn10 fusion zone, while elements in the CuSn10/18Ni300 and the 18Ni300/CoCr fusion zones showed significantly gradient transitions. Compared with other regions, the width of the CuSn10/18Ni300 interface and the CuSn10 region expand significantly. The mechanisms of materials fusion and crack generation at the 316L/CuSn10 interface were discussed. In addition, FGM structures without macro-crack were built by only altering the deposition subsequence of 316L and CuSn10, which provides a guide for the additive manufacturing of FGM structures.
The slow degration of iron limits its bone implant application. The solid solution of Zn in Fe is expected to accelerate the degradation. In this work, mechanical alloying (MA) was used to prepare Fe-Zn powder with supersaturated solid solution. MA significantly decreased the lamellar spacing between particles, thus reducing the diffusion distance of solution atoms. Moreover, it caused a number of crystalline defects, which further promoted the solution diffusion. Subsequently, the MA-processed powder was consolidated into Fe-Zn part by laser sintering, which involved a partial melting/rapid solidification mechanism and retained the original supersaturated solid solution. Results proved that the Fe-Zn alloy became more susceptible with a lowered corrosion potential, and thereby an accelerated corrosion rate of (0.112±0.013) mm/year. Furthermore, it also exhibited favorable cell behavior. This work highlighted the advantage of MA combined with laser sintering for the preparation of Fe-Zn implant with improved degradation performance.
Ti185 alloy is widely used in key industrial fields such as aerospace due to its excellent mechanical properties. The traditional method of preparing Ti185 alloy will inevitably appear “β fleck”, resulting in the decrease of mechanical properties, and the high price of V element limits the wide application of Ti185. In this paper, a low-cost master alloy V-Fe powder is used, a dense block is prepared by spark plasma sintering (SPS) technology, and a high-performance Ti185 alloy is prepared by controlling the sintering parameters. XRD and SEM were used to investigate the phase and microstructure of the samples prepared under different parameters. The compressive strength and friction properties of the directly prepared samples were studied. The samples with a sintering temperature of 1350 °C and a holding time of 30 min exhibited the most excellent comprehensive performance, with the highest compressive strength and lowest friction coefficient of 1931.59 MPa and 0.47, respectively.
In this work, a novel ultrahigh-strength Al−10Zn−3.5Mg−1.5Cu alloy was fabricated by powder metallurgy followed by hot extrusion. Investigations on microstructural evolution and mechanical properties of the fabricated samples were carried out. The results show that the grain size of sintered samples matches with the powder particles after ball milling. The relative densities of sintered and hot extruded samples reach 99.1% and 100%, respectively. Owing to the comprehensive mechanism of grain refinement, aging and dispersion strengthening, the ultimate tensile strength, yield strength and elongation of the Al−10Zn−3.5Mg−1.5Cu alloy after hot extrusion and subsequent heat treatment achieve 810 MPa, 770 MPa and 8%, respectively.
Massive vanadium additions as hard phases in powder metallurgy high-speed steels (PM HSS) lead to higher cost and bad machinability. In this study, ultrahigh alloy PM HSS with CPM121 (10W−5Mo−4Cr−10V−9Co, wt.%) as the basic composition, was directly compacted and activation sintered with near-full density (>99.0%) using pre-oxidized and ball-mixed element and carbide powders. Niobium-alloyed steels (w(V)+w(Nb)=10 wt.%) show higher hardness and wear resistance, superior secondary-hardening ability and temper resistance. But excess niobium addition (>5 wt.%) leads to coarsened carbides and deteriorated toughness. EPMA results proved that niobium tends to distribute in MC carbides and forces element W to form M6C and WC carbides. Further, the role of rotary forging on properties of niobium-alloyed steels (S3) was researched. After rotary forging with deformation of 40%, the bending strength and fracture toughness of niobium-alloyed steels could be further improved by 20.74% and 43.86% compared with those of sample S3 without rotary forging, respectively.
In this paper, 15Cr-ODS steels containing 0, 1 wt%, 2 wt% and 3 wt% Al element were fabricated by combining wet-milling and spark plasma sintering (SPS) methods. The microstructure and mechanical properties of ODS steel were investigated by XRD, SEM, TEM, EBSD and tensile tests. The results demonstrate that the Al addition significantly refines the particle precipitates in the Fe−Cr matrix, leading to the obvious refinement in grain size of matrix and the improvement of mechanical properties. The dispersion particles in ODS steels with Al addition are identified as Al2O3 and Y2Ti2O7 nanoparticles, which has a heterogeneous size distribution in the range of 5 nm to 300 nm. Increasing Al addition causes an obvious increase in tensile strength and a decline in elongation. The tensile strength and elongation of 15Cr-ODS steel containing 3 wt% Al are 775.3 MPa and 15.1%, respectively. The existence of Al element improves the corrosion resistance of materials. The ODS steel containing 2 wt% Al shows corrosion potential of 0.39 V and passivation current density of 2.61×10−3 A/cm2(1.37 V). This work shows that Al-doped ODS steels prepared by wet-milling and SPS methods have a potential application in structural parts for nuclear system.
Combining sintering additive with field assisted sintering, stereolithographical dense Si3N4 ceramics was successfully fabricated. Owing to a large amount of polymer during the stereolithography, the green parts have the characteristics of low powder loading and high porosity. Adjusting the process parameters such as sintering temperature and soaking time can effectively improve the density of the specimens. The stress exponent n of all specimens is in a range of 1 and 2, which is derived from a modified sintering kinetics model. The apparent activation energy Qd of stereolithographic Si3N4 ceramics sintered with applied pressures of 30 MPa, 40 MPa, and 50 MPa is 384.75, 276.61 and 193.95 kJ/mol, respectively, suggesting that the densification dynamic process is strengthened by raising applied pressure. The grain boundary slipping plays a dominating role in the densification of stereolithographic Si3N4 ceramics. The Vickers hardness and fracture toughness of stereolithographic Si3N4 ceramics are HV10/10 (1347.9±2.4) and (6.57±0.07) MPa·m1/2, respectively.
In order to predict the powder flow law of the injection molding process of MgTiO3 ceramic parts with complex structures, a constitutive model and numerical simulation method for MgTiO3 ceramic injection molding were established based on the Hunt method. The material parameters of MgTiO3 such as elastic modulus, Poisson ratio, glass transition temperature, thermal conductivity and specific heat capacity were measured. Based on the fitting curve and the material parameters measured, the cross-WLF viscosity model and P-V-T model required for MgTiO3 ceramic injection molding were optimized. Furthermore, the influence of process parameters on mold filling flow and distribution of parts defects was researched. It was found that the gate position, injection speed and melt temperature have greater influence on mold filling flow and the packing process has an obvious effect on parts’ defects. On this basis, the MgTiO3 ceramic parts injection molding experiment verification was carried out. By comparing the experimental results with the simulated results, it is found that the deformation error is within 1.5% and the density error is within 1%. Therefore, this research provided theoretical guidance for the engineering application of MgTiO3 ceramic parts fabricated by injection molding.
Carbon quantum dots (CQDs), which contain a core structure composed of sp2 carbon, can be used as the reinforcing phase like graphene and carbon nanotubes in metal matrix. In this paper, the CQD/Cu composite material was prepared by powder metallurgy method. The composite powder was prepared by molecular blending method and ball milling method at first, and then densified into bulk material by spark plasma sintering (SPS). X-ray diffraction, Raman spectroscopy, infrared spectroscopy, and nuclear magnetic resonance were employed to characterize the CQD synthesized under different temperature conditions, and then CQDs with a higher degree of sp2 were utilized as the reinforcement to prepare composite materials with different contents. Mechanical properties and electrical conductivity results show that the tensile strength of the 0.2 CQD/Cu composite material is ∼31% higher than that of the pure copper sample, and the conductivity of 0.4 CQD/Cu is ~96% IACS, which is as high as pure copper. TEM and HRTEM results show that good interface bonding of CQD and copper grain is the key to maintaining high mechanical and electrical conductivity. This research provides an important foundation and direction for new carbon materials reinforced metal matrix composites.
Fe−6.5Si soft magnetic composites (SMCs) with hybrid phosphate-silica insulation coatings have been designed to improve their comprehensive property via chemical coating combining sol-gel method in this work. The microstructure and magnetic performance of the Fe−6.5Si SMCs with hybrid phosphate-silica insulation coatings were investigated. The hybrid phosphate-silica coatings with high heat resistance and high withstand pressure, formed on the surface of the Fe−6.5Si ferromagnetic powders, were found stable in the composites. Compared with Fe−6.5Si SMCs coated by single phosphate or single silica, Fe−6.5Si SMCs with hybrid phosphate-silica show much higher permeability and lower core loss. The work provides a new way to optimize the magnetic performance of soft magnetic composites.