Microstructure and mechanical properties of additive manufactured W-Ni-Fe-Co composite produced by selective laser melting

Tungsten composites face severe challenges in machining complex structures due to tungsten's high melting temperature. To explore solutions that enable fabrication of complex W composite parts by additive manufacturing, W-6Ni-2Fe-2Co (W90), W-12Ni-4Fe-4Co (W80) and W-18Ni-6Fe-6Co (W70) composites were consolidated by selective laser melting (SLM). The effects of laser process parameters and chemical compositions on densification, microstructures, phases, and tensile properties were investigated. With the increase of laser energy density, the density of the composite increases. Near full density with an absence of cracks and pores was achieved in the SLM-processed W70 composites. The typical microstructure consisted of un melted polyhedral W particles and the surrounding W-Ni-Fe-Co matrix with W dendrites. Alternating layered fine dendrite and coarse dendrite zones were visible in side views of the composites. The tensile properties of the W70 composite had a pronounced improvement with the increase of laser energy density. A maximum ultimate tensile strength of 1198 MPa was obtained in the SLM-processed W70 composite with elongation of 9.5%. The SLM-processed W-Ni-Fe-Co composites pave the way for new refractory metal alloys and complex shaped parts fabrication by additive manufacturing.     

Selective Laser Melting of in-situ TiC/Ti5Si3 composites with novel reinforcement architecture and elevated performance

A novel Selective Laser Melting (SLM) process was applied to prepare bulk-form TiC/Ti₅Si₃ in-situ composites starting from Ti/SiC powder system. The influence of the applied laser energy density on densification, microstructure, and mechanical performance of SLM-processed composite parts was studied. It showed that the uniformly dispersed TiC reinforcing phase having a unique network distribution and a submicron-scale dendritic morphology was formed as a laser energy density of 0.4 kJ/m was properly settled. The 96.9% dense SLM-processed TiC/Ti₅Si₃ composites had a high microhardness of 980.3HV_0.2, showing more than a 3-fold increase upon that of the unreinforced Ti part. The dry sliding wear tests revealed that the TiC/Ti₅Si₃ composites possessed a considerably low friction coefficient of 0.2 and a reduced wear rate of 1.42 × 10^− 4 mm³/Nm. The scanning electron microscope (SEM) characterization of the worn surface morphology indicated that the high wear resistance was due to the formation of adherent and strain-hardened tribolayer. The densification rate, microhardness, and wear performance generally decreased at a higher laser energy density of 0.8 kJ/m, due to the formation of thermal cracks and the significant coarsening of TiC dendritic reinforcing phase.     

Formation of scanning tracks during Selective Laser Melting (SLM) of pure tungsten powder: Morphology,geometric features and forming mechanisms

Due to the high melting point and high heat conductivity, selective laser melting (SLM) of tungsten is still challenging. To have a better understanding of SLM tungsten parts, the effects of processing parameters such as laser power and scanning speed on scanning tracks formation of pure tungsten powder were investigated. As linear energy increased with increasing laser power and decreasing scanning speed, the height and contact angle of scanning tracks gradually reduced, while the width and penetration depth increased. Owing to the good wetting and spreading, the flow front of scanning tracks gradually became smooth and stable with the increased linear energy. However, the transverse cracks induced by large temperature gradient and high cooling rate appeared on the surface of the scanning tracks at linear energy of more than 1.75 J/mm. A maximum temperature of 4630.27 °C and high cooling rate of 8.6 × 10⁶ °C/s were obtained during SLM process of tungsten powder when the linear energy was 1.75 J/mm. This work provides scientific guidance for SLM-processed tungsten parts.     

Cracking investigation of W and W(C) films deposited by physical vapor deposition on steel substrates

This paper presents the investigation of the cracking of coatings deposited on steel substrates. The coating on substrate systems consisted on pure tungsten films (W) and films of solid solutions of carbon in tungsten [W(C)], which were deposited by direct current reactive magnetron sputtering on stainless steel substrates. The systems were strained uniaxially with a microtensile device adapted to a scanning electron microscope. The mechanical response was analyzed from the experimental results: the straining of the samples showed an evolution of the density of cracks in the coating, which was described trough an empirical equation based on the Weibull distribution function. The density of cracks, which corresponds to the crack saturation of the coating, appeared to vary inversely with coating thickness. Critical parameters relative to their mechanical stability were also determined from the experimental results: the strain energy release rate for crack extension through the film, G^f_c, and the fracture toughness, K^f_Ic, of the coatings. These values are included between 0.2 and 14 J m⁻², and between 0.1 and 2.5 MPa m^−1/2. The fracture resistance of W and W(C) coatings was found to be correlated to their thickness and microstructure.     

Binder jet printed WC infiltrated with pre-made melt of WC and Co

WC-Co was made via binder jet additive manufacturing of tungsten carbide followed by melt infiltration with a Co-WC infiltrant. The goal of the study was to achieve fully densified parts in near-net shape with minimal shrinkage while keeping the Co content low. The exact amount of infiltrant was determined in order to fully densify with minimum shrinkage based on the actual volume taken up by WC powder in the preform based on theoretical density, the bounding volume of prints after shrinkage, and the volume from the infiltrant. The eutectic nature of the infiltrant enabled melting at much lower temperature compared to the melting temperature of pure Co. The density, microstructure, grain size, hardness, and fracture toughness were characterized. The shrinkage and net shaping were assessed with light scans. A detailed look at the fracture mechanics was assessed. This approach achieved highly dense WC-Co parts in net-shape with Co vol% of near 29 (Co wt% ~19), density of 96.2% theoretical, hardness of 8.34 GPa, grain size of 7.7 μm, magnetic saturation of 0.5 T, room temperature thermal conductivity of 125 W/mK, and fracture toughness of 24.7 MPa·m^1/2.     

Densification behavior,microstructure evolution,and wear performance of selective laser melting processed commercially pure titanium

This work presents a comprehensive study of the densification behavior, phase and microstructure development, hardness and wear performance of commercially pure Ti parts processed by selective laser melting (SLM). An in-depth relationship between SLM process, microstructures, properties, and metallurgical mechanisms has been established. A combination of a low scan speed and attendant high laser energy density resulted in the formation of microscopic balling phenomenon and interlayer thermal microcracks, caused by a low liquid viscosity, a long liquid lifetime, and resultant elevated thermal stress. In contrast, using a high scan speed produced the disorderly liquid solidification front and considerably large balling, due to an elevated instability of the liquid induced by Marangoni convection. A narrow, feasible process window was accordingly determined to eliminate process defects and result in full densification. The phase constitutions and microstructural characteristics of SLM-processed Ti parts experienced a successive change on increasing the applied scan speeds: relatively coarsened lath-shaped α → refined acicular-shaped martensitic α′ → further refined zigzag-structured martensitic α′, due to the elevated thermal and kinetic undercooling and attendant solidification rate. The optimally prepared fully dense Ti parts had a very high hardness of 3.89 GPa, a reduced coefficient of friction of 0.98 and wear rate of 8.43 × 10^?4 mm³ N^?1 m^?1 in dry sliding wear tests. The formation of an adherent, plastically smeared tribolayer on the worn surface contributed to the enhancement of wear performance.     

The effect of hot isostatic pressing on thermal conductivity of additively manufactured pure tungsten

The crack-healing behaviors and microstructure evolution of pure tungsten produced by laser powder bed fusion (LPBF) were studied and compared before and after post hot isostatic pressing (post-HIP) treatment. An average thermal conductivity of 133 W·m⁻¹·K⁻¹ at room temperature (RT) was obtained after HIP, which was 16% higher than that of as-built sample (115 W·m⁻¹·K⁻¹). Although the HIP process had little effect on density, it resulted in a large grain size of >300 μm accompanied by a decrease in dislocation density and crack healing, which led to a substantial improvement of thermal conductivity of pure tungsten. The positive correlation between relative density and thermal conductivity of as-built tungsten was reported.     

Microstructures and mechanical properties of FeCoCrNi high entropy alloy/WC reinforcing particles composite coatings prepared by laser cladding and plasma cladding

FeCoCrNi HEA coatings with 20% mass fraction of WC reinforcing particles were prepared by two different cladding methods, laser cladding (LC) and plasma cladding (PC). The microstructure of HEA matrix and WC particles of LC and PC coatings were discussed respectively. For HEA matrix, dendritic morphology was observed in both coatings. For WC particles, a few granular (Cr,W)₂C carbides around WC particles in LC coatings, and a large number of crystal and fishbone Fe₃W₃C carbides around WC particles in PC coatings. Mechanical properties as hardness and wear resistance of the two kinds of coatings were also investigated. The interstitial solution strengthening effect of C element is stronger in PC coating, and the hardness of HEA matrix in LC coatings is twice that of in PC coating, which shows a strong retention force on WC particles. The friction coefficient of LC coating is lower and stable, with the volume wear rate of 0.7 × 10⁻⁵ mm⁻³/N·m, showing high wear resistance. PC coatings have poor wear resistance due to decarbonization and oxidation of WC particles and reduction of retention force of HEA matrix, with the volume wear rate of 8.29 × 10⁻⁵ mm⁻³/N·m. The wear mechanism of both coatings were also discussed.     

Effect of processing parameters on the density,microstructure and strength of pure tungsten fabricated by selective electron beam melting

Pure tungsten samples were prepared by the selective electron beam melting (SEBM) process. The effect of the SEBM process parameters on the density, microstructure and compression strength of pure tungsten was studied. In addition, the influence of substrate preheating temperature during SEBM was studied. A processing window for additive manufacturing of pure tungsten by SEBM was preliminarily determined. Pure tungsten samples with relative density of 99.5% and without obvious pores and microcracks were successfully fabricated. The as-built pure tungsten samples showed strong columnar grain structures. Compression strength along the columnar grains in the build direction was measured to be 1560 MPa. Fracture occurred predominantly along the columnar grain boundaries by decohesion, in addition to brittle transgranular fracture. Refinement and strengthening of the columnar grain boundaries are expected to improve the compression strength of the SEBM-fabricated pure tungsten.     

Influence of processing parameters on the microstructure and tensile property of 85 W-15Ni produced by laser direct deposition

The plate-like shape 85 W-15Ni parts were produced by laser direct deposition technology with different processing parameters (laser power and scanning speed). The influence of processing parameters and their corresponding laser energy density on the microstructural characterization, phase composition and tensile property of 85 W-15Ni samples was investigated. The results show that the relative density of samples increased with the laser energy density and the densification trend started to slow as the laser energy density reached 380–400 J/mm³, though the highest density value was obtained with laser energy of 425 J/mm³. With the increase of laser energy density, more disorder and fine W dendrites existed at the bonding region between deposition layers and more WW grain boundaries formed at the central region of the layer. The 85 W-15Ni samples produced with different processing parameters consisted of W and γ-Ni phase. To improve the tensile property, it is necessary to increase the laser energy density to obtain denser structure and reduce the residual pores or gaps. However, the excessive laser energy density resulted in the formation of more WW grain boundaries that were detrimental to the tensile property. The best tensile properties were obtained at the laser energy density of 395 J/mm³.     

激光熔化沉积制备2A50锻造铝合金组织性能

工艺参数的协同调控决定了沉积工件的组织与性能,在锻造铝合金零件激光增材修复工程应用方面具有重要研究价值。采用OM、SEM、XRD等试验方法,研究能量密度对激光沉积成形2A50铝合金构件组织与性能的影响规律。结果表明：当能量密度低于200 J/mm2时,成形效果较差且产生粉末球化、未熔合等凝固缺陷;随着能量密度的提高,沉积试样底部和顶部一次枝晶间距均明显缩短、平均硬度由85.7 HV提高至92.1 HV;过高的能量密度输入会导致熔池内部分低熔点合金元素蒸发形成气孔缺陷、同时削弱了合金元素的固溶强化效果。在优化的能量密度（333J/mm2）条件下,激光沉积成形2A50锻造铝合金构件获得了较优的综合力学性能,其屈服强度、抗拉强度和延伸率分别为85 MPa、207 MPa和14%。为航空重大装备关键零部件的激光增材修复探索出一条行之有效的技术途径。     

Preparation,microstructure, and properties of tungsten alloys reinforced by ZrO2 particles

Tungsten alloys reinforced by in-situ tetragonal zirconia (W–ZrO₂) were developed via the azeotropic distillation method combined with the powder metallurgy method. The microstructure and abrasive wear properties were studied. The in-situ ZrO₂ particles in the tungsten matrix were obtained by the decomposition of zirconium nitrate after liquid–liquid incorporation of (NH₄)₆H₂W₁₂O₄₀ and Zr(NO₃)₄ aqueous solution. The ZrO₂ particles were distributed evenly in the tungsten matrix, which refined tungsten powders and the grains of tungsten alloys significantly. The density and Vickers hardness of the tungsten alloys decreased with increasing ZrO₂ mass fraction. However, the wear resistance increased firstly and then decreased with increasing ZrO₂ mass fraction. The optimal amount of ZrO₂ for improving wear property is 3%, with the wear resistance of W–3% ZrO₂ improving by approximately 20%–40% compared with that of pure tungsten. The proper amount of ZrO₂ particles can efficiently prevent microcutting to protect the tungsten matrix, thereby enhancing the wear resistance of tungsten alloys.     

Effect of processing parameters on the densification,microstructure and crystallographic texture during the laser powder bed fusion of pure tungsten

Laser Powder Bed Fusion is a leading additive manufacturing technology, which has been used successfully with a range of lower melting point materials (titanium alloys, nickel alloys, steels). This work looks to extend its use to refractory metals, such as those considered in this paper where the behaviour of pure tungsten powder is investigated. A strategy for fabricating high density parts was developed by creating a process map in which the effect of laser energy density was studied. The process quality was assessed using different techniques including light optical microscopy, XCT, SEM and EBSD. The results showed that the laser energy density was adequate to process tungsten to produce functional parts. The bulk density and optically determined densities, under different process conditions, ranged from 94 to 98%, but there was evidence of micro cracks and defects in specimens due to micro- and macro-scale residual stress. Analysis of the microstructure and local crystallographic texture showed that the melt pool formed under the laser beam favoured solidification in a preferred orientation by an epitaxial growth mechanism. The EBSD local texture analysis of the tungsten specimens showed a <111>//Z preferential fibre texture, parallel to the build direction.     

In situ fabrication of TiC-NiCr cermets by selective laser melting

In situ synthesis of TiC-NiCr cermets during selective laser melting (SLM) was investigated. Powder mixture consisting Ti, C and Ni80Cr20 solid solution was prepared by milling process to attain the nominal composition of TiC-40 wt% NiCr. Feedstock powder was milled for 7 h by a high energy ball mill. SLM process was applied at three scanning speeds and the effect of input energy density on phase formation, microstructure and hardness of resultant samples was evaluated. Morphology of the powders and microstructure of in situ fabricated cermets were investigated by field emission electron microscopy (FESEM), and structural evolution during SLM was studied by X-ray diffractometery (XRD). The results showed that TiC can be synthesized during SLM process even at relatively low energy densities and the formation mechanism of TiC phase is affected by the input energy density. Relative density of the samples increased from 88.24% to 98.53% with increasing the energy density from 138.7 to 346.7 Jmm⁻³. The sample fabricated by SLM process using the maximum energy density of 346.7 Jmm⁻³showed the highest mean microhardness value of 1289 HV.     

Friction and wear behaviours of in situ reduced graphene oxide reinforced zirconia ceramic

The tribological behaviour of zirconia composites reinforced by in situ reduced graphene oxide (IrGO) was investigated by a rotating ball-on-plate configuration at room temperature, and was followed by a comparison with composites reinforced by pre-reduced graphene oxide(rGO). The results indicate that both the friction and wear resistance of ceramics increase with the incorporation of graphene oxide (GO). IrGO is better for enhancing the tribological performance of zirconia than rGO. The wear rate decreases by up to one order of magnitude (from 2.33 × 10⁻⁵ mm³·N⁻¹·m⁻¹ to 4.66 × 10⁻⁶ mm³·N⁻¹·m⁻¹) with 0.5 wt% GO. A protective tribofilm containing reduced graphene oxide forms at wear surfaces of ceramics, and the protruding-out of rGO is more pronounced. The main wear mechanism changes from severe delamination to plastic deformation and micro-cracking with increasing GO content. The analysis of wear tracks for ceramics with GO additives by Raman mapping reveals a decrease in the tetragonal-monoclinic phase transformation after sliding wear, which is regarded as the intrinsic reason for the improved wear resistance.     

Effect of Al2O3 content and swaging on microstructure and mechanical properties of Al2O3/W alloys

Al₂O₃-reinfored tungsten alloys were fabricated by powder metallurgy method and hot swaging technology. The investigation was made on the microstructure, relative density, nano-hardness and fracture toughness (K_IC) of the sintered and swaged Al₂O₃/W alloys. The swaging process and addition of Al₂O₃ are beneficial to comprehensive properties of the sintered and swaged alloys. After swaging, the Al₂O₃/W alloys can achieve the full density. According to the nano-indentation test and three-point bend test, the swaged W-0.25 wt% Al₂O₃ alloy possesses the highest hardness of 7.02 GPa, the greatest modulus of 435.09 GPa and the maximum fracture toughness of 21 MPa·m^1/2. The observation of fracture morphology shows that the recrystallization behavior and grain growth occur above 1400 °C in the swaged pure W alloy, which leads to recrystallization brittleness. At the same time, the microstructure of the swaged W-0.25 wt% Al₂O₃ alloy does not change apparently.     

聚碳酸酯激光透射焊接力学性能分析

为研究聚碳酸酯激光透射焊接力学性能,采用10 W半导体端泵浦全固态激光器进行了透明与不透明聚碳酸酯的焊接.采用光学金相显微镜(OM)、场发射扫描电子显微镜(SEM)、显微硬度仪和电子万能实验拉力机分析测试了接头的显微形貌、断口形貌、显微硬度分布及拉伸抗剪强度.结果表明,随着激光面能量的增加,接头的连接形式由焊接接头演化为类似粘接接头;分层和气孔是影响接头力学性能的主要原因,通过控制面能量,可获得力学性能良好的焊接试样,在面能量为0.23 J/mm²时,拉伸抗剪强度达到44 MPa,约为母材强度的68%,且接头的显微硬度与母材相当;接头的断裂机制为混合断裂机制.     

Sintering and sliding wear studies of B4C-SiC composites

Dense boron carbide (B₄C) – silicon carbide (SiC) composites were obtained by spark plasma sintering technique at 1800°C with 3 wt% and 6 wt% aluminium oxide (Al₂O₃) additives. Addition of sintering additives results in formation of aluminium silicate (Al₂SiO₅) liquid phase which accelerates sintering kinetics and helps in obtaining high density ~ 99%. Microstructures reveal uniformly distributed SiC particles in B₄C matrix. Increase in alumina from 3 wt% to 6 wt% results in decrease in hardness from 35.1 ± 0.8 to 33.7 ± 0.9 GPa, and increase in fracture toughness from 5.9 ± 0.4 to 6.5 ± 0.4 MPam^0.5. Using a ball-on-disk tribo tester under dry unlubricated conditions at 5, 10 or 15 N load, influence of alumina content on friction and wear properties of B₄C-SiC composites was investigated against SiC counterbody with a linear speed of 0.08 m/s for 60 min. The coefficient of friction (COF) increased from 0.25 to 0.65 with load, and the influence of alumina on frictional behaviour appeared to be negligible. With increase in load, wear volume of the composites increased from 7.5 × 10⁻² mm³ to 16.1 × 10⁻² mm³ for B₄C-10 wt% SiC - 3 wt% Al₂O₃ and from 4.7 × 10⁻² mm³ to 14.8 × 10⁻² mm³ for B₄C-10 wt% SiC - 6 wt% Al₂O₃ composites. Microcracking, abrasion and pull-outs contributed as major wear mechanisms of composites in selected wear conditions. The relation between wear behaviour and mechanical properties of sintered composites is discussed.     

Densification,microstructure and tribomechanical properties of SPS processed β-SiAlON bonded WC composites

Densification, microstructure and tribomechanical properties of spark plasma sintering (SPS) processed β-SiAlON (20–40 wt%) bonded WC matrix composites have been reported. All the specimens achieved almost their theoretical density values after SPS at 1750 °C for 25 min under 40 MPa. Incorporation of β-SiAlON in WC significantly altered the densification trend of the composites resembling that of pure β-SiAlON. Microstructural investigations using scanning and transmission electron microscopy revealed formation of principally equiaxed, micron sized WC grains surrounded by the sub-micron to micron sized β-SiAlON phase. The interface region between WC and β-SiAlON was found to be free of any reaction product. Energy dispersive X-ray spectrum confirmed presence of characteristics elements in both WC and β-SiAlON phases in the composite. The maximum Vickers hardness (~18 GPa) and fracture toughness (~6.8 MPa-m^0.5) under 10 kgf were obtained for the 30 wt% β-SiAlON/WC composite. These were almost 6% and 50% higher, respectively, than those obtained for pure WC. Indentation size effect (ISE) analyses of some selected specimens indicated moderate sensitivity towards ISE (Meyer's exponent = 1.802) of the 30 wt% β-SiAlON/WC composite and higher true hardness (~15.4 GPa) than those obtained for both the constituent phases. The load dependence of fracture toughness of some selected specimens has also been reported. Unlubricated wear studies under 30 N up to 250 m using ball-on-disc configuration indicated ~46–55 times higher specific wear rate of the β-Si₃N₄ ball when rubbed against the composites compared to that (~8 × 10⁻⁶ mm³/N-m) obtained against pure WC. Formation of compacted flaky tribo-layer within the wear track of the composites was evidenced.     

Processing of WC/W composites for extreme environments by colloidal dispersion of powders and SPS sintering

Tungsten and tungsten carbide are materials with high thermomechanical response that are used or have been proposed for extreme environment applications such as first plasma face, or cutting tools. The high melting temperature and strong bonding energy of both materials force the use of powder metallurgical processes and non-conventional sintering routes to achieve dense parts. Consequently, a high dispersion and close contacts of the starting powders are required. In this paper tungsten and tungsten carbide powders are colloidally processed and mixed to achieve composite powders that are sintered later by Spark Plasma Sintering. Starting micrometric tungsten carbide and nanosized tungsten powders are dispersed in water at pH 3. By using a cationic dispersant, the surface charge of the nanosized W suspended in water reverses to positive, ensuring its attachment to the carbide surfaces and the good dispersion of the two phases when both slurries are mixed.Composite powders with volumetric rations of 50WC/50W, 80WC/20W and 90WC/10W as well as pure WC and W are sintered by SPS following the dimensional change of the specimens during the process. It has been proved that complete coverage of the micronic WC by the nanosized W powders, achieved with this colloidal approach, makes the tungsten govern the initial sintering stages. The derivative of the sintering curves is used to detect the solid state reactive sintering temperature of W₂C. After sintering, XRD and SEM observations indicate that all the mixture compositions yield to ceramic materials with different W₂C/WC ratios, depending on the initial compositions. Dispersion of the two phases is high and no remaining W is detected. Flexure tests at room temperature show that composite materials present a slightly lower fracture strength than pure WC.