Microstructure and properties of 6FeNiCoSiCrAlTi high-entropy alloy coating prepared by laser cladding

The content of each constituent element in the newly developed high-entropy alloys (HEAs) is always restricted in equimolar or near-equimolar ratio in order to avoid the formation of complex brittle phases during the solidification process. In this study, a 6FeNiCoSiCrAlTi high-entropy alloy coating with simple BCC solid solution phase has been prepared by laser cladding on a low carbon steel substrate. The microstructure, hardness and magnetic properties have been investigated. The experimental results show that the tendency of component segregation in the conventional solidification microstructure of multi-component alloy is effectively relieved. The microstructure of the coating is mainly composed of equiaxed polygonal grains, discontinuous interdendritic segregation and nano-precipitates. EBSD observation confirms that the polygonal grains and interdendritic segregation have similar BCC structure with lots of low angle grain boundaries at the interface. The microhardness of the coating reaches 780 HV_0.5, which is much higher than most of the HEAs prepared by other methods. In addition, the coating shows excellent soft magnetic properties.     

Microstructure and wear resistance of laser clad cobalt-based alloy multi-layer coatings

Multi-layer Co-based alloy (HMSP 2537) were deposited on Ni-based superalloy plate with a TJ-TL-T5000 type CO₂ laser. Sections of such coatings were examined to reveal their microstructure and phase composition using optical microscope, scanning electron microscope (SEM), and X-ray diffractometer (XRD). The hardness and wear resistance of the coatings were tested. The results showed that the prime phase (γ-Co dendrite) and other phases, such as CrNi, Co₇W₆, and Cr₂₃C₆ existed in the coatings. Dendrite or cellular microstructures were observed perpendicular to the interface, and coarsening microstructures were obtained as more layers deposited. Dendrite paralleling to laser scan speed was also found near the top surface of the last layer. Fine microstructures of γ-Co dendrite and lamellar eutectic in inter-dendritic regions strengthened the coatings. Microhardness and wear resistance of the coatings were much higher than that of substrate but slightly decreased with layers increased.     

Microstructure of yttric calcium phosphate bioceramic coatings synthesized by laser cladding

The yttric calcium phosphate (CaP) coatings were in situ prepared on pure titanium substrate by laser cladding. The morphologies and phases constitution of CaP coatings were studied by electron probe microanalysis, X-ray diffraction and so on. The bonding state between the coating and the substrate is fine metallurgical combination, and the addition of yttria can fine the structure and increase the tensile strength of the coatings. The X-ray result shows that the coating is composed of the phases of HA, α-Ca₂P₂O₇, β-Ca₂P₂O₇ and CaTiO₃.     

Microstructure and wear properties of TiCN/Ti coatings on titanium alloy by laser cladding

Titanium carbide nitride (TiCN) reinforced Ti coating was fabricated on the surface of Ti–6Al–4V alloy by laser cladding method. Microstructure and wear properties at the surface of the coating in atmosphere were investigated. Three zones can be distinguished of the coating: the clad zone (CZ), the heat affected zone (HAZ) and the substrate. The clad zone is composed of TiCN dendrites, TiO₂ and Ti. A metallurgical bonding between the coating and the substrate was obtained. The microhardness and wear resistance of the TiCN/Ti coating are significantly improved. The average hardness of the coating is about 3 to 6 times of that of the substrate. The friction coefficients of the substrate and the coating are 0.48 and 0.34 respectively. The friction coefficient of the Ti–6Al–4V substrate was insensitive to the normal load, while that of the cladded TiCN/Ti coating was very sensitive to the normal load. The wear mass losses of the cladded samples are much lower than that of the substrate whatever the normal load is.     

Composition design and laser cladding of Ni-Zr-Al alloy coating on the magnesium surface

The cluster line criterion was used for optimized design of a Ni-Zr-Al alloy used as coating on the AZ91HP magnesium alloy by laser cladding. Results show that the coating mainly consists of an amorphous, two ternary intermetallic phases with Ni₁₀Zr₇ and Ni₂₁Zr₈ type structures resulting in high hardness, good wear resistance and corrosion resistance. The interface between the clad layer and the substrate has good metallurgical bond.     

Fabrication of nanoporous manganese by laser cladding and selective electrochemical de-alloying

Fabrication of nanoporous Cu-Mn alloy coatings was investigated by a two-step process involving high power laser cladding of a homogeneous Cu40Mn60 alloy coatings followed by selectively electrochemical de-alloying. Auger mapping results indicate that nanoporous manganese was obtained by selective electrochemical etching of the less active Cu component owing to the passivation of the more active manganese in potassium nitrate solution. The surface morphology of the porous Mn was a ribbon-like structure, different from interconnected bicontinuous nanopores that are usually obtained by de-alloying. The influence of de-alloying time, electric potential and temperature on the formation of nanoprosity is systematically investigated. Nanopore sizes can be tailored to be less than 100 nm. Under optimal etching conditions the nanopore size was below 25 nm. The surface area of the nanoporous manganese layer was enhanced by up to 990 times compared with that of a polished sample.     

H13钢激光熔覆Co基涂层组织及热疲劳性能

针对热作模具用H13钢工况下易产生热疲劳失效的问题,采用Nd:YAG激光器在H13钢表面制备Co基合金涂层。利用光学显微镜、扫描电镜、能谱仪和X射线衍射仪,对涂层组织、合金元素分布及物相组成进行检测。利用显微硬度计和热震实验法,测试热疲劳对Co基合金涂层和淬火回火态H13钢硬度影响。结果表明,Co基合金涂层从底部到表层,依次为平面晶、胞状晶、树枝晶和等轴晶。Co基合金涂层物相主要由-Co和M23C6相组成,热疲劳后涂层表面形成M2O3和M3O4(M=Fe,Co,Cr)氧化物。Co基合金涂层硬度最高可达706HV0.2且呈梯度降低;热循环1000次后,Co基合金涂层表面硬度降低24.4%,H13钢表面硬度降低37.7%,Co基合金涂层硬度下降幅度低于H13钢。热循环1000次后,Co基合金涂层表面未发现明显热裂纹,H13钢表面形成大量网状热裂纹。Co基合金涂层中,Cr元素形成致密Cr2O3氧化膜使其热疲劳性能优于H13钢。     

Modification of tribology and high-temperature behavior of Ti-48Al-2Cr-2Nb intermetallic alloy by laser cladding

In order to improve the tribology and high-temperature oxidation properties of the Ti-48Al-2Cr-2Nb intermetallic alloy simultaneously, mixed NiCr-Cr₃C₂ precursor powders had been investigated for laser cladding treatment to modify wear and high-temperature oxidation resistance of the material. The alloy samples were pre-placed with NiCr-80, 50 and 20%Cr₃C₂ (wt.%), respectively, and laser treated at the same parameters, i.e., laser output power 2.8 kW, beam scanning speed 2.0 mm/s, beam dimension 1 mm × 18 mm. The treated samples underwent tests of microhardness, wear and high-temperature oxidation. The results showed that laser cladding with different constitution of mixed precursor NiCr-Cr₃C₂ powders improved surface hardness in all cases. Laser cladding with NiCr-50%Cr₃C₂ resulted in the best modification of tribology and high-temperature oxidation behavior. X-ray diffraction (XRD), optical microscope (OM), scanning electron microscopy (SEM) and energy-dispersive spectrometer (EDS) analyses indicated that the formation of reinforced Cr₇C₃, TiC and both continuous and dense Al₂O₃, Cr₂O₃ oxide scales were supposed to be responsible for the modification of the relevant properties. As a result, the present work had laid beneficial surface engineering foundation for TiAl alloy applied as future light weight and high-temperature structural candidate materials.     

Phase composition and tribological properties of Ti-Al coatings produced on pure Ti by laser cladding

Ti-Al coatings with ∼14.7, 18.1, 25.2 and 29.7 at.% Al contents were fabricated on pure Ti substrate by laser cladding. The laser cladding Ti-Al coatings were analyzed with X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray energy dispersive spectroscopy (EDS). It was found that with the increase of Al content, the diffraction peaks shifted gradually to higher 2θ values. The laser cladding Ti-Al coatings with 14.7 and 18.1 at.% Al were composed of α-Ti and α₂-Ti₃Al phases, while those with 25.2 and 29.7 at.% Al were composed of α₂-Ti₃Al phase. With the increase of Al content, the cross-sectional hardness increased, while the fracture toughness decreased. For the laser cladding Ti-Al coatings, when the Al content was ≤18.1 at.%, the wear mechanism was adhesive wear and abrasive wear; while when the Al content ≥25.2 at.%, the wear mechanism was adhesive wear, abrasive wear and microfracture. With the increase of Al content, the wear rate of laser cladding Ti-Al coatings decreased under 1 N normal load, while the wear rate firstly decreased and then increased under a normal load of 3 N. Due to its optimized combination of high hardness and high fracture toughness, the laser cladding Ti-Al coating with 18.1 at.% Al showed the best anti-wear properties at higher normal load.     

激光沉积修复TA15合金的组织和力学性能

采用激光沉积修复方法对TA15槽损伤试样进行了修复试验,考察了修复试样的组织和力学特点,发现激光修复试样组织是由基材的双态组织经由热影响区过渡到修复区的网篮组织。修复区组织为粗大的原始柱状晶,晶粒内为/网篮组织,晶内片层取向随机, 宽0.4~0.5 m。分析了修复区和坡面存在熔合不良缺陷的原因,并给出了纠正措施。修复试样的室温静拉伸结果表明,试样的抗拉强度接近工业锻件水平,表明修复区与基材之间形成了致密冶金结合,但由于修复区粗大柱状晶组织,试样的冲击韧性比基材稍差。     

激光沉积修复TA15合金的组织和力学性能

采用激光沉积修复方法对TA15槽损伤试样进行了修复试验,考察了修复试样的组织和力学特点,发现激光修复试样组织是由基材的双态组织经由热影响区过渡到修复区的网篮组织。修复区组织为粗大的原始柱状β晶,晶粒内为α/β网篮组织,晶内α片层取向随机,宽0.4～0.5μm。分析了修复区和坡面存在熔合不良缺陷的原因,并给出了纠正措施。修复试样的室温静拉伸结果表明,试样的抗拉强度接近工业锻件水平,表明修复区与基材之间形成了致密冶金结合,但由于修复区粗大柱状晶组织,试样的冲击韧性比基材稍差。     

Effect of nano-Y2O3 on microstructure of laser cladding cobalt-based alloy coatings

Cobalt-based alloy coatings with and without nano-Y₂O₃ particles produced by a 5 kW CO₂ laser on Ni-based superalloy were introduced. Solidified microstructure, phase compositions and distribution of nano-particles were investigated by using optical microscope, SEM, TEM and XRD. The results showed that metastable phases, such as γ-Co and Cr₂₃C₆ existed in cobalt-based alloy coatings. Another two phases (Y₂O₃ and ?-Co) were found by adding nano-Y₂O₃. Without nano-Y₂O₃, rapid directional solidified microstructure of columnar dendrite appeared. Fine and short dendritic microstructure and columnar to equiaxed transition (CET) occurred by adding nano-Y₂O₃ particles. With the increase in amount of nano-Y₂O₃, fully equiaxed crystallization appeared and the formation mechanism was analyzed. The results also showed that the sub-microstructure of the coatings changed from dislocation to stacking fault by adding nano-Y₂O₃.     

铁基合金激光熔覆层高温润滑磨损性能

为提高40Cr合金钢的表面耐磨性,采用预置激光熔覆法在40Cr基体表面制备铁基合金涂层, 利用扫描电镜观察分析熔覆层显微组织形貌,用显微硬度仪测试熔覆层截面显微硬度,用摩擦磨损试验机测定在润滑条件下基体、熔覆层的摩擦系数随温度变化的规律。研究结果表明:熔覆层与基体实现良好冶金结合,熔覆层横截面微观组织呈现平面晶、树枝晶和胞状晶分布;熔覆层硬度值介于617.5~926.6 HV0.2之间,基体硬度介于205.2~278.2 HV0.2之间;在200 ℃以下,熔覆层摩擦系数在磨程中趋于平稳,在0.1附近轻微波动,小于基体平均摩擦系数;当温度超过200 ℃,油膜分解,引发润滑失效,磨损方式向干摩擦转化,磨损机理从微切削磨损主导向粘着磨损、磨粒磨损和氧化磨损复合磨损方式转化。     

激光熔覆FeCrNiMnMoxB0.5高熵合金涂层组织与耐蚀性能

为了获得性能优异的涂层材料,采用激光熔覆的方法在Q235钢基体上制备了不同Mo含量的FeCrNiMnMoB0.5系高熵合金涂层,着重探究Mo对高熵合金组织与性能的影响。通过金相、X射线衍射、扫描电镜、硬度计、电化学工作站分别研究了高熵合金涂层的显微组织、相结构、显微硬度及耐蚀性能。结果表明：FeCrNiMnMoB0.5系高熵合金组织为树枝晶,主要由fcc相和少量金属间化合物构成。当添加少量Mo时,涂层的硬度较低,为290 HV,随着Mo的增加,晶格畸变加剧,出现少量金属化合物,涂层硬度最大增加到658 HV。在模拟饱和盐水泥浆溶液中FeCrNiMnMo0.4B0.5表现出较好的耐腐蚀性。