Study of dilution in laser cladding of a carbon steel substrate with Co alloy powders

High power diode laser with coaxial powder injection was used to deposit single tracks of cobalt alloy on to a carbon steel plate in order to study dilution. Two different methods to evaluate dilution are proposed and validated: dilution results to be proportional to the average percent or iron present in the clad. To study the correlations between dilution and processing parameters, clads were produced in different processing conditions. Dilution is correlated with the specific energy, and equation to estimate the average iron contamination of the clads was found. ‘Trial and error’ method was applied to improve this estimation. A statistically better prediction of the iron contamination is obtained when the combined parameter P^2.5/F⁴ is used. Dilution influences clad microstructure and thus hardness of the final coating, which decreases on increasing dilution. Phase distribution is also affected by dilution, Fe and C contamination stabilises α-fcc phase.     

Characterization of Individual Microneedles Formed on Alloy Surfaces by Femtosecond Laser Ablation

Cross-sectional microstructural analyses of micron/nano-sized structures (termed microneedles) formed by low and high fluence pulse laser ablation of AISI 4340?steel, Ti6Al4V, and Al 5754 alloy specimens were performed. Dependence of length scale and orientation of microneedle microstructures on energy absorptance during laser irradiation, heat transfer direction, absorptivity, and thermal conductivity of the material was established. Microneedle nucleation and growth process were explained based on penetration depths, redeposition of ablated material, and ablation rates.     

激光功率对碟阀表面熔覆Ni 基合金涂层性能的影响

采用半导体激光器在2205双相不锈钢表面激光熔覆Ni基合金涂层.借助扫描电镜、电化学综合测试仪和硬度测试仪等,探讨了激光功率对涂层稀释率、微观组织、耐腐蚀性能及硬度的影响.结果表明:激光功率越大,涂层稀释率越大,熔覆层与基体元素发生更多的对流扩散;熔覆层的耐腐蚀性能随激光功率的增加而降低,当激光功率为2.7 kW时,熔覆层的自腐蚀电位最低,为-0.46 mV,腐蚀电流最小,为3.47×10-5 A/cm2. 硬度测试实验表明,激光熔覆Ni基合金涂层硬度最高达680 HV,约为基体硬度的2.5倍.      

Modeling of Heat Transfer and Fluid Flow in the Laser Multilayered Cladding Process

The current work examines the heat-and-mass transfer process in the laser multilayered cladding of H13 tool steel powder by numerical modeling and experimental validation. A multiphase transient model is developed to investigate the evolution of the temperature field and flow velocity of the liquid phase in the molten pool. The solid region of the substrate and solidified clad, the liquid region of the melted clad material, and the gas region of the surrounding air are included. In this model, a level-set method is used to track the free surface motion of the molten pool with the powder material feeding and scanning of the laser beam. An enthalpy–porosity approach is applied to deal with the solidification and melting that occurs in the cladding process. Moreover, the laser heat input and heat losses from the forced convection and heat radiation that occurs on the top surface of the deposited layer are incorporated into the source term of the governing equations. The effects of the laser power, scanning speed, and powder-feed rate on the dilution and height of the multilayered clad are investigated based on the numerical model and experimental measurements. The results show that an increase of the laser power and powder feed rate, or a reduction of the scanning speed, can increase the clad height and directly influence the remelted depth of each layer of deposition. The numerical results have a qualitative agreement with the experimental measurements.     

Studies on Direct Laser Cladding of SiC Dispersed AISI 316L Stainless Steel

In the present study, attempts have been made to develop SiC dispersed (5 and 20 wt pct) AISI 316L stainless steel matrix composite by direct laser cladding with a high power diode laser. Direct laser cladding has been carried out by melting the powder blends of AISI 316L stainless steel and SiC (5 and 20 wt pct) and, subsequently, depositing it on mild steel (0.15 pct C steel) in a layer by layer fashion to develop a coupon of 100 mm² × 10 mm dimension. A continuous, defect-free (microcracks and micro- or macroporosities), and homogeneous microstructure is formed, which consists of a dispersion of partially dissolved SiC (leading to formation of very low fraction of Cr₃C₂ and Fe₂Si) in grain-refined austenite. The microhardness of the clad layer increases from 155 VHN to 250 to 340 VHN (for 5 wt pct SiC dispersed) and 450 to 825 VHN (for 20 wt pct SiC dispersed) as compared to 155 VHN of commercially available AISI 316L stainless steel. The corrosion rate in 3.56 wt pct NaCl solution is significantly reduced in 5 wt pct SiC dispersed steel; however, 20 wt pct SiC dispersed steel showed a similar behavior as the commercially available AISI 316L stainless steel. The processing zone for the development of a defect-free microstructure with improved properties has been established.     

Hydrogen Uptake Enhancement and Accelerated Hydrogen Re-embrittlement of Cd-plated AISI 4340 Steel Bolts Coupled with IN718 Nuts

Hydrogen re-embrittlement on anodically coated high strength steels could be of great concern because the uptake of hydrogen from the corrosion process can cause component failure. A scratched Cd-coated AISI 4340 steel membrane has been coupled with different materials reproducing crevice conditions, and the hydrogen uptake has been measured using a modified Devanathan?CStachurski permeation apparatus. Experimental tests proved that, in presence of a crevice, metals nobler than cadmium strongly enhance local hydrogen reduction on exposed steel areas, thus possibly favoring brittle failure of high strength steel components during service. Therefore, the coupling of uncoated nuts made of noble passive alloys (like Inconel) to Cd-plated AISI 4340 steel bolts should be avoided.     

Microstructure and wear properties

The laser surface cladding technique was used to formin situ Fe-Cr-Mn-C alloys on AISI 1016 steel substrate. In this process mixed powders containing Cr, Mn, and C with a ratio of 10:1:1 were delivered using a screw feed, gravity flow carrier gas aided system into the melt pool generated by a 10 kw CO₂ laser. This technique produced ultrafine microstructure in the clad alloy. The microstructure of the laser surface clad region was investigated by optical, scanning, and transmission electron microscopy and X-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying elements which, in turn, produced a fine distribution of complex types of carbide precipitates in the ferrite matrix because of the high cooling rate. An alloy of this composition does not show any martensitic or retained austenite phase. In preliminary wear studies the laser clad Fe-Cr-Mn-C alloys exhibited far superior wear properties compared to Stellite 6 during block-on-cylinder tests. The improved wear resistance is attributed to the fine distribution of metastable M₆C carbides.     

Microstructure and wear properties of laser clad Fe−Cr−Mn−C alloys

The laser surface cladding technique was used to formin situ Fe-Cr-Mn-C alloys on AISI 1016 steel substrate. In this process mixed powders containing Cr, Mn, and C with a ratio of 10∶1∶1 were delivered using a screw feed, gravity flow carrier gas aided system into the melt pool generated by a 10 kw CO₂ laser. This technique produced ultrafine microstructure in the clad alloy. The microstructure of the laser surface clad region was investigated by optical, scanning, and transmission electron microscopy and X-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying elements which, in turn, produced a fine distribution of complex types of carbide precipitates in the ferrite matrix because of the high cooling rate. An alloy of this composition does not show any martensitic or retained austenite phase. In preliminary wear studies the laser clad Fe-Cr-Mn-C alloys exhibited far superior wear properties compared to Stellite 6 during block-on-cylinder tests. The improved wear resistance is attributed to the fine distribution of metastable M₆C carbides.     

Analysis of the laser-cladding process for stellite on steel

Laser-cladding experiments have been performed with STELLITE 6 powder on mild steel substrates, using a 1.5 kW linearly polarized continuous wave CO₂ laser as a heat source. The clad height, the mass efficiency, the dimensions of the melt pool, as well as the global absorptivity, were measured as functions of the powder feed rate and the scanning speed. A quantitative analytical model of the process is proposed, based on the overall mass and energy balance. It allows the calculation of the mass efficiency and of the global absorptivity, taking into account the incorporation of the powder into the melt pool as well as the energy absorbed by the powder jet and the substrate. It successfully explains the experimental results and demonstrates the role played by the melt pool inclination with respect to the substrate. A processing diagram is given to find rapidly the optimal laser treatment conditions and the desired clad height. It is discussed with respect to the other limiting conditions of the process, the geometrical maximum powder efficiency, the porosity, the dilution, and the maximum power of the laser installation.     

Mechanical Properties and Metallurgical Characterization of Dissimilar Welded Joints between AISI 316 and AISI 4340

In the present study, the influence of six different process parameters and three interactions on joint tensile strength, toughness, fusion zone microhardness variation are studied during dissimilar tungsten inert gas welding between austenitic stainless steel AISI 316 and alloy steel AISI 4340. Detailed experimental study using fractional factorial experimental design and subsequent statistical analysis show that higher tensile strength, toughness can be achieved using ER 309 filler material and suitably selecting the other process parameters and heating conditions. Addition of small proportion of hydrogen in shielding gas increases the heat transfer efficiency, melting and subsequent penetration. Preheating of AISI 4340 material reduces the chance of solidification cracking and post-heating helps to improve the joint mechanical property. Microstructural observations show that improper selection of process parameters may lead to micro-pores and degrade the joint quality. Successful joining of the dissimilar materials greatly depends on the selection of optimum process parameters, filler material and shielding gas.     

Effect of Hardfaced Interlayer Thickness and Low Hydrogen Ferritic Capping on Ballistic Performance of Shielded Metal Arc Welded Armour Steel Joints

Armour grade quenched and tempered steel closely confirming to AISI 4340 is well known for its superior ballistic performance and hence used in the fabrication of combat vehicles. The traditional fillers like austenitic stainless steel showed poor ballistic performance of these welded joints as compared to the base metal. Attempts have been made to deposit hardfaced interlayer between austenitic stainless steel weld metals. Though this method, marginal improvements in ballistic performance can be yielded, and cracks were observed in between base metal and hardfaced layer. Thickness of the hardfaced interlayer plays a vital role for the effective ballistic performance. Thus, an attempt has been made to investigate the effect of hardfaced interlayer thickness on ballistic performance of armour steel welds. The results of effect of buttering, low hydrogen ferritic (LHF) filler and three different hardfaced layer thicknesses (4, 5.5 and 7 mm) on ballistic performance of shielded metal arc welded armour steel joints were given.     

Analytical electron microscopy of grain boundaries in high-strength steels

Phosphorus could be detected at prior austenite grain boundaries (PAGB) in high-strength alloy steels quenched and tempered at 500°C when using a VG's HB 501 dedicated field emission STEM but not with a conventional JEOL 4000FX STEM. No phosphorus was detected at PAGB's in the as-quenched materials or away from PAGB's in tempered materials of either type. The grain boundary coverage of phosphorus was, assuming a specimen thickness of 80 nm, 0.7 monolayers for the 3.5NiCrMoV rotor steel and 0.4 monolayers for the AISI 4340 steel. The grain boundary concentration of phosphorus, assuming a specimen thickness of 80 nm and a segregated layer thickness of 1 nm, for the 3.5NiCrMoV rotor steel was 6 wt% and for AISI 4340 4 wt%. Compared to the bulk concentration of about 0.01 wt% this means that the enrichment factor of P to the grain boundaries was several hundred times (610 respectively 370). Our measurements showed no correlation between the stress corrosion crack growth rate and the grain boundary phosphorus concentration. The yield strength, however, decreased after tempering while the phosphorus concentration at the grain boundaries increased.     

Gas metal arc welding of duplex stainless clad steel plates

The 2205 duplex stainless + DH36 clad steel plate was welded by gas metal arc welding (GMAW),and the welding performance of the clad steel plate was investigated.The results show that the adaptability of the welding procedure for the base metal of carbon steel,the transition layer,and the cladding material is excellent.The test results indicate that the phase proportion and component dilution of the GMAW-welded joints of clad steel plate can be effectively controlled to yield joints with good mechanical properties and corrosion resistance.     

Workability of commercial-purity titanium and 4340 steel during equal channel angular extrusion at cold-working temperatures

The deformation and failure of commercial-purity (CP) titanium (grade 2) and AISI 4340 steel (tempered to R _c 35) during equal channel angular extrusion were determined at temperatures between 25 °C and 325 °C and effective strain rates between 0.002 and 2.0 s⁻¹. The CP titanium alloy underwent segmented failure under all conditions except at low strain rates and high temperatures. By contrast, the 4340 steel deformed uniformly except at the highest temperature and strain rate, at which it also exhibited segmented failure. Using flow curves and fracture data from uniaxial compression and tension tests, workability analysis was conducted to establish that the failures were a result of flow localization prior to the onset of fracture. This conclusion was confirmed by metallographic examination of the failed extrusion specimens.     

真空轧制825合金/X65钢复合板的组织性能

 针对油气输送领域对耐H2S腐蚀油气复合管线的需求,采用真空轧制复合技术成功制备出825镍基合金/X65高强管线钢复合板。真空轧制复合技术是基于真空电子束焊接和热轧复合所开发出的一种新型复合技术,在高真空、高温和强塑性变形条件下,复合界面实现优异的冶金结合。采用X65/825合金/825合金/X65的4层对称复合轧制模式,并对复合界面的微观组织和力学性能特征进行分析。研究表明,复合界面连续平直,无孔洞和裂纹等缺陷,镍、铬和铁元素在界面两侧发生明显的扩散,另外复合界面生成一条连续的厚度约为1 μm的TiC薄带,在结合界面离散分布少量的颗粒状Al2O3化合物。界面平均剪切强度为404 MPa,拉剪断裂在复合界面处。     

Crack Arrest Toughness of Two High Strength Steels (AISI 4140 and AISI 4340)

The crack initiation toughness (K _c ) and crack arrest toughness (K _a ) of AISI 4140 and AISI 4340 steel were measured over a range of yield strengths from 965 to 1240 MPa, and a range of test temperatures from -53 to +74°C. Emphasis was placed onK _a testing since these values are thought to represent the minimum toughness of the steel as a function of loading rate. At the same yield strengths and test temperatures,K _a for the AISI 4340 was about twice as high as it was for the AISI 4140. In addition, theK _a values showed a more pronounced transition temperature than theK _c values, when the data were plotted as a function of test temperature. The transition appeared to be associated with a change in fracture mechanism from cleavage to dimpled rupture as the test temperature was increased. The occurrence of a “pop-in” behavior at supertransition temperatures has not been found in lower strength steels, and its evaluation in these high strength steels was possible only because they are not especially tough at their supertransition temperatures. There is an upper toughness limit at which pop-in will not occur, and this was found for the AISI 4340 steel when it was tempered to its lowest yield strength (965 MPa). All the crack arrest data were identified as plane strain values, while only about one-half of the initiation values could be classified this way.     

真空制坯热轧钛/钢复合板工艺及性能

钛/钢复合板的需求量日益增多，真空制坯热轧复合法（VRC）是制备高性能钛/钢复合板的有效工艺。介绍了钛/钢复合板制备工艺的国内外现状和工艺特点。依托863重点项目“钛/钢复合板研究与生产技术开发”和十三五重大课题“容器板轧制复合原理与关键技术”，利用真空制坯热轧复合法（VRC）在实验室和钢厂进行了一系列钛/钢复合板的轧制试验，对复合板的界面组织与力学性能进行了分析。实验室制备的钛/钢复合板，界面生成了明显的TiC层，未发现氧化物等杂质，断口有大量韧窝生成，复合界面平均拉剪强度达到了230MPa。钢厂试生产的钛/钢复合板，宽幅达到3500mm，界面生成连续的β- Ti层，拉剪断口未检测到氧化物，拉伸、冲击、弯曲等力学性能均满足国家标准，剪切强度均在196MPa以上，已达国内领先水平。     

In625镍基合金粉末激光熔覆参数研究

为了确定在ZG06Cr13Ni4Mo不锈钢板上激光熔覆In625镍基合金粉末的最佳生产工艺参数,采用响应曲面分析法设计并开展一系列的激光熔覆参数实验,并利用Desigh-Expert软件中Response Surface模块对最终数据进行方差定量分析。通过金相显微镜对多道搭接熔覆试样的表面形态和横截面组织进行了观察和定性分析,从而确定熔覆工艺的最佳参数组合。结果表明,在维持送粉量不变的条件下,熔覆层高度对激光功率与扫描速度的响应都比较明显;熔覆工艺的最佳参数组合为激光功率2000 W,送粉量84 g·min?1,扫描速度5 mm·s?1,在此参数下获得的熔覆试样具有高质量的熔覆层,无气孔和裂纹,且表面光滑。     

Microscopic observations of adiabatic shear bands in three different steels

Microscopic observations are made of the shear band material in three different steels: (1) an AISI 1018 cold-rolled steel (CRS), (2) a structural steel (HY-100), and (3) an AISI 4340 vacuum arc remelted (VAR) steel tempered to either of two hardnesses, RHC 44 or 55. To produce the shear bands, specimens were subjected to large shear strains at relatively high strain rates, ≈10³/s, resulting in essentially adiabatic deformation conditions. It was found that whenever the shear band led to fracture of the specimen, the fracture occurred by a process of void nucleation and coalescence; no cleavage was observed on any fracture surface, including the most brittle of the steels tested (RHC = 55). This is presumably due to the softening of the shear band material that results from the local temperature rise occurring during dynamic deformation. Differences in shear band behavior between the various microstructures are also described. Formerly Research Assistant, Brown University     

laser cladding of Ni-Al bronze on Al alloy AA333

The present study investigates the effects of laser-processing parameters, such as laser power, traverse speed, powder-feed rate, and flow rate and species of assisting gas, and material prop-erties, such as substrate surface condition, on laser cladding of Ni-Al bronze on Al alloy AA333. The proper processing parameters were determined experimentally and are discussed in terms of their effects on laser-clad quality and microstructure as observed using optical and scanning electron microscopes. Despite a large difference in melting points between the cladding material, Ni-AI bronze (MP = 1063 °C), and the substrate, Al-alloy AA333 (MP = 577 °C), clads of thickness from 1.2 to 2.5 mm that are crack-free and had good fusion were achieved. The substrate surface condition and the flow rate and species of assisting gas were found to be important for clad formation. A sandpaper-polished substrate absorbs less energy at the molten pool front and facilities reducing dilution. A large flow rate of assisting gas, such as helium, also has an effect on reducing dilution. A laser-generated molten-pool model was developed to explain the preceding experimental results. Y. LIU formerly Research Associate, Center for Laser Aided Materials Processing, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign.