Microstructure and mechanical properties of high power diode laser (HPDL) treated cast aluminium alloys

The appliance and development of modern technologies in the areas of surface engineering can be extended by laser surface treatment, especially using high power diode laser (HPDL) for remelting, feeding and/or alloying. The purpose of this work was to determine technological and technical conditions for tungsten carbide (WC) ceramic powder feeding into the surface layer of the laser treated Al–Si–Cu cast aluminium alloys with high power diode laser, as well as to investigate the microstructure and ceramic powder particle distribution in the surface layer. Special attention was devoted to monitoring of the layer morphology of the investigated material and on the particle occurred. Light and scanning electron microscopy as well as X‐Ray diffraction were used to characterize the microstructure of the remelted zone. A wide range of laser powers was choose and implicated by different process speed rates. Also one powder in form of tungsten carbide was used for feeding with the middle particle size of 80 µm. As the main findings there was found that, the obtained surface layer is without cracks and defects as well as has a comparably higher hardness value compared to the non remelted material. The hardness value increases according to the laser power used so that the highest power applied gives the highest hardness value in the remelted layer. Also the distribution of the tungsten carbide particles is good, but there are still possibilities for further modelling. The major purpose of this work is to study the effect of a high power diode laser melting on the cast Al–Si–Cu alloys structure to provide application possibilities for automotive and aviation industry.     

Electrochemical performance and microstructural evolution of laser deposited Al‐Sn coating in acidic environment

Outstanding performance of materials is one of the requirements of modifying the existing materials in order to meet a global demand necessary in technology innovation. Direct laser metal deposition technique due to excellent properties has replaced conventional techniques in modifying the surface of materials. The interplay between melting, fusion and optimised laser parameters are very important factors to be considered when using laser metal deposition technique. ASTM A29 steel property was enhanced through this technique by Al‐Sn reinforcements. A 3‐kW continuous wave ytterbium laser system was employed by this process. This research investigated the microstructure and corrosion properties of fabricated laser metal deposited ASTM A29 steel. The resulting microstructure and content of the inconsequential elements in the coatings fabricated were studied to obtain the results achieved. Observation of the microstructure showed typical phase of acicular α′‐martensite attributed to rapid cooling of the molten pool. The electrochemical behaviour was investigated in 1 M HCl solution at 27 °C via potentiodynamic polarization technique. The fabricated coatings had corrosion rate of 0.03435 mm/yr, current density of 3.95 ? 10^?6 A/cm², and polarization resistance of 7093.4 Ω ? cm². While the control had corrosion rate of 16.308 mm/yr and polarization resistance of 8.0631 Ω ? cm².     

Effects of scanning speed on creep behaviour of 316L stainless steel produced using selective laser melting

Increasing numbers of critical components for the aerospace and power industries are fabricated using additive manufacturing (AM). To increase the productivity of high‐temperature components by AM, the scanning speed needs to be maximized without sacrificing the required creep properties. In this study, five rectangular blocks were manufactured using selective laser melting while varying the scanning speed from 420 to 980 mm/s. Small punch creep tests were conducted at 650°C using 10 × 10 × 0.5 mm specimens machined from each block. Creep deformation and rupture life were measured. Power law creep constants were also determined. Difference of creep behaviours were explained based on the microstructures showing pore defects and the measured metal density values. A 3D response surface plot was employed to predict the creep life as a function of the scanning speed and the energy density. As a consequence, a scanning speed range of 416 to 572 mm/s is recommended to maximize productivity and to increase creep resistance.     

Densification and microstructural evaluation during laser sintering of M2 high speed steel powder

Abstract

In the present work, the densification and microstructure of M2 high speed steel powder processed by direct laser sintering method was studied. Test specimens were produced using a 200 W continuous wave CO₂ laser beam at different scan rates ranging from 50 to 175 mm s^?1. The building process was performed under argon and nitrogen atmospheres in order to evaluate the role of sintering atmosphere. It was found that the sintered density strongly depends on the laser scan rate and thus on the duration time of the laser beam on the surface of the powder particles. Generally, with a decrease in the scan rate higher densification was obtained. However, formation of large cracks and delamination of the sintered layers is feasible at low scan rates. The results also demonstrated that sintering under argon atmosphere yields better densification compared to a nitrogen atmosphere, in particular at higher scan rates. The microstructure of laser sintered parts consisted of large and elongated pores parallel to the building direction. The metal matrix structure was found to be heterogeneous, i.e. carbon rich austenite was formed due to carbon segregation. This structure consisted of fine cellulars or dendrites of martensite and retained austenite. This article describes the influence of manufacturing parameters on the densification of laser sintered M2 high speed steel powder. The microstructural features of the processed parts are also addressed.     

Laser surface hardening of H13 steel in the melt case

Laser heating caused a melting layer to form on the H13 steel, which usually has bad thermal conductivity and diffusivity. Therefore, the modified Ashby–Eastering heat-transfer equation was used to provide the temperature field for laser surface hardening in the melt. When the laser hardened H13 steel through surface melting, the basic microstructure of the dendrites was surrounded by an extremely fine lamellar structure in the melt layer. It is clear that the contours of the melting point isotherms and the critical phase transition temperature of H13 in the quenched and as-received conditions were comparable in the temperature distribution field under different laser energy densities. When the laser moves on, the phase transition temperature of H13 is raised and it becomes higher than the A₁ temperature because the heating rate during laser processing is usually >10⁴ °C/s. The larger the grain size or the more heterogeneous the structure, the higher the temperature and the longer the duration required for transforming the steel into austenite.     

Coatings on graphite crucibles used in melting uranium

Standard coatings for graphite crucibles used for melting uranium have generally been zirconia based and have been applied as a paint or by flame spraying. These coatings do not provide adequate protection at the temperatures normally required for melting uranium alloys. Yttria provides superior protection above 1300°C but becomes less satisfactory above 1450°C when applied directly on graphite. The utilization of a protective niobium/zirconia bilayer coating between the yttria and the graphite results in improved performance at 1500°C. Yttria has been satisfactorily applied both by plasma spraying and by brush applying a stable suspension. When the protective niobium layer is used, coating adherence after melting is excellent and multiple use of coatings is practical. The coatings adhere to graphite with a high coefficient of thermal expansion (CTE) (≈7 μm m^-1 °C^-1) much better than to standard crucible grade graphite (≈4μm m^-1 °C^-1). A single Nb/Y₂O₃-coated high CTE graphite crucible has been used for seven melts at 1450°C without repair or increased carbon contamination.The yttria paint coating is cost effective when compared with flame- or plasma-sprayed zirconia.     

Stability of Surface‐Enhanced Ultrahydrophilic Metals as a Basis for Bioactive rhBMP‐2 Surfaces

For several years the treatment of metals like cp titanium and 316L stainless steel with concentrated chromosulfuric acid at high temperatures (230‐240 °C) has formed the basis for preparing ultra‐hydrophilic priming coats on these metals (Jennissen et al. Materialwiss. Werkstofftech. 30, 838‐845, 1999). Metals treated in this way have been called surface‐enhanced, displaying a characteristic ultrastructure, and can be easily modified to carry a biocoat of recombinant human bone morphogenetic protein 2 (rhBMP‐2). The major oxide on surface enhanced titanium is TiO₂. Thus this TiO₂‐layer could be responsible for the ultra‐hydrophilic properties of the priming coat. Irradiation of TiO₂ layers by ultraviolet light (Wang et al., Nature 388, 431‐432, 1997) has been shown to endow these layers with ultra‐hydrophilic properties (i.e. contact angles of ～ 0°). However the ultra‐hydrophilic TiO₂‐layers produced by irradiation are unstable and revert to the original high contact angles of ～ 70° within several days. The question of whether the ultra‐hydrophilic surfaces prepared by the chromosulfuric acid method show long‐term stability was therefore important to answer. In addition the question if rhBMP‐2 immobilized on such a surface will retain its biological activity was of great interest. In this paper it will be shown that ultrahydrophilic titanium mini‐plates retain their ultra‐hydrophilicity with contact angles of 0‐8° unchanged for at least 50 days and support the immobilization of rhBMP‐2 in a biologically active form.     

激光选区熔化成形工艺对304L不锈钢冲击韧性的影响

目的 了解激光选区熔化(SLM)成形工艺参数对304L不锈钢冲击韧性的影响,从而得到304L不锈钢的最佳成形工艺参数。方法 对激光功率300~340 W,激光扫描速度800~1 500 mm.s^?1条件下的激光选区熔化成形304L不锈钢开展冲击试验,通过表面硬度、微观组织及断口形貌观察对冲击韧性的影响规律进行分析。结果 SLM成形304L不锈钢微观组织为跨越熔池生长形成的不规则柱状晶粒,成形工艺参数对试样表面硬度影响不显著;随着激光功率的增大和激光扫描速度的降低,304L不锈钢断面致密程度提高,孔洞类缺陷尺寸减少且数量减少,冲击韧性增大,冲击功最大值为141.9 J。结论 基于冲击试验结果,在激光体能量密度为100~140 J/mm³的条件下,304L冲击韧性稳定在138 J左右,为SLM成形304L材料的最佳成形参数区间。     

超高性能混凝土与普通混凝土的黏结抗冻性能

对147个超高性能混凝土与普通混凝土的100mm×100mm×100mm立方体黏结试件进行了冻融循环后的黏结性能研究,测量了冻融后试件的相对动弹性模量、质量损失率以及劈裂抗拉强度,研究了超高性能混凝土中的钢纤维掺量、普通混凝土的强度等级、黏结面形式、试件的浇筑方向等因素对黏结试件抗冻性能的影响。结果表明,冻融循环结束后,所有黏结试件中的超高性能混凝土部分都没有出现损伤,超高性能混凝土可以作为普通混凝土结构的理想外围护材料;随着冻融循环次数的增加,黏结试件的相对动弹性模量逐渐减小,质量损失率先降低后增加,黏结试件的劈裂抗拉强度线性下降;影响黏结试件冻融后劈裂抗拉强度下降速度的关键因素是超高性能混凝土中的钢纤维掺量和黏结面的形式。     

Influence of processing parameters of selective laser melting on high‐cycle and very‐high‐cycle fatigue behaviour of Ti‐6Al‐4V

Orthogonal experiment design together with the analysis of variance was used to examine the processing parameters (laser power, scan speed, layer thickness and hatch spacing) of selective laser melting (SLM) for superior properties of SLM parts, in which nine groups of specimens of Ti‐6Al‐4V were fabricated. The results clarify that the influence sequence of individual parameter on the porosity is laser power > hatch spacing > layer thickness > scan speed. Ultrasonic fatigue tests (20 kHz) were conducted for the SLMed specimens in high‐cycle fatigue (HCF) and very‐high‐cycle fatigue (VHCF) regimes. The S‐N data show that the fatigue strength is greatly affected by the porosity: the group with the smallest porosity percentage having the highest fatigue strength in HCF and VHCF regimes. Then, the tests on the validation group were performed to verify the optimal combination of SLM processing parameters. Moreover, the observations by scanning electron microscopy revealed that fatigue cracks initiate at lack‐of‐fusion defects in the cases of surface and internal crack initiation.     

Sublimation epitaxy of AlN layers on 4H-SiC depending on the type of crucible

Epitaxial layers of aluminum nitride ≤335 μm thick have been grown at temperatures of 1900 and 2100 °C on 10×10 mm² (0 0 0 1)-oriented α(4H) silicon carbide (SiC), with growth times of 1 and 4 h, via sublimation-recondensation in a RF-heated graphite furnace. The source material was polycrystalline AlN. The sublimation process was performed in three types of graphite (C) crucible: C1, C2 with inner diameters of 35 and 51 mm, respectively, and C3 with the same inner diameter as C1, but coated with a layer of TaC. The surface morphology reflects the hexagonal symmetry of the substrate, suggesting an epitaxial growth for samples grown in C1 and C3 crucibles for all growth conditions. The same symmetry is observed for AlN layers grown in the C2 crucible, but only at 2100 °C. X-ray diffraction analyses confirm the epitaxial growth of AlN samples with the expected hexagonal symmetry. A high-resolution X-ray diffractometer was used to assess the quality of the single crystals. A full width at half maximum of 242 arcsec was achieved for an AlN layer grown in the crucible coated with TaC.     

Einfluss unterschiedlicher Oberflächenzustände vor dem Plasmanitrieren auf Eigenschaften und Zerspanungsverhalten des Schnellarbeitsstahls S 6‐5‐2

Surface states and wear behavior of drills of ground, sandblasted and plasmanitrided samples and drills made of AISI M2 high speed steel In the present work the effect of different surface conditions on plasma nitriding response of AISI M2 high speed steel was investigated. The plasma nitriding of ground and sandblasted samples and drills was performed at temperatures of 400°C and 500°C for two gas mixtures: 5 vol.% N₂ and 76 vol.% N₂ in hydrogen. Surface layers were characterized before and after plasma nitriding concerning the microstructure, roughness, microhardness, chemical composition, phase composition and residual stress states. Machining tests were carried out with drills during which drilling forces and flank wear have been measured. A significant effect of the surface state prior to nitriding on residual stress states and the properties of the nitrided layer and untreated core has been observed. Thinner nitrided layers on ground and sandblasted samples were attributed to high compressive residual stress states and a stress affected diffusion of nitrogen and carbon. In the machining tests, sandblasted drills exhibited the best performance. Lower nitrogen concentrations in the gas atmosphere without the formation of a compound layer gave the lowest drill flank wear for sandblasted surfaces while higher nitrogen concentrations led to a reduction of drilling forces and torque.     

Experimental and numerical research on high‐temperature thermal insulation performance of lightweight porous ceramic/nanomaterial composite structure

The thermal protection structure of hypersonic vehicles must meet the design requirements of high efficiency and light weight, and its heating surface must also be able to withstand thermal erosion by high‐speed and high‐temperature airflow. In this paper, a light‐weight porous ceramic material and a lightweight nanoscale thermal insulation material with excellent thermal insulation performance are combined to form an integrated thermal protection structure. Experimental study and numerical simulation of the structure's high‐temperature thermal insulation performance are carried out. The experimental results show that a composite sheet made from a 20 mm‐thick lightweight porous ceramic material and a 10 mm‐thick nanomaterial exhibit a temperature drop of 85 % between its back surface and front surface in four thermal environments (1200, 1000, 800 and 600 °C) at 1800 s. This indicates excellent thermal insulation performance of the composite sheet. In addition, the operating temperature limit (<1000 °C) is obtained through high‐temperature thermal performance tests on single‐layer nanomaterial sheets and scanning electron microscopy results. This provides an important basis for determining and optimizing the thickness ratio of the two materials in composite structure.     

Sub‐Micron Anisotropic InP‐based III–V Semiconductor Material Deep Etching for On‐Chip Laser Photonics Devices

Two InP‐based III–V semiconductor etching recipes are presented for fabrication of on‐chip laser photonic devices. Using inductively coupled plasma system with a methane free gas chemistry of chlorine and nitrogen at a high substrate temperature of 250 °C, high aspect ratio, anisotropic InP‐based nano‐structures are etched. Scanning electron microscopy images show vertical sidewall profile of 90° ± 3°, with aspect ratio as high as 10. Atomic Force microscopy measures a smooth sidewall roughness root‐mean‐square of 2.60 nm over a 3 × 3 μm scan area. The smallest feature size etched in this work is a nano‐ring with inner diameter of 240 nm. The etching recipe and critical factors such as chamber pressure and the carrier plate effect are discussed. The second recipe is of low temperature (?10 °C) using Cl₂ and BCl₃ chemistry. This recipe is useful for etching large areas of III–V to reveal the underlying substrate. The availability of these two recipes has created a flexible III–V etching platform for fabrication of on‐chip laser photonic devices. As an application example, anisotropic InP‐based waveguides of 3 μm width are fabricated using the Cl₂ and N₂ etch recipe and waveguide loss of 4.5 dB mm^?1 is obtained.

     

The impact of aging and drag-finishing on the surface integrity and corrosion behavior of the selective laser melted maraging steel samples

The maraging steel components fabricated using the selective laser melting process exhibit remarkable static strength. However, high pore density and large surface imperfections impede their overall mechanical and chemical performance. Thus, the components are often post-treated with mechanical- and thermal-based treatments to overcome their inherent imperfections and enhance their final mechanical properties. Although the post-processing treatments are useful in enhancing the selective laser melted components’ mechanical performance, their effect on corrosion behavior is not comprehensively evaluated. In this study, the selective laser melting prepared maraging steel samples’ corrosion behavior was examined in the as-built condition and compared with the post-processed samples subjected to aging and drag finishing operations. Compared to the as-built condition, both aging and drag-finishing post-processing treatments increased the selective laser melting samples’ corrosion even though the surface integrity was improved.     

Analysis on the dynamic extension for transverse surface cracks in the as-cast steel slab at high temperatures

The fracture toughness test is carried out at high temperatures (800–1200 °C) to study the critical value of J-integral (J_IC) for the as-cast steel slab. Extended finite element method (XFEM) is used in an Abaqus model to study the dynamic extension of transverse surface cracks during the straightening and bending process of the as-cast steel slab. Results show that the ductile fracture toughness decreases from 26.75 to 3.46 N/mm as the temperature increases from 800 to 1200 °C. Transverse surface cracks (vertical to the casting direction) on the outer and inner arc surface of the slab extend 27.3 mm and 23.4 mm for the maximum when the slab is bending and straightening, respectively. The extended length is analysed corresponding to the position where the slab passes through bending and straightening rollers. The results show that the extended length increases largely at the end of the bending or straightening process, which indicates the stress distribution at the bending roller 15#, and the straightening roller 59#, 60# are overlarge and should be optimized.     

Electrical and mechanical properties of electrically conductive polyethersulfone composites

Electrically conductive polyethersulphone (pes) composites containing carbon fibres, nickel fibres, stainless steel fibres or aluminium flakes at various volume fractions up to 40% were fabricated and tested. For electromagnetic interference (emi) shielding effectiveness > 50 dB, the minimum filler volume fraction was 40% for carbon fibres of length 200 or 400 μm, 20% for nickel or stainless steel fibres, and 30% for aluminium flakes. The tensile strength first increased and then decreased with increasing filler content, such that the highest tensile strength occurred at 30 volume% (vol%) for carbon fibres (of length 200 or 400 μm) as the filler and at 10 vol% for nickel or stainless steel fibres. However, for carbon fibres (of length 100 μm) and aluminium flakes, the tensile strength increases up to at least 40 vol%. The best overall performance was provided by aluminium flakes at 40 vol%; the resistivity was 7 × 10⁻⁵ Ω cm, the emi shielding effectiveness was > 50 dB and tensile strength was 67 MPa. The resistivity of the aluminium flake composites was not affected by heating in air at 140°C for up to at least 144 h.     

Diamond fibre microheaters

Abstract

Free standing diamond fibres with 125 μm diameter tungsten wire or carburised steel cores coated by chemical vapour deposition with ～ 35 μm thick diamond were heated by passing an electric current through the cores. Tungsten wire cored fibres were heated to 1000°C in a vacuum without any visible change in the fibres. In air the diamond oxidised above ～550°C. The diamond fibres with the carburised steel core reached a maximum temperature of ～ 200°C before melting at a local hot spot. This core was also ferromagnetic and thus the fibre had a unique combination of properties. Oxidation produced a large increase in the diamond surface area and this effect might be used to enhance heat transfer in diamond fibre sensors and in diamond fibre-metal matrix composite thermal conductors.     

Mechanical properties and corrosion behavior of selective laser melted 316L stainless steel after different heat treatment processes

Irregular grains, high interfacial stresses and anisotropic properties widely exist in 3D-printed metallic materials, and this paper investigated the effects of heat treatment on the microstructural, mechanical and corrosion properties of 316 L stainless steel fabricated by selective laser melting. Sub-grains and low-angle boundaries exist in the as-received selective laser melted (SLMed) 316 L stainless steel. After heat treatment at 1050 °C, the sub-grains and low-angle boundaries changed slightly, and the stress state and strength decreased to some extent due to the decrease of dislocation density. After heat treatment at 1200 °C, the grains became uniform, and the dislocation cells vanished, which led to a sharp decline in the hardness and strength. However, the ductility was improved after recrystallization heat treatment. The passive film thickness and corrosion potential of the SLMed 316 L stainless steel decreased after heat treatment, and the pitting potential also decreased due to the accelerated transition from metastable to steady-state pitting; this accelerated transition was caused by the presence of weak passive films at the enlarged pores after heat treatment, especially for an adequate solid solution treatment.     

High-temperature titanium nitridation kinetics

Titanium samples 60.0 mm in length and 3.0 × 0.3 mm in cross section were heated in a nitrogen gas atmosphere for 60 min at temperatures from 1300 to 2100°C. At temperatures below 2000°C, the titanium nitridation process comprised two stages. The lower temperature limit of exponential nitridation kinetics was determined to be ~1000°C. At temperatures above the melting point of the metal, the presence of liquid phase in the bulk of the material has no significant effect on the titanium nitridation process.