Joining of cemented carbides to steel by laser beam welding

Welding of dissimilar materials such as steel and cemented carbides (hardmetals, cermets) is particularly challenging e.g. because mismatches in their thermal expansion coefficients and thermal conductivities result in residual stress formation and because of the formation of brittle intermetallic phases. Laser beam welding of cemented carbides to steel appears as an attractive complementary technique to conventional brazing processes due to its high precision, high process speed, low heat input and the option of welding without filler. Here a laser welding process including pre‐heat treatment and post‐heat treatment was applied successfully to joining as‐sintered and nitrided hardmetals and cermets to low alloyed steel. The microstructure and mechanical properties of the welds are investigated by microscopy, X‐ray diffraction, microhardness measurements, and bending tests. The results reveal that the three‐step laser beam welding process produced crack‐free and non‐porous joints. Nitridation of the cemented carbides results in a significant reduction of the amount of brittle intermetallic phases. The mechanical properties of the joints are competitive to those of the conventional brazed steel‐cemented carbide joints.     

Laser surface alloying of 1045 AISI steel using Ni–CrB2 powder

Abstract

Laser surface alloying is a process whose purpose is to improve the surface properties by incorporating alloying elements into the surface. The advantages of using laser for surface treatment are: formation of a non-equilibrium or amorphous phase as well as homogenisation and refinement of the microstructure, all without affecting the substrate properties. Powder (50 wt-%Ni–50 wt-%CrB₂) was injected into a melt pool created by a CW–CO₂ laser on AISI1045 steel plates. In order to alloy the entire surface, the sample was scanned at scan speeds in the range of 600–6000 mm min^–1 and the laser power was in the range of 1750–2500 W. The powder feed rate was 1·6 g min^–1, the laser beam was 2 mm in diameter, with 60% overlap between successive laser paths. Metallographic cross-sections were made of the samples. For each sample the following properties were characterised: layer depth, microhardness (HV), layer microstructure and composition. It has been found that the scan speed and the laser power affect the depth of the melt pool, the microstructure, the hardness and the treated layer composition. The laser boronised surface exhibits better wear resistance than D2 tool steel hardened to 59 ± 1 HRC. This will be discussed based on numerical analysis of the laser/material interaction.     

Mikrostrukturelle und mechanische Eigenschaften von laserstrahlgeschweißtem Magnesiumfeinblech AZ31‐HP mit und ohne Zusatzwerkstoffe

Microstructural and mechanical properties of laser welded sheets of magnesium AZ31‐HP with and without filler wires This paper describes Nd:YAG laser beam welding experiments carried out on rolled 2.5 mm thick magnesium sheet AZ31‐HP. For the butt welds in flat position, filler wires AZ31X and AZ61A‐F were used, diameter 1.2 mm. The microstructure and mechanical properties of the different laser beam welded joints were examined and compared with one another. The obtained results show that the laser beam welding of AZ31‐HP sheet is possible without hot crack formation, both without and with filler wires. The determined tensile strength, ductility, fracture toughness and microhardness of laser beam welded joints without filler wire were not effected by AZ31X nor AZ61A‐F. By use of these filler wires loss of zinc was minimized and the shape of weldments was optimized. The values of fracture strength, yield strength and microhardness of the joints and base material are quite similar. It is found that the ductility of the joints is lower than the base materials due to the heterogeneous microstructure of the fusion zones and geometrical notches of the weld seams. Both, weld and base material of AZ31‐HP, showed stable crack propagation. Furthermore, for base material slightly lower fracture toughness values CTOD than for the joints were determined.     

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.     

2Cr13钢激光表面合金化的组织和性能分析

2Cr13钢通过激光表面合金化达到表面改性的目的。利用扫描电镜观察了在不同工艺参数下，激光表面合金化的强化效果及显微组织。用显微硬度计检测了激光合金化组织的显微硬度。结果表明，2Gr13钢激光表面合金化能大大提高其表面的硬度和耐磨性，使它的应用更为广泛。     

Structural and optical properties of nitrogen‐iron co‐doped titanium dioxide films prepared via sol‐gel dip‐coating: Effect of urea and iron nitrate concentration in the sol

In this study, nitrogen‐iron co‐doped titanium dioxide films were prepared via sol‐gel dip‐coating method using urea and iron nitrate as nitrogen and iron source, respectively. Nonmetal doping of TiO₂ have some disadvantages such as massive charge carrier recombination and losing the photo‐catalytic capability. Three different nitrogen‐iron co‐doped titanium dioxide sols with different urea and iron nitrate concentration were prepared. The resulting sols were homogeneous and transparent, and no precipitation was observed in any of them. It was observed that the film prepared with middle urea‐iron nitrate concentration sol got opaque in a short time after the dip‐coating process. All prepared films were characterized by X‐ray diffraction, X‐ray photoelectron spectroscopy, scanning electron microscopy, confocal microscopy and ultraviolet–visible (UV‐Vis) spectroscopy. It was found that the concentration of the urea and iron nitrate in the sol had an effect on the crystal structure, microstructure, surface morphology and optical properties of the resulting films. Samples with middle concentrations had amorphous structure and bigger particle size. It was seen that sample with higher iron amount has lower band‐gap. It is concluded that we can prepare transparent anatase, transparent amorphous and opaque amorphous titanium dioxide films by changing the urea and iron nitrate concentration in the sol.     

Maßgeschneiderte Tribologie durch Laseroberflächenbehandlung

Tailored tribology by laser surface treatment There are quite different requirements for tribological properties of surfaces in industry. Both reduction and maximization of friction and/or wear are possible requirements. The friction and wear properties depend on the tribological system consisting of the friction partners, the medium between them and the environment around them. So for each application tailored surfaces with special tribological properties are needed. In the paper examples are presented which deal with the investigation and the implementation of laser based processes to obtain surfaces with very different tribological properties. Wavelengths, output power values, intensity distributions and beam qualities of the available lasers vary in a wide range. Also the available devices for beam formation and beam guidance enable special processes for tailoring properties for particular applications. These processes are for example the single‐layered or multi‐layered laser cladding generating homogeneous or graded claddings, the laser alloying or laser dispersing and the laser stimulated deposition of diamond layers at atmospheric pressure.     

45钢激光合金化后的组织结构及性能

对45钢激光合金化处理可改善其组织结构及性能。研究了45钢不同工艺条件(功率、扫描速度)下激光处理后的组织结构、成分、显微硬度以及耐腐蚀性能。结果表明:45钢合金化后从表层到基体的区域分为合金化区、过渡区和热影响区,其中合金化区由树枝晶和胞状晶组成;合金化区的显微硬度相对于基体有明显提高,且随扫描速度以及功率的增大,合金化区的硬度增大;45钢合金化后耐腐蚀性能得到了较大的提升。     

45 钢表面激光合金化组织分析及硬度测试

目的为了提高45钢表面性能,采用CO2激光器对其表面进行合金化处理。方法利用带有能谱的扫描电子显微镜(SEM/EDS)、金相显微镜、X射线衍射仪、显微/维氏硬度计、扫描电镜等,对合金化层组织及性能进行了观察和分析。结果激光合金化层由合金化区、结合区和热影响区3部分组成,涂层与基体呈冶金结合;涂层主要含Cr3C2,Fe Ni3,Cr23C6,Fe3C相;激光合金化层的显微硬度达1032 HV,约为基体的3.5倍。结论 45钢经激光合金化处理,可改善其表面性能,显著提高其硬度。     

ELECTRON AND LASER BEAM ALLOYING OF STEELS

Electron and laser beams are used to produce surfaces having properties similar to stainless steels by melting and alloying electroplated chromium films on two commercial Fe-based alloy substrates. In this paper, procedures and experimental parameters to form reasonably uniform surface alloys are discussed and microstructures as characterized by optical microscopy, electron probe microanalysis and microhardness testing are documented. Oxidation tests show that isothermal oxidation rate at 900 °C can be significantly reduced by laser surface alloying T91 steel with chromium.     

A Simple Route to Porous Graphene from Carbon Nanodots for Supercapacitor Applications

A facile method to convert biomolecule‐based carbon nanodots (CNDs) into high‐surface‐area 3D‐graphene networks with excellent electrochemical properties is presented. Initially, CNDs are synthesized by microwave‐assisted thermolysis of citric acid and urea according to previously published protocols. Next, the CNDs are annealed up to 400 °C in a tube furnace in an oxygen‐free environment. Finally, films of the thermolyzed CNDs are converted into open porous 3D turbostratic graphene (3D‐ts‐graphene) networks by irradiation with an infrared laser. Based upon characterizations using scanning electron microscopy, transmission electron microscopy, X‐ray photoelectron spectroscopy, X‐ray diffraction, Fourier‐transform infrared spectroscopy, and Raman spectroscopy, a feasible reaction mechanism for both the thermolysis of the CNDs and the subsequent laser conversion into 3D‐ts‐graphene is presented. The 3D‐ts‐graphene networks show excellent morphological properties, such as a hierarchical porous structure and a high surface area, as well as promising electrochemical properties. For example, nearly ideal capacitive behavior with a volumetric capacitance of 27.5 mF L ^{? 1} is achieved at a current density of 560 A L ^{? 1}, which corresponds to an energy density of 24.1 mWh L ^{? 1} at a power density of 711 W L ^{? 1}. Remarkable is the extremely fast charge–discharge cycling rate with a time constant of 3.44 ms.     

Influence of Ultrafine‐Grained Layer on Gaseous Nitriding of Large‐Sized Titanium Plate

In this paper, mechanical shot blasting on a large sized titanium plate is conducted to induce severe plastic deformation, which generates an ultrafine‐grained surface layer. The effect of an ultrafine‐grained layer on nitriding is evaluated at nitriding temperatures from 600 to 850 °C. The structural phases and mechanical property improvements are investigated and compared to those of a coarse‐grained specimen by using X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, and mechanical property measurements. The results indicate that an ultrafine‐grained layer enhances the nitriding kinetics and produces a thicker nitrided layer than that of a coarse‐grained plate at the same gaseous nitriding temperatures. The improved kinetics are attributed to a greater number of grain boundaries and defects introduced into the titanium plate surface by the mechanical shot blasting treatment. Meanwhile, the surface and cross‐sectional hardness values improve compared to the coarse‐grained plate due to the thicker nitrided layer resulting from deeper nitrogen diffusion.     

功率对纯钛薄片激光封孔成形及接头性能的影响

为实现直径为0.1 mm微孔的良好封接,采用低功率脉冲激光对0.40 mm厚的工业纯钛(CP-Ti Grade 1)进行了封焊,采用扫描电镜、显微硬度计及光学显微镜,研究了激光功率对焊缝表面成形、焊接接头横截面形貌和显微硬度的影响规律,分析了接头的微观组织结构.结果表明:当激光功率不低于19.2 W时,微孔可以实现完全封接,且熔核直径随着激光功率的增大而增大,但随着激光功率增大,焊缝凹陷和烧蚀现象越加严重;当激光功率增大时,焊缝表面显微硬度呈上升趋势,激光功率为19.2 W时,焊缝成形良好,且最大显微硬度值可以达到290 HV;焊缝中心微观组织为针状α、锯齿状α以及板条状α晶,焊缝上边缘为锯齿状α晶,而焊缝下边缘为细小的锯齿状α和针状α晶.     

Material characterization and corrosion behavior of hybrid coating Ti−Al−Si−Cu/Ti−6Al-4V composite

Titanium alloy grade 5 is a grade of the titanium material that is in-demand in the marine, aerospace, biomedical and turbo machinery industries. It offers great properties such as being light weight, good corrosion performance and great strength. However, some of the other properties, namely: its low hardness and poor tribological performance, has limited its various industrial application. Developments have focused on the enhancement of the surface properties without altering the bulk of the material. This has led to the laser metal deposition technique categorized under the additive manufacturing processes. It is a feasible technique that operates on layer-by-layer additive processing to manufactures whole parts or repair local damages in components. This study aims to ascertain the optimum processing conditions of the process by varying the laser intensity and scanning speed between 0.9 kW–1 kW and 1.0 m/min–1.2 m/min, respectively, while maintaining all the other process parameters. The specimens were produced by utilizing the ytterbium laser system to conduct laser surface alloying of Ti−Al−Si−Cu/Ti-6Al-4 V. Metallographic preparations, characterizations to conduct laser metal deposition (LMD), microhardness and corrosion test were conducted. It was deduced that Ti-9Si-3Cu alloy had the best optimum performance at 1 kW and 1.0 m/min. The hardness and corrosion were optimum improved at reduced scan speeds and increased laser power.     

Effect of beam interaction time on laser alloying with pulsed Nd–YAG laser

Abstract

Surface alloying of aluminium with nickel was carried out using a pulsed Nd–YAG laser. The effect of beam interaction time on laser alloying of aluminium with pulsed Nd–YAG laser has been studied. It was found that the beam interaction time of a pulsed laser has a significant effect on microstructure and properties of alloyed layers. The results indicated that with changes in the beam diameter, higher thickness of alloyed layer and higher microhardness are both obtained at a lower effective interaction time. When travel speed changes, the same conditions are obtained at a higher effective interaction time.     

Selective Laser Post-Treatment on Titanium Cold Spray Coatings

The aim of the present work is to investigate the feasibility and effects of a selective postdeposition laser treatment on titanium coatings. Commercially pure titanium grade 2 powders were deposited by means of a cold spray process on aluminum alloy AA2024-T3 sheets. The surface treatment of the coating was realized using a 220 W diode laser. The influence of heat input and dimensional features of coating layer and substrate was assessed by an experimental campaign conducted following a design of experiments approach. Optical and scanning electron microscopy analysis of the microstructure of the deposited and treated material as well as microhardness measurements showed the formation of a compact layer of titanium oxide on the coating surface and the preservation of the temper state of the aluminum substrate.     

High-temperature thermal barrier coating formation by laser alloying of CP-Ti with pre-placed Ni-SiC coating

High-temperature thermal barrier coating was created on CP-Ti using a pre-placed Ni-SiC layer by laser alloying technique. The coating was developed using 80% Ni + 20% SiC, 50% Ni + 50% SiC and 60% Ni + 40% SiC, and the latter two compositions are found to be efficient in producing a uniform layer. The 100% SiC pre-placement was also used. A flaw-less coating of 0.4–0.6 mm thickness was produced at a lower power density of 1.3 to 1.9 × 10⁵ W cm^–2. Very high power density of 2.5–3.0 × 10⁵ W cm^–2 is inefficient to produce uniform coating. The laser alloyed coating consists of dendrites and intermetallic precipitates. The degree of dendrite population depends upon the coating composition and laser processing conditions. The coating hardness was 600–1200 HV, which is three to six times higher than the base titanium. Uniform hardness was obtained for the coatings produced at a laser power density of 1.3 × 10⁵ W cm^–2. The titanium silicide (TiNiSi, Ti₅Si₃, TiSi) and nickelide (NiTi₂) phases formed on the laser-alloyed coating surface was confirmed by X-ray analysis. These intermetallic phases can improve high-temperature properties of titanium and its alloys. The effect of laser power density and coating composition on the alloying depth alloying width, hardness and microstructure are discussed. The present work investigated the microstructure evolution, hardness and compound phases by means of optical and scanning electron microscopy, Vickers hardness testing, EDXRD and SIMS analysis. A 5 kW CW CO₂ laser was used for laser alloying experiments.     

Employment of fiber laser technology to weld austenitic stainless steel 304 l with aluminum alloy 5083 using pre-placed activating flux

The overlapping welding was carried out in keyhole mode between austenitic stainless steel 304 l and aluminum alloy 5083 using a low power fiber laser in continuous irradiation. The significant content of magnesium as the alloying element with low boiling point and high vapor pressure inside the AA 5083 matrix can induce the spatter formation and depression on surface of the weld beads upon laser beam absorption and temperature growth which can deteriorate the mechanical properties and appearance of the joints. To reduce these defects, a variety of single and multi-components activating fluxes including oxide-based TiO₂ and halide-based CaF₂ flux powders were pre-placed on the surface of welding material prior to laser welding. The EDX and XRD analyses in addition to microhardness and shear tests were carried out to characterize the joints. The obtained results showed that, the oxide and halide activating fluxes can significantly improve the joints' strength up to 1.48 and 1.85 times in average respectively compared with autogenous joint. It was deduced that the simultaneous effect of significant decrease in joints' surface depression leading to welds' geometry improvement in addition to less formation of interfacial Fe–Al intermetallics, were the major causes for considerable strength improvements.     

LASER SURFACE TREATMENTS FOR PROMOTING CORROSION RESISTANCE OF A CARBON STEEL

The effect of laser treatment and laser alloying on corrosion resistance of 1045 steel has been studied. Various ways of surface alloying have been investigated by using continuous CO₂ laser beam: i) irradiation of chromium painted surfaces; ii) irradiation of Ni and Cr-electroplated surfaces; iii) direct injection of Ni and Cr powders into the melt pool. A high and uniform level of alloying in the surface layer can be achieved in the cases of electroplated surfaces and direct powder injection. It requires, however, an appropriate choice of irradiation conditions (such as beam power, beam traverse speed, beam defocusing and degree of overlapping) that provide remelted layer of a limited and fairly uniform thickness. It has been found that in order to achieve corrosion behavior of laser treated surfaces similar to that of austenite type 304 stainless steel chromium and nickel contents in the alloyed layer are to be higher than those of 304 steel.     

A comparative study of laser beam welding and laser-MIG hybrid welding of Ti-Al-Zr-Fe titanium alloy

Ti-Al-Zr-Fe titanium alloy sheets with thickness of 4 mm were welded using laser beam welding (LBW) and laser-MIG hybrid welding (LAMIG) methods. To investigate the influence of the methods difference on the joint properties, optical microscope observation, microhardness measurement and mechanical tests were conducted. Experimental results show that the sheets can be welded at a high speed of 1.8 m/min and power of 8 kW, with no defects such as, surface oxidation, porosity, cracks and lack of penetration in the welding seam. In addition, all tensile test specimens fractured at the parent metal. Compared with the LBW, the LAMIG welding method can produce joints with higher ductility, due to the improvement of seam formation and lower microhardness by employing a low strength TA-10 welding wire. It can be concluded that LAMIG is much more feasible for welding the Ti-Al-Zr-Fe titanium alloy sheets.