Problems in laser repair cladding a surface AISI D2 heat-treated tool steel

The aim of the present work is to establish the relationship between laser cladding (LC) process parameters (power, process speed, and powder feed rate) and AISI D2 tool steel metallurgical transformations, with the objective of optimizing the processing conditions during real reparation. It has been deposited H13 tool steel powder on some steel substrates with different initial metallurgical status (annealed or tempered) using a coaxial LC system. The microstructure of the laser clad layer and substrate heat-affected zone (HAZ) was characterized by optical microscopy, scanning electron microscopy, and electron backscattered diffraction. Results show that the process parameters (power, process speed, feed rate, etc.) determine the dimensions of the clad layer and are related to the microstructure formation. Although it is simple to obtain geometrically acceptable clads (with the right shape and dimensions) in many cases some harmful effects occur, such as carbide dilution and non-equilibrium phase formation, which modify the mechanical properties of the coating. Specifically, the presence of retained austenite in the substrate–coating interface is directly related to the cooling rate and implies a hardness diminution that must be avoided. It has been verified that initial metallurgical state of the substrate has a big influence in the final result of the deposition. Tempered substrates imply higher laser absorption and heat accumulation than the ones in annealed condition. This produces a bigger HAZ. For this reason, it is necessary to optimize the process conditions for each repair in order to improve the working behaviour of the component.     

Selective laser sintering of single-phase powder Cr-V tool steel

Presented is positive experience from selective laser sintering (SLS) of cylindrical steel specimens (3.0% C, 3.0% Cr, 1.0% Si, 12.0% V, Fe balance) 30 mm long and 5 mm in diameter by rapid prototyping. It was demonstrated that monolithic steel material could be successfully fabricated by this technology. Differential thermal analysis (DTA), scanning electron microscopy (SEM), and x-ray diffractometry (XRD) were used to study the microstructure, phase, and chemical composition of the source material and obtained specimens. Low-melting cementite-based eutectic was found to provide the liquid phase sintering of powder tool steel. The porosity of the green sintered specimens did not exceed 5%. The mean hardness value of sintered specimens was 825 HV.     

新版碳素工具钢和高速工具钢国家标准的变化

介绍了2008年修订发布的GB/T 1298—2008《碳素工具钢》及GB/T 9943—2008《高速工具钢》2项新标准,并分别与相应旧版本做了对比分析,指出了新、旧标准之间的差异。该2项新标准与相对应的前一版本相比,在标准名称、适用范围以及技术内容方面都有不同程度的变化。两项新标准的优点是技术内容上更加完善,适用范围更广,并与现行的相关标准保持了较好的协调关系,更便于贯彻执行。     

Surface modification of hot work tool steel by high-power diode laser

Laser surface modification, although for a number of years, is a technology, which is still in its infancy. This paper presents the results of laser alloying influence on structure and properties of the surface of the X38CrMoV5-3 hot work steel, carried out using the high-power diode laser (HPDL). Niobium carbide powder was used for alloying. In the paper, investigation results are presented of the melting and alloying of ceramic particles on the surface layer. Results of the metallographic examinations qualitative X-ray phase analysis and the mechanical properties are presented.     

Study on the stability of plasma-polymerized fluorocarbon ultra-thin coatings on stainless steel in water

Metallic stents are thin wire mesh tubes implanted in partially blocked arteries to prevent the obstruction of blood flow and to act as internal scaffolding. Mainly made of 316L stainless steel, they are prone to localized corrosion attack in long-term applications which in turn causes the release of potentially toxic ions such as chromium and nickel. To improve the long-term performance and safety of stainless steel stents, a multi-step process was previously developed in our laboratory to isolate the stainless steel surface from the body fluid by coating the metal with an ultra-thin cohesive and adherent plasma-polymerized fluoropolymer coating (Teflon-like ultra-thin coating). In this context, the aim of the present study was to evaluate the stability of the Teflon-like ultra-thin coating in deionised (D.I.) water using static immersion tests. Deionised water was selected as ageing medium mainly because it did not contain any compounds that might deposit onto the surface and interfere with further surface analyses. After periods of immersion in D.I. water ranging from 1 to 4 weeks, chemical and morphological characterization of the coating, both in its bulk and surface, was carried out by atomic force microscopy (AFM), ellipsometry, Fourier-transform infrared spectroscopy (FTIR), contact angle measurements (CA) and X-ray photoelectron spectroscopy (XPS). Results have shown that upon ageing, there is a decrease in the fluorine concentration and an oxidation of the coating. The degradation of the coating could probably result from the water, which penetrates the coating and induces changes in the chemical composition and microstructure.     

Performance of a cutting tool made of steel matrix surface nano-composite produced by in situ laser melt injection technology

Steel-matrix (105WCr6 steel) surface nano-composites with (Ti,W)C micron-sized and (Fe,W)₆C nano-sized carbide precipitates were produced by in situ laser melt injection technology with subsequent heat treatment. The microhardness of a 1 mm thick nano-composite layer was found to be higher than that of the initial matrix. The machinability of the surface nano-composite by a cubic boron nitride (CBN) wheel was found lower, but still reasonable compared to the initial matrix. Cutting tools produced from our new nano-composite by the CBN wheel were found to have higher wear resistance, longer tool life and provided lower cutting forces against a C45 steel workpiece compared to the initial matrix of the nano-composite.     

A study of fiber laser welding of galvanized steel using a suction method

In this paper, we describe a method employing a suction device adapted to provide a negative pressure zone on the surface of the keyhole to allow the highly-pressurized zinc vapor to escape. The microstructure and properties of the lap joint were studied, and the distribution of zinc element in the joints was analyzed using synchrotron radiation X-ray. A high-speed video camera was used to record the dynamic behavior of the laser-induced plasma plume and the zinc vapor. Experimental results demonstrated that this suction method can not only facilitate venting of the high pressure zinc vapor from the molten pool and the keyhole, but also stabilizes the laser-induced plasma plume during the welding process. A lap joint with a good surface quality and excellent mechanical strength was obtained using this method.     

Surface modification of DF-2 tool steel under the scan of a YAG laser in continuously moving mode

Pulsed Nd:YAG laser with focus diameter of 1.06 μm was used to treat successively and continuously the surface of DF-2 cold work tool steel plate. Surfaces irradiated under a large range of laser setting parameters resulted in very different morphologies and microstructures. Roughness of the treated surface was measured by Talysurf profilometer, whilst surface morphologies and microstructures were investigated by scanning electron microscopy (SEM). Change of crystalline structure and composition of the treated surface layers before and after treatments were studied by energy dispersion spectroscopy (EDX). Cross-sectional micro-hardness of the treated layers was also analyzed. Measured micro-hardness values clearly indicated the performance improvement of the treated surface owing to the formation of martensite. Penetration depth of the micro-hardness varied greatly with the laser irradiating parameters that recursively results in different microstructures. Suitable control of the laser irradiation parameters allowed the accomplishment of highest micro-hardness at the outermost surface.     

Temperature control in laser brazing of a steel/aluminium assembly using thermographic measurements

One way of making car bodies lighter is to introduce some aluminium parts in place of steel. Steel and aluminium can be joined by laser braze welding. As in other types of thermal joining, inter-metallic phases may weaken the joint. In laser braze welding, these appear as a thin layer of brittle compounds at the steel/seam interface. Their formation is related to temperature. It has been shown that, if the layer is less than 10 μm thick, the joint is not compromised [Kreimeyer M., Sepold G. Laser steel joined aluminium-Hybrid structures, Proceedings of ICALEO'02, Jacksonville, USA; 2002]. Not only can temperature gradient be calculated by numerical simulation, but it is also possible to measure the surface temperature by thermography. We show here how thermography may be used to control temperature during laser braze welding.     

Modeling of temperature field and pool formation during linear laser welding of DP1000 steel

A rotary-Gauss body heat source was employed in the study to model the laser welding of DP1000 steel. The condition of heat dissipation during the welding has a significant effect on the temperature field as well as the shape and size of the laser weld. A series of welding experiments were performed, and good agreement was observed between the calculated weld dimensions and the experimental results. The microhardness values across the welded joint were measured to determine the range of the soft zone in the heat affected zone (HAZ) and simultaneously the temperature range experienced in this region. The results indicated that the soft zone significantly affects the mechanical performance of the welded joint. The width of the soft zone and its distance from the weld center increase with increasing laser power, while the width of the soft zone and its distance from the weld center decrease with increasing welding speed.     

Effect of the initial microstructure on the plasma nitriding behavior of AISI M2 high speed steel

The nitriding behavior of AISI M2 steel was studied on samples previously submitted to two different heat treatments in order to investigate the effects of the initial microstructure on the thickness and hardness of nitrided layer. Prior to nitriding, one group of samples was fully annealed while the other group was quenched and tempered, thus acquiring the lowest and highest hardness respectively. Plasma nitriding was performed at 450 °C for 8 h with a mixture of N₂ and H₂ in a plasma reactor working under floating potential. Structural and mechanical properties of nitrided layers were characterized using X-ray diffraction (XRD), optical microscopy and microhardness testing. Variations in surface roughness were obtained by 3D surface profilometry analysis. The thicker nitrided layer was obtained for the fully annealed samples, in which the nitrided layer is composed of γ′-Fe₄N and ε-Fe_2-3N phases plus a diffusion zone. For the hardened-tempered samples, the nitrided region mainly consisted of a diffusion zone. Plasma nitriding increased the surface hardness of the fully annealed samples by 330% and that of the quenched-tempered samples by 50%. The nitrided depth was also estimated using cross-sectional microhardness profiles; giving about 140 µm and ∼ 70 µm for the fully annealed and quenched-tempered samples, respectively. Due to the grain to grain nitrogen diffusion, plasma nitriding also increased the surface roughness. The largest roughness was obtained for the fully annealed samples, in accordance with the largest nitrided depth. The difference in the nitriding behavior was explained on the basis of the microstructural aspects of the substrates such as the concentration of the freely dispersed alloying elements and the level of compressive residual stresses.     

A double-point moving source model for predicting seam geometry in laser welding

A theoretical double moving point source model, based on the superposition principle, is proposed for predicting the weld seam geometry produced by a CW Nd:YAG laser in a constrained overlap configuration on a martensitic stainless steel in a transitional regime between pure conduction and keyhole welding. This intermediate regime is modelled by varying the power balance between the two point sources along with their relative distance. Tests show that the main geometrical features of the weld bead (penetration depth and resistance length) are comparable to the predicted values (error less than 5%). Finally the model can be also profitably used in order to predict the temperature field around the molten pool.     

Selective laser melting of Fe-Ni-Cr layer on AISI H 13 tool steel

An attempt to fabricate Fe-Ni-Cr coating on AISI H13 tool steel was performed with selective laser melting. Fe-Ni-Cr coating was produced by experimental facilities consisting of a 200 W fiber laser which can be focused to 80 μm and atmospheric chamber which can control atmospheric pressure with N₂ or Ar. Coating layer was fabricated with various process parameters such as laser power, scan rate and fill spacing. Surface quality and coating thickness were measured and analyzed. Three different surface patterns, such as type I, type II and type III, are shown with various test conditions and smooth regular pattern is obtained under the conditions as 10 μm of fill spacing, 50–350 mm/s of scan rate and 40 μm of fill spacing, 10–150 mm/s of scan rate. The maximum coating thickness is increased with power elevation or scan rate drop, and average thickness of 10 μm fill spacing is lower than that of 40 μm fill spacing.     

Study on the strength and failure modes of laser welded galvanized DP980 steel lap joints

The mechanical properties of different sections of the laser welded galvanized high strength dual phase (DP) 980 steel lap joint such as the hardened zone (fusion zone and the grain coarsened heat affected zone (HAZ)), the softened zone (subcritical HAZ), and the base material were determined through mini-tensile tests. The numerically-predicted load–displacement curve and the sample rotation angle were verified by experimental measurements. The FE model considering the non-homogeneous mechanical properties was built based on the weld cross section geometries that were obtained under various laser welding conditions. The numerically-predicted von Mises equivalent strain concentrations and failure modes of the galvanized DP980 lap joints with respect to different laser welding conditions exhibit reasonable agreement with the experimental results.     

A simplified energy-based model for laser welding of ferritic stainless steels in overlap configurations

A theoretical model is developed for predicting the weld shape produced by a Continuous Wave (CW) Nd:YAG laser in a constrained overlap configuration on a ferritic stainless steel, and verified by means of experiments. Tests demonstrate that, as assumed in the modelling phase, penetration depth is linearly dependent on the energy density input, within the hypothesis of conduction dominated welding. Penetration depth determines the weld resistance length at the interface since the weld profile is found to change from approximately semicircular to parabolic when the energy density input is varied in the range from 22 J/mm² to 32 J/mm².     

Characteristics of laser aided direct metal/material deposition process for tool steel

Laser aided direct metal/material deposition (DMD) process builds metallic parts layer-by-layer directly from the CAD representation. In general, the process uses powdered metal/materials fed into a melt-pool, creating fully dense parts. Success of this technology in the die and tool industry depends on the parts quality to be achieved. To obtain designed geometric dimensions and material properties, delicate control of the parameters such as laser power, spot diameter, traverse speed and powder mass flow rate is critical. In this paper, the dimensional and material characteristics of directed deposited H13 tool steel by CO₂ laser are investigated for the DMD process with a feedback height control system. The relationships between DMD process variables and the product characteristics are analyzed using statistical techniques. The performance of the DMD process is examined with the material characteristics of hardness, porosity, microstructure, and composition.     

Hybrid laser arc welding of X80 and X120 steel grade

The aim of the present work was to investigate the possibilities of hybrid laser arc welding regarding reliable production of longitudinal welds of high strength pipe steels X80 and X120 and to evaluate achievable mechanical properties of laser hybrid welds. The study focused on weld toughness examination in low temperature range up to ?60°C. Suitable filler materials were identified in the context of this task. It could be shown that metal cored electrodes guaranteed sufficient Charpy impact toughness at low temperature for both investigated materials. Modern arc welding technologies such as modified pulsed spray arc were used to promote deeper penetration of the filler material into the narrow laser welding gap. Edge preparation with a 14 mm deep root face was considered as optimum, because no penetration of the filler material could be detected beyond this depth limit, and therefore, any metallurgical influences on the weld metal properties through the welding wire could be excluded.     

MAG和激光扫描-电弧复合焊X80钢接头组织和性能

针对不同预热条件下X80M钢管道全自动焊接接头,开展了裂纹尖端张开位移(crack tip opening displacement, CTOD)的差异性研究. 采用单边缺口3点弯曲试样进行CTOD试验,按照GB/T21143-2014标准和ISO15653-2018标准推荐的公式分别进行CTOD值的计算. 结果表明,两种计算公式结果不同,采用ISO 15653-2018标准推荐公式计算CTOD值结果与GB/T21143-2014标准相比平均偏高35%,差异出现的原因是两种标准的修正方式侧重点不同,即GB/T21143-2014标准针对裂纹长度变化进行修正,而ISO15653-2018标准针对测试温度的影响进行修正. 预热温度为80 ℃的接头试样CTOD值整体较低,其中按GB/T21143-2014标准计算有3个试样低于标准,最低为0.16 mm,按ISO 15653-2018标准计算有1个试样低于标准,最低为0.20 mm;预热温度为120 ℃时,所有试样在两种标准下计算的CTOD值均满足标准要求,焊接接头断裂韧性更好. 相同预热条件下,熔合区试样CTOD值整体高于焊缝区试样,且CTOD最大值均出现于熔合区,具有较好的断裂韧性,熔合区CTOD值受组织不均匀性的影响,不同位置间存在较大差异,存在性能的不均匀性.

     

Modification of nitride layer on cold-work tool steel by laser melting and friction stir processing

The microstructural control of the nitrided case on a cold-work tool steel (SKD11) plate by laser melting and friction stir processing (FSP) was studied. The laser melting and the FSP were used as a pretreatment for the nitriding to refine the microstructure of the SKD11. The diffusion zone of the nitrided case on the SKD11 plate contained thick brittle boundary lines consisting of local formed nitride particles. On the other hand, the microstructure of the diffusion zone was very uniform and the thick brittle boundary lines disappeared as a result of the combined treatment of the laser melting and the FSP before the nitriding process.     

低碳钢激光预熔活性焊接法

以8 mm厚的低碳钢为研究对象,在氧气的保护下,用小功率光纤激光在待焊焊件表面进行预熔处理,使表面熔化生成一层氧化层,然后用TIG焊或激光电弧复合焊覆盖氧化层,研究工艺参数对焊缝成形的影响.结果表明,激光预熔后进行覆盖焊接,电弧没有收缩,熔深增加到1.5倍左右,表面成形良好.随着电流的增加,熔深增加,但激光预熔后的焊道增加更快.随着焊接速度的增加,熔深减小.随着激光预熔功率的增加,熔深增加.随着复合焊接中激光功率的增加,熔深增加.焊缝含氧量的增加,使得表面张力温度系数由负变正,是低碳钢激光预熔活性焊接熔深增加的主要原因.