Laser surface modification of Ti-6Al-4V alloy

Hardening of Ti-6Al-4V alloy with laser surface melting (LSM) and laser surface alloying (LSA) techniques was attempted. Both LSM and LSA were carried out in a nitrogeneous atmosphere. Niobium, molybdenum and zirconium were used as alloying elements in the LSA. A hardness increase was observed for both LSM and LSA. Maximum hardness was obtained for LSM and zirconium alloy addition. In LSM, hardness increased almost three-fold in comparison to the substrate, which has a Vickers hardness of 350, by the formation of TiN in the region of 100 m melt depth. Hardness then decreased slowly and reached a minimum of 580 VHN at the maximum melt depth of 750 m. However, hardness for the zirconium alloy addition was uniform throughout the melted zone. Ageing treatments were performed for all specimens at 450C and different ageing times. Hardness measurements and X-ray diffraction were utilized to delineate the features associated with the hardening of the melted zone.     

Laser surface modification of 316 L stainless steel with bioactive hydroxyapatite

Laser-engineered net shaping (LENS?), a commercial additive manufacturing process, was used to modify the surfaces of 316 L stainless steel with bioactive hydroxyapatite (HAP). The modified surfaces were characterized in terms of their microstructure, hardness and apatite forming ability. The results showed that with increase in laser energy input from 32 J/mm² to 59 J/mm² the thickness of the modified surface increased from 222 ± 12 μm to 355 ± 6 μm, while the average surface hardness decreased marginally from 403 ± 18 HV_0.3 to 372 ± 8 HV_0.3. Microstructural studies showed that the modified surface consisted of austenite dendrites with HAP and some reaction products primarily occurring in the inter-dendritic regions. Finally, the surface-modified 316 L samples immersed in simulated body fluids showed significantly higher apatite precipitation compared to unmodified 316 L samples.     

Laser surface modification of a 13.5% Cr, 0.6% C steel

A 13.5% Cr, 0.6% C steel, with an initial microstructure of chromium carbides in a ferrite matrix, was heat-treated by scanning a high-power laser beam over the surface. The aim was to compare the physical and chemical properties produced by this type of selective surface treatment with those resulting from a conventional furnace desensitization and quench-hardening heat treatment. Surface heating homogenized the carbon originally bound in the carbides sufficiently to produce martensite, giving hardening to levels comparable with a conventional heat treatment. Chromium-rich zones, carbides and retained austenite were also detected in the heated microstructure. Surface melting produced complete homogenization of both carbon and chromium, which resulted in the retention of large amounts of austenite in the microstructure on cooling to room temperature. Subsequent refrigeration at — 196 °C transformed some of the austenite to martensite. Pitting corrosion and local reductions in hardness were observed adjacent to treated areas under certain conditions, due to precipitation of secondary carbides and elevated tempering, respectively.     

Laser surface modification of metals and alloys

In order to understand the properties of solids, the materials scientist frequently needs to characterize the microstructure of a material on as fine a scale as possible. Characterization here is used in its most general sense, i.e. to describe as completely as possible the morphology, crystallography and elemental composition of the material under investigation. Although instruments have been developed over the years that can provide this information, they generally have one common limitation, namely spatial resolution. Analytical electron microscopy (AEM) is the term which is now applied to a new group of analytical techniques used in conjunction with a transmission electron microscope and/or a scanning-transmission electron microscope that can provide high spatial resolution (about 20 nm) analyses from microvolumes of material in thin electron-transparent specimens. The basic principles of microanalysis using AEM are discussed, together with examples of application in both metallurgical and ceramic studies.     

Duplex surface treatment of stainless steel X12CrNi 18 8

Described technology of stainless steel duplex surface treatment is based on the plasma nitriding of the component in micropulse plasma and subsequent coating by Ni and composite Ni/diamond film. The formed duplex coating is characterized by very good mechanical properties, e.g., an excellent abrasion resistance, a low friction coefficient and a high hardness. __________ Translated from Problemy Prochnosti, No. 1, pp. 129–132, January–February, 2008.     

Laser surface modification of UNS S31603 stainless steel. Part II: cavitation erosion characteristics

Austenitic stainless steel UNS S31603 was laser surface alloyed with various elements (Co, Ni, Mn, C, Cr, Mo, Si) and alloys/compounds (AlSiFe, Si₃N₄ and NiCrSiB) as presented in Part I together with the microstructures and the corrosion characteristics of the alloyed specimens. In Part II, the cavitation erosion characteristics of the alloyed specimens in 3.5% NaCl solution at 23°C were studied by means of a 20 kHz ultrasonic vibrator at a peak-to-peak amplitude of 30 μm. The hardness profile and the compositional profile of the alloyed layers were investigated by a Vickers hardness tester and by EDX respectively. The cavitation erosion resistance of specimens alloyed with AlSiFe, C and NiCrSiB were highest, reaching 11.1, 10.5 and 7.9 times that of the substrate respectively. The damage mode was identified to be ductile fracture for specimens containing austenite as the major phase, and brittle fracture when the major phase was ferrite or intermetallic. Cavitation erosion was initiated at the phase boundaries where there was an abrupt change in mechanical properties (e.g. hardness) and then propagated into the weaker phase. It was also noted that large improvement in cavitation erosion resistance and corrosion resistance could not be simultaneously achieved in the present study.     

Laser surface modification of polymers to improve biocompatibility

The effect on neutrophil chemokinesis and fibroblast adhesion of changing surface topography was examined using two polymeric substrates; polycarbonate and polyetherimide, modified by laser treatment to produce pillars of varying dimensions on the surfaces of these materials. The dimensions for the pillars were 7,25 or 50 m square, 0.5, 1.5 or 2.5 m deep. Human neutrophils were isolated, by centrifugation, on ficoll from heparinized whole blood obtained from healthy volunteers. Isolated neutrophils were exposed to the surfaces for 20 min and tracked using image processing and analysis techniques. The mean speed for each cell on each surface was calculated and this data statistically analysed using multivariate analysis of variance to determine any significant effect on speed of movement due to the surface topography. Compared to the potent stimulator FMLP all surfaces did not stimulate significant cell movement, but within the groups some surfaces had more effect on cell movement than others, and were stimulating cells to move faster than on the same untextured surface. Surface topography can stimulate neutrophils to move at different speeds across a surface. L929 fibroblasts were incubated on the surfaces for 48 h and then examined using scanning electron microscopy to study fibroblast position and adhesion with respect to the pillars. No pattern of orientation with respect to the pillars were observed and fibroblasts spread and elongated whether in contact with the pillars or on a smooth area of the material.     

不锈钢表面的化学改性与防冻性能研究

以硅氧烷为疏水剂,通过液相、气相沉积方法在不锈钢表面成膜,构建稳定的疏水性表面。通过静态接触角及滚动角对其进行表征,对改性后的不锈钢进行相同条件的冰冻,比较其融冰时间,研究其防冻性。结果表明:十六烷基三甲氧基硅烷液相成膜的疏水性膜性能最好,接触角为118o,滚动角13o;十八烷基三甲氧基硅烷气相成膜的不锈钢片防冻能力最佳,融冰时间最短60s,不锈钢表面沉积疏水膜能够提高其防冻性能。     

Laser surface hardening of thin steel slabs

The transformation hardening of steels by surface heating by a c.w. laser beam has been studied. We examined the surface treatment of thin steel slabs by a suitable mathematical model of the thermal transient induced by laser beam heating. The laser parameters for surface hardening of such samples and the resulting microstructures are discussed. Hardening depths calculated from the mathematical model fit well with experimental results.     

Influence of surface integrity on fatigue strength of 40CrNi2Si2MoVA steel

Influence of surface integrity (including surface roughness, residual stresses, and microstructure in surface) on fatigue limit of 40CrNi2Si2MoVA steel specimens is investigated comprehensively in this work according to a systematic consideration. The surface integrity of specimens is changed due to several widely used manufacturing procedures: heat-treatment, grinding, electro-polishing, hard chromium plating and shot peening. In comparison with specimen electro-polished after grinding, the specimen without polishing has 10% lower fatigue limit due to higher surface roughness; while shot peening improves the fatigue limit for about 36% due to inducing of compressive residual stress field in the surface and transferring the fatigue crack source from surface to interior. The fatigue limit of specimen with decarburized layer after grinding is lower about 13%, but the shot peening can eliminate its detrimental effect. Hard chromium plating decreases the fatigue limit dramatically. The shot peening carried before plating can improve the fatigue limit of specimen and cause it to get to a level even higher than that of specimen without plating.     

离子束辅助沉积技术(IBAD)在冷作模具表面改性上的应用

IBAD是离子注入及沉积技术发展起来的材料表面改性方面的综合技术,工模具经IBAD处理后现场使用寿命可提高2～4倍。材料经IBAD处理后改性层的硬度明显提高、摩擦系数减小、耐磨性增强;改性层是由(Ti2N+TiN)、(α-Fe+TiN)、(α-Fe(N)+Cr7C3)三层次组成,与基体间附着性好、晶粒细化呈纳米相。     

Prediction of stress history in carburized and quenched steel cylinders

Abstract

Quenching and residual stresses were evaluated in carburized (0·18% core carbon) steel cylinders. A variety of experimentally generated data including dilatation strains, temperature-dependent elastic-plastic properties, and thermal profiles from oil and water quenching were used as input data for a finite-element program. The program was designed specifically for cylinders and allowed determination of the stress history during quenching and the development of the residual-stress profiles. Special emphasis was given to evaluating the factors that lead to the final residual-stress patterns including the degree of plastic flow and the phase transformations for the carburized and high-carbon steels. To verify the results, residual stresses were evaluated by X-ray diffraction methods and compared with those determined by the finite-element program. The X-ray results corroborated those found with the finite-element program.

MST/18     

Dynamic reliability behavior for sliding wear of carburized steel

In this paper two types of the dynamic reliability model are proposed and compared with the Weibull distribution of sliding wear. These models are concerned with the relationship between the hazard function and the dynamic reliability. One considers a differential equation form, called the DE model, the other takes an algebraic dependence, AE model.The physical problem used in the simulation deals with the sliding of multiple ring-disk system. The disk is carburized, and the ring is assumed to be much harder than the disk. The dynamic reliability and failure rate variation of the disk can be estimated by the critical wear depth during sliding. The results show that the predictions followed DE model and Weibull distribution are a little conservative. Some physical interpretations of the models are presented. Both models are recommended due to their acceptable RMS error, physical     

Laser surface melting of a complex high alloy steel

The effect of laser surface melting (LSM) on the microscopic morphology of a complex high alloy steel has been examined in single track experiments. Different combinations of power (1–4 kW) and scan speed (500–1000 mm min⁻¹) were selected in order to obtain a range of depths of melt penetration. An increase in the depth of melt penetration has correlated with rise in the estimated surface temperature induced by laser melting. Within the strongly refined structure of the laser melted alloy, the interdendritic spacing, λ varied from 0.29 μm to 0.97 μm with increase in the depth of melt penetration. The microhardness in the laser surface melted region has been shown to decrease linearly with λ^−0.5.     

Laser surface melting of electro-metallurgic WC/steel composites

Laser surface melting was used to treat electro-metallurgic WC/steel composites. The microstructure and properties of the melted zone were investigated. It was found that a homogenous and fine microstructure was formed in the melted zone. A significant phase precipitated was the herringbone eutectic carbide of Fe₃W₃C. The volume fraction of the eutectics was related to the thermal absorption increasing with the decrease of scanning rate. Preheating was beneficial to form the orientation dendrite structure and to increase the volume fraction of the eutectics. The laser melted surface possessed better microhardness and wear resistance compared to the substrate. The volume fraction of the eutectics played an important role on the wear resistance which increased with the increase of the volume fraction of the eutectics. The wear resistance was mainly depended on the content of the primary WC, as increased with the increase of the content of WC. The improvement of interface bonding, fine eutectic precipitation and structure refinement etc. were regarded as the micro-mechanism of enhancing the surface properties.     

Plasma transferred arc surface modification of a low carbon steel

The potential of Plasma Transferred Arc (PTA) hardfacing goes beyond the surface welding of superalloys. This work evaluated low carbon steel surface modification by PTA deposition of fine WCoC carbides, and mixtures of Fe powders and 5–35 wt% carbides. Characterization included visual inspection, optical and scanning electron microscopy, X-ray diffraction and microhardness profiles. PTA processing allowed for the dissolution of carbides confirmed by X-ray diffraction, leading to homogeneous microstructures. Microstructures varied from a Widmanstätten morphology to a typical dendritic solidification structure upon the WCoC content. Surface soundness depended on powder preparation and composition. Sound surfaces exhibiting hardness up to 700 Hv were obtained for the 35 wt% WCoC powder mixture.     

The tensile properties of carburized and uncarburized low carbon mild steel

The properties of a low carbon mild steel in monotonic tension loading were compared in the plain normalized and the carbo-nitrided slowly cooled conditions. The application of a carbo-nitriding process raised the yield strength of the steel to that of the nominal tensile strength in the uncarburized samples, and increased the nominal tensile strength to a value 45% above that of the plain normalized steel, whilst still retaining a good measure of ductility. The fractures for the plain samples were cup and cone type whilst those for the reinforced samples revealed slant mode fractures.     

Laser surface hardening of AISI 01 tool steel and its microstructure

Abstract

The effects of laser surface hardening on AISI 01 tool steel samples were studied by changing the laser operating parameter combinations and the initial steel microstructure. Both melted and solid state transformed regions were produced, and then studied using optical microscopy, analytical electron microscopy, X-ray diffraction, and measurements of micro hardness to investigate the hardening mechanisms and the development of compressive residual stresses. The results indicate that hardened case depths up to 0·6 mm can be obtained using a laser beam operated at a power of 500 W and a scan rate of 2·1 mm s^?1, but that different amounts of retained austenite and undissolved carbides are observed for different beam powers. Quenched and tempered AISI 01 steel samples, with initial hardness values in the range 30–40 HRC, are better suited for laser surface hardening compared with the samples with initial hardness of 48–50 HRC, because the formation of an over tempered region adjacent to the hardened zone can be avoided.

MST/901