Papers by Keyword: Laser Surface Melting

Abstract: Rapidly solidified thin micro-alloyed surface layers are generated by laser melting of plasma thermal sprayed steel surfaces. Samples of carbon steel are plasma sprayed with fine nickel and aluminum powders. Laser surface melting generated a thin localized molten pool of metal with steep horizontal thermal gradient. The latter triggered intense vortex formation in the molten pool which thoroughly mixed the nickel and aluminum powders within the molten pool in a fraction of a second. As the sample is moved away with a predefined velocity, the cold substrate quenched the melt pool, generating rapidly solidified micro-alloyed surfaces. A 2.5 kilowatts continuous carbon dioxide laser was used for surface melting; laser power was maintained at 800 watts while the samples were moved with respect to the laser beam at linear velocities in the range of 100-200 mm/min. The technique generated metallurgical bonded novel surfaces. Depth of the laser modified layer was achieved in the range of 0.2-0.4 mm. Refined microstructures of pre-austenite grain size in the range of 4±2 µm were generated. Micro-hardness measurements of the modified layer indicate an almost three times enhancement in the hardness values; the latter are, in general indicative of mechanical strength of the material. The shape of the solid/liquid interface of the advancing molten pool determines the orientation of the growing dendrites; at higher velocities of sample translation with respect to the laser beam, these are almost parallel to the sample surface. The orientation of the dendrites, the uniformity in surface alloying (within fraction of a second) and the resulting hardness values are explained with the help of the modeled shape of the liquid metal pool. The laser processed material proved to be a flexible technique to synthesize novel surfaces for surface sensitive applications.

Abstract: The article aims to comprehend the microstructural changes, in Plasma Transfer Arc (PTA) deposited M2 high speed steel (HSS) hardfacings upon incorporation of 10 wt% Mo alloying during deposition followed by laser surface melting. PTA deposited hardfacings were produced over 4140 steel. Then Mo alloyed and unalloyed PTA deposits were subjected to laser surface melting (LSM) process. A comprehensive microstructural characterization for all the resultant structures was carried out. Optical metallography using appropriate etching reagents and SEM microscopy in conjunction with XRD techniques were employed to ascertain the matrix structure and carbides morphology. The PTA microstructure was close to equilibrium structure of M2 HSS containing mixture of ferrite/austenite/martensite along with MC, M₂C and M₆C type carbides. While the LSM of M2 HSS caused higher fraction of martensite and finer grains in the structure resulting in increment in hardness. 10-wt% Mo addition changes the carbides from MC and rod like M₂C to fibrous M₂C and fishbone like M₆C carbides. The LSM of Mo alloyed M2 HSS PTA deposits led to an overall decrease in the fraction of M₆C carbides and fibrous M₂C carbides accompanied by a decrease in hardness.

Abstract: 316L stainless steel is used in various industrial applications including chemical, biomedical and mechanical industries due to its good mechanical properties and corrosion resistance. Recycling of 316L stainless steel scrap without significantly reducing its value has received recently great attention because of the environmental regulations. In the current work, 316L stainless steel scrap was recycled via casting using Skull induction melting technique. The casted products subsequently subjected to laser surface melting process to improve its surface properties to be used for harsh environment. The results showed defect free surfaces with homogeneous microstructures. Nano size grains were also obtained due to rapid solidification process. Such nano size grains are preferred for extending the usage of the 316L stainless steel in new applications.Corresponding author: E-Mail: elgazzar.ha@gmail.com

Abstract: Attempt was made to improve the surface hardness and wear properties of AISI M2 high speed tool steel. Laser surface melting (LSM) of tool steel was conducted with 2.2 KW Nd:YAG laser as heating source. Conventional hardening of the tool steel was applied. Characterizing the LSM, with optical and field emission scanning electron microscopy and surface hardness technique was used to evaluate the micro-hardness and mechanical behaviour of different regions of melting pool. AISI M2 tool steel is approximately HV 260, hardness after conventional treatment was 850 HV and the hardness after laser surface heat treatment is around 900 HV. It was found that there is a considerable influence of the laser power density and scanning velocity on the melted zone dimensions and the re-solidified structure. Increasing laser energy and reducing the laser scanning rate results in deeper and wider melt pool formation.

Abstract: A nodular cast iron (NCI) has been surface melted using the high power direct diode laser (HPDDL) with a quasi-rectangular laser beam spot and the uniform distribution of power. The effect of a heat input and a shielding gas on the quality of surface melted layers (SMLs) has been investigated. The microstructure of the SMLs has been assessed by optical microscopy, scanning electron microscopy and X-ray diffraction (XRD). Comparative erosion tests between the SMLs and as-received NCI have been performed following the ASTM G 76 standard test method. The HPDD laser surface melting of the NCI enables to produce non-porous layers having a hardness up to 1000 HV. It has been determined that the hardness of SMLs depends on the amount of cementite and residual austenite in the fusion zone. The SMLs produced in an argon atmosphere contain higher volume fraction of austenite, than those produced in nitrogen, and consequently have the lower hardness. With increasing heat input the hardness increases, as the result of more complete dissolution of graphite and the higher amount of cementite. The SMLs exhibited significantly higher erosion resistance than the as-received NCI for erodent impact angle of 30°, and slightly lower at 90°.

Abstract: Hastelloy C-276 is a nickel-based super alloy which has inbuilt corrosion resistance and exhibits low wear resistance. In this work, an attempt has been made to improve the tribological properties of this alloy without sacrificing the corrosion properties by laser surface melting in argon atmosphere. The results indicated better grain refinement at optimum laser parameters of 1.5 kW and 300 mm/min thereby exhibiting a maximum hardness of 447 HV.The corrosion rate for the entire laser treated samples showed a decreasing trend with a minimum value of 2.07172 x10^-2 mm/year, compared to base alloy. This paper demonstrates that laser surface melting is a viable method to improve the wear and corrosion properties of Hastelloy C-276.

Abstract: A KrF excimer laser was introduced for laser surface melting (LSM) of the aluminium alloys AA2124-T4 and AA6061-T4. The microstructural and compositional analysis was conducted using SEM, low-angle XRD, and TEM; the corrosion behaviour of as-received (AR) and laser-treated (LT) samples was evaluated by electrochemical techniques and immersion test in a 3.5% NaCl solution. A melted layer consisting of a re-solidified layer with refined microstructure and dissolution of intermetallic particles (IMPs), and a thin film of aluminium oxides at the top, was formed after LSM for both alloys. The corrosion resistance of both alloys was improved after LSM. The results of immersion test showed different corrosion behaviour for LT AA2124 and LT AA6061. The delamination of the melted layer was observed for AA2124 but was not observed for AA6061 after exposure to 3.5% NaCl solution for 24 h. This was attributed to the formation of copper-rich segregation bands in the melted layer of AA2124 due to higher content of copper in AA2124 than AA6061. A significant number of micro-pores were present in the melted layer for AA2124 treated with high number of laser pulses, leading to the decrease of the corrosion resistance.

Abstract: The present work investigated the effect of laser surface melting (LSM) process on micro arc oxidation (MAO) coating prepared in silicate electrolytes (Si-MAO) and aluminate electrolytes (Al-MAO). Plasma intensity of laser spots, element content and phase constituents of coatings were analyzed. During LSM remelting process, the formation of heat sources and micro molten pools on the surface of MAO coating were discussed. The results showed that the moderate plasma was formed more easily on the Si-MAO than Al-MAO coating, owing to the temperature threshold of melting oxide layer was decreased for silicon-containing MAO coating during LSM treatment. Besides, the Si-MAO coating of laser-treated had low porosity and smooth surface.

Abstract: The present paper deals with the microstructure and hardness distribution in width and in depth of the surface layer of steel Ch18N10T GOST (AISI 321, EN X6CrNiTi 18-10) after surface melting by continuous wave CO2 laser. Light microscopy, XRD analysis and Vickers hardness testing (HV5 and HV0,05) have been used in our research. Phase analysis shows disturbance of the mono-phase initial austenitic structure in the treated layer. The structure of the melted pool consists of austenite with dendrite morphology and δ-ferrite situated in the dendrites’ cores. The ferrite has been clearly identified by XRD analysis. As a result from fast heating and cooling, ferrite, obtained by diffusionless sliding mechanism, was observed along the austenite grains’ boundaries in the heat affected zone. The presence of small inclusions of supposed Ti carbide, non-identified by XRD analysis, was also observed. The durometric investigations show that the surface hardness in the melted zone is in the range 180-210 HV5 while that of the basic metal is about 270 HV5.

Abstract: In the present study, surface melting of a magnesium alloy, ZE41, was performed with an Nd:YAG laser, using different laser parameters (scan rate and beam type). The microstructure of the laser treated and untreated specimens were characterised by optical and scanning electron microscopy. The degree of microstructural refinement and melt depth was found to be a function of the laser scan rate. Electrochemical characterisation of the different laser treated specimens along with the untreated alloy was performed using electrochemical impedance spectroscopy. The laser treated specimens and untreated alloy showed similar corrosion resistance.