Laser Transformation Hardening Research Articles

A comparative study of laser fluence effect on surface modification and hardness profile of austempered ductile iron

The impact of laser transformation hardening (LSH) and Laser surface melting (LSM) on an austempered ductile iron (ADI) alloy was investigated. Various laser fluences (J mm−2) were irradiated on the ADI specimens, obtained by different beam powers and scanning speeds to reach the optimum parameters to attain a surface with a good combination of high hardness and deep hardened depth, together with obtaining minimal distortion and crack-free surface. ADI surfaces were treated by means of high-power Nd:YAG laser in continuous wave mode. A detailed investigation of the obtained microstructure was characterized via optical and scanning electron microscopy attached to an EDX analyzer as well as X-ray diffraction analysis. Such a wide laser fluence range has a considerable effect on the achieved microstructure. A process parameter map was established which indicated that three different laser treatments were induced under such values of laser fluences: a laser transformation hardening, a partial laser melting, and a complete laser melting. The results revealed that to produce a hardened microstructure, specimens have to be operated within a very small range of laser fluence from 9.6 up to 29 J mm−2, achieving surface hardness values around 900 HV0.1 and depths of approximately 184–700 μm, respectively. Meanwhile, partial melting of the treated layer took place at a medium energy density range from 37 to 112 J mm−2, by a further increase in laser fluence (120–360 J mm−2), complete melting occurred which resulted in a deeper treated layer that reached 3.5 mm with an increase in microhardness value to almost 1100 HV0.1 at 360 J mm−2. The obtained microstructure in case of low laser fluence consisted of an austenite dendritic structure with undissolved graphite nodules, however, at medium and high laser fluence, the microstructure contains large cementite plates, iron-silicon carbides, a small area of ledeburite structure, and some retained austenite. The quantitative amounts of such phases directly depend on the applied laser fluence. Such phases were identified by means of EDX analysis.

Laser transformation hardening of metal sheets under air and water environment

Laser transformation hardening of metal sheets under air and water environment

AN EXPERIMENTAL INVESTIGATION OF LASER TRANSFORMATION HARDENING OF UNALLOYED TITANIUM USING ND:YAG LASER

This research paper presents the investigation on laser transformation hardening (LTH) of unalloyed titanium of 1.6mm thickness sheet, nearer to ASTM Grade 3 of chemical composition was investigated using 2KW CW Nd: YAG Laser. The effects of laser process parameters: laser power750-1250W), scanning speed (1000-3000 mm/min) and focal point position (-10mm to -30mm) on the hardened bead profile parameters such as hardened bead width (HBW) and hardened depth (HD) was investigated using Response Surface Methodology (RSM). The experimental design is based on an independent quadratic Box-Behnken Design (BBD) matrix. The reduced linear and quadratic polynomial equations for predicting hardened bead width and hardened depth of bead profile were developed. Adequacies of models were examined by Analysis of Variance (ANOVA) technique. The results designate that the mathematical models developed predict the responses adequately adequate within the limits of hardening parameters being employed. It is recommended that the regression equations can be used to find optimum laser hardening conditions for desired criterion.

Laser transformation hardening of various steel grades using different laser types

Laser surface hardening has become a promising replacement technique for conventional heat treatment. The laser is a heat source, deployed with optics to modify the surface along with the desired geometry. Laser density and its interaction time can be controlled and varied depending upon the final requirement at the surface. Increased hardness, diffusion-less phase transformation, self-quenching, uniform case depth over the entire scanned area are the reasons for laser-assisted hardening to become very popular and successful. Another reason for its success is that there is no bulk transformation of the material; only selected regions are subjected to laser irradiation for better surface improvements. This paper is a precis of various investigations done with a gas type CO2 laser, a solid-state Nd: YAG laser, high-power diode laser and fiber laser (only Continuous Wave mode) over a variety of steel grades. Laser wavelength, power intensity, transverse/scanning speed, focal plane distance, surrounding atmosphere, pre-treated surface and surface absorptivity are parameters involved in laser surface hardening technique. By varying these independent variables, the desired surface property is achieved on the substrate subjected. A significant analysis of various experimental investigations done in this area is provided. Particular reference is made to surface hardening using different lasers as mentioned above. Although laser hardening is a potential technique for enhancing the surface properties, multi-track hardening/back tempering of the large surface area is still a concern and needs some attention. The main reason is a decrease in hardness in the overlap region. Various studies indicate that a single-track laser hardened region results in 3- or 4-times higher hardness than that of the original or base metal hardness. In contrast, in the overlapped region, the hardness value decreases, i.e., the difference between the hardness values at the single-track hardened region and an overlapped region is around 100–400 HV. This article reviews the chronological development of laser transformation hardening method explicitly by outlining the effect of process parameters on hardened depth and width, hardening influence in wear and corrosion resistance, numerical simulation and experimental validation techniques. Particular reference is made to CW (continuous wave) type laser hardening.

Combining laser transformation hardening and ultrasonic impact strain hardening for enhanced wear resistance of AISI 1045 steel

A medium-carbon steel AISI 1045 was hardened by a laser heat treatment (LHT), a multi-pin ultrasonic impact treatment (UIT), and two combined laser-ultrasonic surface treatments (UIT + LHT and LHT + UIT processes) to improve the wear resistance of the surface layer. The paper is focused on detecting the relationship between the surface roughness and microstructure, phase state, hardness, residual stress of the specimens hardened by the LHT, UIT and combined UI + LHT/LHT + UIT processes, and their wear and friction behaviors. The hardened plane specimens were examined after the short-term (15 min) and long-term (45 min) wear tests under oil-lubricated conditions in the quasi-static and dynamic loads. The analysis of the surface morphologies and cross-section profiles of the worn tracks were assessed by an optical 3D measurement system and scanning electron microscopy (SEM). The wear mechanisms are also discussed. Results showed that the combined LHT + UIT treatment provides a lower surface roughness (Ra ~0.25 μm) and a highest surface hardness (~11,000 HV0.05) while the combined UIT + LHT treatment leads to the deeper hardened zone by 1.5–2 times. As compared to the untreated specimen (dynamic tests), the wear loss magnitudes were respectively decreased by ~5%, 35%, 42%, and 84% after the UIT, LHT, combined UIT + LHT and LHT + UIT processes. The LHT + UIT-processed specimen has the highest wear resistance owing to the formation of the fine-grained martensitic microstructure, more flattened surface microrelief, and the absence of the oxide film.

Investigation into laser refurbishment and transformation hardening of cast iron forming dies for the automotive industry

Forming dies used in the automotive industry are shaped as such that they impart a predetermined contour or a profile to the workpiece. Due to subsequent assembly requirements of sheet-formed body structure components, the dimensional accuracy of the components is of vital importance. The dimensional accuracy of the forming dies is, therefore, of equal importance. The sheet metal forming process involves severe contact conditions, with the draw die radius generally being the most affected zone. Due to intense local tribological conditions, wear damage is observed. The possible laser refurbishment of wear damaged cast iron die radii as well as laser transformation hardening of these areas have been investigated. The refurbishment is seen as a remedy to an existing problem and transformation hardening as a precautionary wear-inhibiting solution to the problem. Due to the high carbon content of the cast iron and the dilution—although low—obtained during the cladding process, the resulting build-up susceptibility to cracking can be high. A stainless steel buffer layer between the die substrate and the overlay material was introduced. It was found that the H11 overlay material hardness complied with the specific requirement. The laser transformation hardening was studied at different parameter sets to obtain the required surface hardness, and laser hardening was successfully implemented on a forming die that was put in service.

Application of Response Surface Methodology for Modeling of Laser Transformation Hardening of Commercially Pure Titanium ASTM Grade3

In the presented study, the laser transformation hardening of commercially pure titanium sheet material of thickness being 1.6mm is investigated using CW, 1.6 kW solid State Nd: YAG laser. A Full Factorial Design (FFD) with Response Surface Methodology (RSM) is employed to establish, optimize and to investigate the relationships of three laser transformation hardening process parameters such as laser power, scanning speed, and focused position on laser hardened bead profile parameters such as hardened bead width, hardened depth, heat input and power density. In this work, Laser Transformation Hardening (LTH) with high precision and an optimum desired minimum value of hardened depth of 241 microns has been accomplished with heat input = 150 J/cm and power density = 294.08 W/mm2 for the laser process parameters of lower beam power: 750 Watts, high scanning speed: 3000 mm/min and a defocused beam of –30 mm. Effects of laser process parameters on laser hardened bead geometries were carried out using RSM. Results indicate that the scanning speed has a positive effect on all hardened bead dimensions while the laser power has a positive effect particularly on hardened bead width as compared to hardened depth and heat input. The optimum laser hardening conditions are identified sequentially to minimise hardened depth, heat input, power density and maximum hardened bead width. The validation results demonstrate that the developed models are accurate with low percentages of error.

Study on Hardness and Microstructural Properties of Ti-6Al-4V Alloy using Overlapped Multipass Laser Transformation Hardening under Optimized Test Conditions

In this research paper, overlapped multipass Laser Transformation Hardening (LTH) of Ti-6Al-4V titanium alloy sheet of 2 mm thickness was analyzed experimentally for uniformly intense, CW spherical beam moving with constant speed using 2 kW Nd: YAG laser. Experiments were conducted for optimized two sets of laser process parameters: 1. High Laser Process Parameter (HLPP), Lp = 800 Watts, Ss = 3000 mm/min, Fp = -10 mm, with heat input 180 J/cm and 2. Low Laser Process Parameter (LLPP), Lp = 600 Watts, Ss = 2000 mm/min, Fp = -10 mm, with heat input 160 J/cm respectively having same Fp = -10 mm. The maximum, minimum and average hardened depths of 0.27, 0.19 and 0.23 mm respectively, achieved for HLPP were found to be minimum, as compared to the maximum, minimum and average hardened depths of 0.38, 0.29 and 0.33 mm, respectively, for LLPP. Measurements of Vickers micro-hardness survey of the hardened zone of the laser processed Ti-6Al-4V alloy are presented. Vickers micro-hardness of an as-received two-phase (α+β) Ti-6Al-4V titanium alloy is 328 HV. The results showed that Vickers micro-hardness on top of the surface (TS), in hardened or fusion zone (Fz), at the interface of Fz -Haz, in the heat affected zone (Haz) is higher than the bulk material. The high hardness values of 450 HV and 445 HV were investigated on the top surface for high and low laser process parameters respectively. This can be the quality characteristics of the dissolution of small amounts of oxygen, nitrogen, and carbon with hard martensite α' (transformed β) formation, thereby ensuring an increase in wear resistance of laser treated hardened surface of Ti-6Al-4V considerably in relation to the untreated or base alloy.

Status of laser transformation hardening of steel and its alloys: a review

The state of laser transformation hardening in surface modification of steel and its alloys is reported in this review paper. The laser hardening process involves the use of high-intensity laser radiation to heat the surface of steel into the austenitic region rapidly. Due to high rates of heat transfer, steep temperature gradients result in rapid cooling by conduction. This causes the transformation from an austenite to a martensite microstructure without the need for external quenching and generates a hard wear-resistant surface. The introduction of high-power lasers, such as neodymium-doped yttrium aluminum garnet (Nd:YAG), diode and fiber lasers, has made laser-assisted hardening an attractive candidate for surface hardening processes. Recently, fiber lasers appeared as a competitor to Nd:YAG and diode lasers for material processing. This review paper is a synopsis of the fundamentals of laser hardening, outlining some of its benefits compared with those of conventional hardening techniques and its industrial applications. A selective review of the experimental and numerical research carried out on steel in this area is also presented. Finally, it is concluded that the overall state of research on laser hardening in surface engineering of steel is well developed, and a still-growing industrial application is observed.

基于光束离散的激光相变硬化蠕墨铸铁温度场数值模拟

基于光束离散的激光相变硬化蠕墨铸铁温度场数值模拟

Experimental Investigation on Hardness Properties of Laser Hardened Bead Profile of Commercially Pure Titanium Grade3 Using Nd:YAG Laser

This research paper presents the laser transformation hardening (LTH) to improve the surface hardness of commercially pure titanium, nearer to ASTM Grade3 chemical composition of 1.6[Formula: see text]mm thickness sheet using a CW (continuous wave) 2[Formula: see text]kW, with radiation wavelength [Formula: see text][Formula: see text][Formula: see text]m Nd:YAG laser. Full factorial and response surface design approach in Design Expert 9 software have been discussed and evaluated by statistical regression analysis and analysis of variance. The experiment was carried out as the full factorial design (FFD) array of 27 with 3 factors, 3 levels, i.e. 3[Formula: see text] experiments. The selected input parameters are: laser power, scanning speed and focused position, and responses are: Vickers Microhardness on top surface, in fusion zone, and in heat affected zone. FFD and response surface methodology (RSM) were applied to evaluate and optimize the effects of laser process parameters on Vickers microhardness of laser hardened surface. The results show that, the hardness of as-received commercially pure titanium is approximately 153[Formula: see text]VHN and the hardness after laser transformation hardened bead geometrical surface is in the range of 200–240[Formula: see text]VHN. The hardness can be increased with the increase in the scanning speed and decrease in the optimum value of laser power i.e. heat input applied. It has been found that the quadratic model is best fitted for prediction of the Vickers microhardness of laser hardened surface. These findings are significant in modern development of hard surface coatings for corrosion and wear resistant applications. Application of experimental results will be considered in the aerospace, marine, chemical, medicine, automobile and the engineering industries.

Determination of optimum regimes of combined laser-ultrasonic hardening and finishing of tool steel AISI D2

To improve the physical and mechanical properties of working surfaces of products, a new method combined laser-ultrasonic surface hardening and finishing using a scanning laser beam ultrasonic multiple impact head was suggested. The surface layers of cold worked tool steel AISI D2 were hardened by laser heat treatment and by ultrasonic impact treatment. The laser transformation hardening process was implemented using a 1 kW solid state fiber-laser with scanning optics and PID closed loop temperature control. The ultrasonic deformation hardening process was carried out by means of a 0.3 kW ultrasonic generator and ultrasonic oscillatory system, which contained a piezoceramic transducer, step-like horn and the impact head with seven cylindrical pins. The single-path processes were studied. This paper is focused on experimental analysis of the effects of heating temperature (1200…1340 °C) and specimen feed rate (40…140 mm/min) used at the laser transformation hardening process on the hardening depth and surface hardness/microhardness of subsurface layer, as well as the effects of the vibration amplitude of ultrasonic horn (15…18 μm) and treatment duration (60…240 s) at ultrasonic impact peening on the surface roughness/waviness and hardness using response surface methods. The experimental plan is based on a miscellaneous design matrix method. The quadratic regression equations for predicting the studied output parameters were developed. The optimum regimes of the laser transformation hardening and ultrasonic impact peening processes were determined based on the highest surface hardness and minimum roughness/waviness. It was established that the process of laser transformation hardening should realize at heating temperature 1250...1300 °C and specimen feed rate 80...100 mm/min, and the ultrasonic impact peening process – at vibration amplitude of ultrasonic horn 18 mm and treatment duration 100...120 s.

Laser transformation hardening effect on hardening zone features and surface hardness of tool steel AISI D2

The relationship of technological input regimes of the laser transformation hardening on change the hardening depth, hardening width, and hardening angle, as well as surface hardness of the tool steel AISI D2 using multifactor experiment with elements of the analysis of variance and regression equations was determined. The laser transformation hardening process implemented by controlling the heating temperature using Nd:YAG fiber laser with scanner, pyrometer and proportional-integral-differential controller. The linear and quadratic regression models are developed, as well as response surface to determine the effect of the heating temperature and feed rate of the treated surface on the energy density of the laser beam, hardening depths, hardening width, hardening angle, and surface hardness are designed. The main effect on the energy density of the laser beam has a velocity laser treatment, on the other hand, the main effect on the geometrical parameters of the laser hardened zone and surface hardness has temperature heating are shown. The optimum magnitudes of the heating temperature (1270 °C) and feed rate of the treated surface (90 mm/min) for laser transformation hardening of the tool steel AISI D2 using fiber laser with scanner were defined.

Prediction of hardness and deformation using a 3-D thermal analysis in laser hardening of AISI H13 tool steel

In this study, a 3-D thermal analysis assisted by a systematic experimental study using a 2kW multi-mode fiber laser was employed to develop predictive models for hardness and thermal deformation in laser transformation hardening of AISI H13 tool steel. Using the thermal model, temperature histories were obtained, which then were processed to compute the effective carbon diffusion time (ECDT) and the effective cooling time (ECT), the two concepts recently proposed by Ki and So [1]. Carefully observing the extensive hardness and deformation measurement data, it was revealed that ECDT is a parameter that correlates well with hardness, and ECT, when extended to the plastic deformation region, shows a strong correlation with deflection angle for both plastic deformation and solid-state phase transformation regions. By finding the minimum specimen thickness that preserves the hardening characteristics of thick steel plates, we were able to increase the deflection angle to∼0.8°, and, after laser hardening, hardness values of up to∼800Hv were obtained. In this study, ECDT and ECT maps for AISI H13 tool steel were computed along with the hardening process window (or heat treatable region), and using the thermal model and experimental data, the minimum specimen thickness that preserves the hardening characteristics of thick plates was determined. This study shows that hardness distributions and thermal deformation behavior can be effectively predicted from a 3-D thermal analysis.

Developments in laser-based surface engineering processes: with particular reference to protection against cavitation erosion

With the increasing demands of economical, reliable and durable hydraulic and pneumatic systems, it is necessary to minimize the material damage from cavitation erosion (CE) when systems are handling cavitating and corrosive fluids. Cavitation erosion is a nausea for many engineering components, such as ship propellers & rudders, turbine, diesel engine, cylinder liner, pump impeller vanes, control valves, hydraulic turbines, bearings, pipes, ultrasonic cleaners and mechanical heart valves, which are exposed to the high-speed flowing or vibratory fluids. This paper reviews the rationale behind the application of laser surface modification for achieving CE resistant surfaces of fluid handling components. The problem of CE may be tackled by enhancing the surface properties of the base materials (ferrous and non-ferrous alloys) with various laser surface modification techniques including laser transformation hardening (LTH), laser surface melting (LSM), laser surface alloying (LSA), laser cladding (LC), laser dispersion (LD) and laser plasma hybrid spraying (LPHS). The CE performance of a variety of laser-surface modified layers/coatings is discussed in this review. In particular, coatings of hard-facing alloys, shape memory alloys, surface metal or intermetallic matrix composites and cermets on ferrous and non-ferrous alloys are included. The mechanisms of the enhancement in cavitation erosion resistance (Re) are discussed.

Erratum to: Modeling of micro-scale fiber laser hardening process and optimization via statistical approximation of the engineering models

Laser transformation hardening is effective technique used for selective hardening of components such as turbine blade, camshafts and gears. Laser hardening provides benefits over other selective hardening processes in terms of thermal distortion, control of process and appearance of component. The fiber laser is relatively recent development. The single/low mode fiber lasers have good beam quality, high wall plug efficiency, fiber delivery and powers up to few hundred watts. The beam quality of single mode fiber lasers enable it to be focused to a spot size of few tens of mm which can yield hardened tracks of 100 to 500 mm. These fiber lasers can be extremely useful in localized micro-scale surface hardening to create hardened patterns for improving the wear resistance. This paper is focused on developing analytical thermal model of moving heat sources and integrating it with kinetic hardening model to capture the metallurgical changes induced by fiber lasers. An ideal surface hardening technique should give widest hardened track at minimum case depth. To address this issue, an optimization methodology based on statistical approximation of the physics-based engineering models has been developed.

Numerical-experimental analysis of the effect of surface oxidation on the laser transformation hardening of Cr–Mo steels

Laser surface hardening is a technology that enables important advantages to be obtained in comparison with conventional techniques in terms of accuracy of the heat affected zone and productivity. Nevertheless, the development of realistic and flexible models has to be fulfilled in order to control the effects of every set of process conditions. Despite many different models having been developed, very few of them deal with the increment of absorption related with the instantaneous value of the layer of oxide growth during the process in a non-protective atmosphere. This work analyzes the problem of oxide formation at the external surface using kinetic relations, whose parameters have been related with the process variables, considering non-equilibrium conditions. Then, the oxide thickness was associated with a value of absorption through an innovative formula that considers the path of the laser radiation in the interface oxide-base material. The thermal calculations obtained by this method have allowed phase changes to be predicted using Avrami law. Both thermal and metallurgical results for different process conditions have been compared with experimental data showing an excellent agreement.

A Comprehensive Model for Laser Hardening of Carbon Steels

This article illustrates the development of a complete and exhaustive mathematical model for the simulation of laser transformation hardening of hypo-eutectoid carbon steels. The authors propose an integrated approach aimed at taking into consideration all the phenomena involved in this manufacturing process, with particular attention to implementing easy mathematical models in order to optimize the trade-off between the accuracy of the predicted results and the computational times. The proposed models involve the calculation of the 3D thermal field occurring into the workpiece and predict the microstructural evolution of the target material exploiting an original approach based on the definition of thermodynamic thresholds, which can be considered as a physical constant of the material itself. Several parameters and phenomena are taken into consideration in order to accurately simulate the process: laser beam characteristics, fast austenization of the steel, and tempering effect due to mutually interacting beam trajectories. The accuracy of the model is presented by means of hardness comparisons between hardness predictions and measurements in single and double paths surface treating of AISI 1040.

Improvement of Slurry Erosion Wear Resistance of 16Cr-5Ni Martensite Stainless Steel by LSA and LTH

Laser surface alloying (LSA), with Wallex-50 and Tribaloy-700 powders, and laser transformation hardening (LTH) reduced the silt erosion rate in 16Cr-5Ni steel. Slurry erosion wear tests were conducted on 16Cr-5Ni steel, LSA-modified steel, and LT-hardened steel at different angles of impingement and velocities at a constant size of silica particles. The LSA-modified steel and LT-hardened steel showed improved wear resistance by about 1.5-3 times as compared to substrate 16Cr-5Ni steel for all angles of impingement. This improvement in hardness and erosion resistance was due to the incorporation of carbides and hard complex phases after LSA and LTH.

Numerical analysis and experimental validation of hardness and microstructural changes during laser transformation hardening of low alloy steel plates

Laser transformation hardening is used to obtain the localized microstructural change, improve wear resistance and hardness to the desired area without affecting the bulk properties. In this study, laser transformation hardening of EN25 steel is performed using a 2 kW continuous wave solid-state neodymium yttrium aluminium garnet laser system. Numerical simulation of laser transformation hardening is carried out by finite element method considering the thermo-physical properties of the material. Based on the numerical simulation the temperature, hardness distribution, hardened depth and width and the phase proportions are determined. The simulation is performed by varying the travel speed of the workpiece. The simulation results are verified with the experimental data and are found to be satisfactory with less than 5% error. It is observed that almost 97% of the phases in the hardened zone is converted into martensite phase with increased hardness about two fold compared to the base material.