Laser Surface Heat Treatment Research Articles

A new approach to control and optimize the laser surface heat treatment of materials

Laser heating is often used to perform the surface treatment by modifying local microstructural and mechanical properties of components having complex geometries. In this study, the laser surface heat treatment of a rotating cylindrical work-piece was investigated using both experimental and numerical modeling approaches, with an aim to correlate and predict the temperature distribution during the process. The depth of the laser affected zone was predicted by solving the transient heat transfer with a moving laser heat source, using finite element analysis. The temperatures derived from the microstructural examination of the experimental specimen were found to closely agree with the predicted results from the numerical simulations. The numerical and experimental results have also led to a new observation, indicating a linear variation of the absorptivity with the laser scan speed. The prediction of the cooling curves from simulation suggested the β→αʺphase transformation and the recovery of the β phase, and the existence of new phases were confirmed through electron microscopy. The rapid cooling during the laser surface treatment was found to induce a flake-structure that consisted of both martensite (αʺ) and regained bcc (β) phase. A new polynomial input power function has been proposed to achieve uniform distribution of the heat penetration along the cylinder axis, saving about 10% of the material wastage.

Microstructural Characterization of Surface Softening Behavior for Cu-Bearing Martensitic Steels after Laser Surface Heat Treatment

The surface hardening and softening behavior of two types of medium carbon martensitic steel (AISI P20-improved and AISI P21) after laser-assisted heat treatment was quantitatively compared. The laser-assisted heat treatment was performed using a high-power diode laser with in situ temperature and laser power control (two-color pyrometer system). For AISI P20-improved steel, the peak hardness value within the hardening zone was approximately 640 HV after laser-assisted heat treatment at a temperature of 1473 K. In other words, the hardness increased by 120% from the base metal level (290 HV). However, for AISI P21 steel, the hardness within the heat-treated zone did not change from that of the base metal (410 HV), despite being accompanied by martensite transformation. Moreover, it was clearly observed that the hardness dropped below the level of the base metal at the boundary between the heat-treated zone and the base metal region, forming a softening zone. This softening behavior was strongly related to coarsening and a looser distribution of Cu precipitates compared with that of the base metal region, despite the same matrix phase (i.e., tempered martensite) existing in the softening zone and in the base metal region.

Fabrication of un-coated transparent superhydrophobic sapphire surface using laser surface ablation and heat treatment

Sapphire is a widely used hard transparent material for optics and protective windows, and a superhydrophobic coating on sapphire can prevent contamination by self-cleaning. However, the coating can be easily degraded according to time and heat. In this research, transparent superhydrophobic sapphire surfaces were fabricated via laser surface ablation, without coating, for good stability and heat resistance. The laser ablated surface showed hydrophilic initially, but the wettability transition to superhydrophobic was achieved after an additional simple heat treatment. Contact angle and transmittance were measured to confirm the superhydrophobicity and transparency, and surface analysis was performed to explain the wetting mechanism.

Laser surface heat treatment of electroless Ni–P–SiC coating on Al356 alloy

Electroless Ni–P–SiC coatings are recognized for their hardness and wear resistance. In the present study, electroless Ni–P coatings containing SiC particles were co-deposited on Al356 substrate. Laser surface heat treatment was performed using 700W Nd:YAG pulsed laser. Effects of different laser operating parameters, such as laser scan rate, laser average power and defocusing distance on microstructures were investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). The results of microstructural characterization indicated that the laser treatment under different operating conditions produced composite coating contained nanocrystallined Ni-based matrix with SiC particles Ni3P, Ni12P5, Ni5P2, Ni8P3 precipitates. The microhardness measurements showed that the hardness of the coating was increased up to 60%, due to laser heat treatment, without effect on base metal.

The Influence of Heat Treatment and Process Parameters Optimization on Hardness and Corrosion Properties of Laser Alloyed X12CrNiMo Steel

Martensitic stainless steels are used in the production of steam turbine blades but their application is limited due to low hardness and poor corrosion resistance. Laser surface alloying and heat treatment of X12CrNiMo Martensitic stainless steel was conducted with the aim of enhancing hardness and corrosion properties. A Rofin Sinar Continuous Wave Nd: YAG solid-state laser was used to alloy the specimens. The electrochemical and hardness properties were studied using potentiodynamic polarization technique and Vickers micro hardness tester. The microstructures of the as-received, post-heated and pre-heated specimens were investigated by a Scanning Electron Microscope (SEM) and Optical Micrograph (OM) respectively. From the experimental results, the post-heated specimens exhibited the highest hardness property as compared to all other specimens. There was also significant improvement in the corrosion resistance of the post heated specimen compared to all other specimens and the substrate as evidenced by higher polarization resistance and lower corrosion rates. From the analysis of grey relational grade model, the significant laser processing parameters were identified. The results showed the influence of laser power and scanning speed on the corrosion rate, hardness and alloyed depth. The predicted results were found to be in good agreement with the experimental results.

Wavelet Packet Decomposition to Characterize Injection Molding Tool Damage

This paper presents measurements of acoustic emission (AE) signals during the injection molding of polypropylene with new and damaged mold. The damaged injection mold has cracks induced by laser surface heat treatment. Standard test specimens were injection molded, commonly used for examining the shrinkage behavior of various thermoplastic materials. The measured AE burst signals during injection molding cycle are presented. For injection molding tool integrity prediction, different AE burst signals’ descriptors are defined. To lower computational complexity and increase performance, the feature selection method was implemented to define a feature subset in an appropriate multidimensional space to characterize the integrity of the injection molding tool and the injection molding process steps. The feature subset was used for neural network pattern recognition of AE signals during the full time of the injection molding cycle. The results confirm that acoustic emission measurement during injection molding of polymer materials is a promising technique for characterizing the integrity of molds with respect to damage, even with resonant sensors.

Detection of Damaged Tool in Injection Molding Process with Acoustic Emission

ABSTRACTThe measurement of acoustic emission (AE) signals during injection molding of polypropylene with new and damaged mold is presented. The damaged injection mold was fitted with a steel insert with cracks induced by laser surface heat treatment. Two resonant piezoelectric AE sensors were attached to the mold via AE waveguides. To improve the mold integrity prediction with smaller defects, AE signal frequency characteristics and a measure of AE signal amplitude probability distribution are implemented. A 5-dimensional feature vector with real-valued explanatory variables is proposed, providing the defining points in an appropriate multidimensional space to characterize the state of injection molding tool. Feature vectors are classified with neural network pattern recognition. The results confirm that presented AE technique offers characterizing the integrity of molds also with resonant sensors.

Integrated Process of Laser-Assisted Machining and Laser Surface Heat Treatment

A process is proposed for integrating the laser-assisted machining (LAM) and laser surface heat treatment (LSHT) in a single operation. Experimental and numerical investigations were carried out. LSHT tests were performed to investigate the effect of the process parameters on the microstructure evolution and hardenability. A methodology and an empirical model for prediction of hardened depth were proposed. A two-dimensional finite element (2D-FE) model was developed to predict the phase transformation during the LAM and LSHT processes. The optimization of the LAM process was also investigated using the developed finite element model.

Laser Surface Heat Treatment and Modification

Abstract Laser surface modification (LSM) techniques provide new pathways to improving the performance of engineered structures and components, either by altering surface metallurgy or chemistry or by adding a functional surface layer.

Analysis of Latest Application Movement of Laser Surface Heat Treatment Technologies

The application of two representative laser surface treatments technologies which are laser shock processing technology and laser coating treatment technology are described. The application of laser shock processing in the fields of aero engine disc, wing panel formation, structural welding and strengthening, nuclear power station pressure container strengthening etc and also evaluates the effects are analyzed. The applications of laser coating treatment in the field of aeronautic manufacturing are analyzed in detail. The development direction of laser shock processing and laser coating treatment are indicated finally.

The Process Study on Laser Heat Treatment for CNC Machine Tool Guide

With the continuous development of CNC technology, CNC machine tools run at the higher speed. Cutting force will be greater, which is bound to exacerbate the wear and tear of the machine tool guides. Therefore, new requirements of numerical control machine tool guide surface hardness are proposed. Using Laser surface heat treatment process to scan the guide surface of CNC Machine Tool, the guide can get high hardness hardened layer. After being casted iron HT300 was heat treated by the laser, the microstructure is homogeneous, very fine martensite. Its Surface hardness can get to HV832. The treatment enhances the surface abrasion resistance, scratch-resistant capacity.

Laser surface hardening of AISI H13 tool steel

An attempt was made to improve the surface hardness and wear properties of AISI H13 tool steel through solid solution hardening and refinement of microstructures using a 200 W fiber laser as a heat generating source. The hardness of laser melted zone was investigated. In order to identify the effect of heat input on the laser melting zone, scanning conditions were controlled. The results show that, the hardness of as-received AISI H13 tool steel is approximately Hv 240, and the hardness after laser surface heat treatment is around Hv 480–510. The hardening depth and width are increased with the increase in the heat input applied. Application of experimental results will be considered in tooling industry.

Nanoscale evaluation of laser-based surface treated 12Ni maraging steel

Maraging steels are used in several high-tech areas. Among them are highly thermo-mechanically loaded vital parts of die casting dies for pressure die casting of aluminium and magnesium alloys. From the economic point of view, the operation life of dies is extremely important to the price of the castings. Operational life can be successfully extended by a regular maintenance of die parts. Laser surfacing is a very promising process for rebuilding of worn out surfaces of vital die parts. In this research, the state in the maraging steel 1.2799 (DIN) after the application of laser surfacing process has been analysed using scanning electron microscope. The analysis revealed diverse microstructure through-depth of the laser-surfaced specimens. On the basis of the estimated size and volume fraction of the nano-precipitates in the individual microstructure zones located through-depth of the heat-affected zone, a through-depth variation of microhardness was predicted. The results are supported by Vickers microhardness tests. It was confirmed that the mechanical properties of the 1.2799 maraging steel strongly depend on the characteristic at the nano or micro level. Some of the results obtained can be also applied to laser surface heat treatment of maraging steels.

Heat transfer analysis of laser heated surfaces — conduction limited case

The laser surface heat treatment technique is widely spread in industry for the treatment of the engineering parts. To improve the heat treatment process, study of the heat transfer mechanism appropriate to the laser heating is necessary. The present study develops an analytical solution to the laser heating process using the Fourier heat conduction model. The temperature distribution inside the substance together with its variations in the direction perpendicular to surface are predicted. The validity of the Fourier theory in laser heating is also discussed. To validate the theoretical predictions, the measurement of surface temperature is carried out. The study is extended to include the effect of the laser output pulse lengths.

A Study on the Thermal and Residual Stress by Welding and Laser Surface Hardening Using a new Two-Dimensional Finite Element Model

The transient thermal stress and residual stress in bead-on-plate welding and laser surface hardening was analysed by employing a new two-dimensional finite element model. In the formulation, the sliced solution domain possessing one element in the welding or hardening direction was introduced to allow the change of longitudinal total strain along the width and depth of the specimen. The longitudinal total strain, which makes the resultant force in the longitudinal direction zero, was used as the boundary condition. To determine the longitudinal total strain, the equations of the compatibility, force equilibrium and moment equilibrium were considered. Calculation of the transient temperature distribution was based on the two-dimensional finite element model of the quasi-stationary condition. By using the proposed model the thermal and residual stresses of the bead-on-plate welding and laser surface heat treatment were successively calculated. Especially in laser surface hardening, the thermal stress was induced mainly from the temperature gradient and martensitic phase transformation, which was found to have a greater influence on the residual stress than the former. The computed results were compared with those obtained by the conventional two-dimensional model of the plane strain condition. The simulation results revealed that the present model could more reasonably describe the thermal and residual stresses than the plane strain model.

A study on residual stresses in laser surface hardening of a medium carbon steel

The transient thermal stress and the residual stress in laser surface-hardening treatment of a medium carbon steel were analysed by employing a new two-dimensional finite element model. In this formulation, a sliced solution domain, having one element in the hardening direction, was introduced to satisfy the self-equilibrium of the resultant force in the hardening direction. By using the proposed model, the thermal and residual stresses in the laser surface heat treatment were successively calculated. The thermal stress was induced mainly by the temperature gradient and the martensitic phase transformation; the phase transformation was found to have a greater influence on the residual stress than the temperature gradient. The simulation results revealed that a compressive residual stress region occurs near the hardened surface of the workpiece and a tensile residual stress region occurs in the interior of the workpiece, whereas the maximum tensile residual stress occurs along the centre of the laser scanning path in the interior region (y = 0). In comparison with the gaussian distribution of the beam power, the square beam mode results in a wider, but shallower, hardened zone. The calculation results also showed that the high-power beam with the high scanning speed is more suitable for laser surface hardening than the low-power beam with the low scanning speed if the heat input per unit length of the workpiece is maintained constant.

Fatigue Characteristics of Laser Surface-Hardened Cast Irons

Contrary to expectations, laser surface heat treatment has a deleterious effect on the fatigue performance of pearlitic gray and ductile cast irons. A 1.2 kW, continuous wave, CO2 gas laser, operating in square beam mode, was employed to heat-treat the surfaces of standard fatigue specimens. Rotational-bending fatigue tests were then conducted on untreated (as-cast) and laser treated specimens. Results indicated that the effect on fatigue behavior of case depth, microstructure and hardness of laser-hardened surface layers were opposite to that observed in carbon and low alloy steels. A fracture model based on the presence of graphite, residual stresses, and strain-induced transformation is postulated to explain the adverse effect of laser hardening of cast irons.

Laser surface heat treatment of AISI 4340 steel: A microstructural study

The effect of laser surface heat treatment on the microstructure of AISI 4340 steel was investigated and compared with conventional furnace heat treatments. The microstructural variables, i.e. prior austenite grain size, substructure of martensite, amount of retained austenite and its distribution and carbide precipitations, were carefully studied using transmission electron microscopy. The two significant observations were the presence of fine cementite particles of 200 Å size and large amounts of thin films of retained austenite in the as-quenched structures. The microstructure exhibited was fully martensitic, the substructure of which contained extensive twins. The average prior austenite grain size was measured to be 35 μm. A comparison with conventional furnace heat treatment (870°C treatment) showed that there was an enhancement in the amount of retained austenite and precipitation of fine cementite in the laser-heat-treated specimens. However, the prior austenite grain size and martensitic substructure remained essentially the same as those of conventional heat treatment.