Temperature gradient mechanism in laser forming of thin plates

To obtain further insight into the deformation of a plate in the laser forming process, the temperature gradient mechanism (TGM) is studied. Through the investigation, it can be found that, under the processing conditions of TGM, the plate not only bends about the x-axis but also about the y-axis. An analytical model estimate of the bending angle about the y-axis is constructed based on the theories of heat transfer and the mechanics of elastoplasticity. Numerical simulations are carried out to investigate the deformation of the plate about the y-axis by choosing the different process parameters. The analytically based estimate is used to suggest suitable starting values for the simulation process of calculated results. The study of the bending about the y-axis may describe more fully the deformation of a plate, which is helpful in high-precision forming.     

Estimation of laser solid forming process based on temperature measurement

By using a moving disc heat source model, an analytical model was developed to describe laser solid forming (LSF) process with the feedback of the surface temperature of the molten pool, which can be used to estimate the geometric characterizations (width and height) of the clad layer rapidly. An on-line temperature measurement system was established and some single-pass cladding experiments were conducted while the molten pool temperature was monitored. It was found that the estimated geometric characterizations agreed well with the experimental results. In addition, the power consumed by conduction, convection, radiation, evaporation and absorption during LSF were also estimated by the model. It was shown that the majority of the total absorbed power was conducted to the substrate. The effective model can not only be used to optimize the processing parameters but also potentially applied to the real-time feedback control.     

An experimental investigation of underwater pulsed laser forming

Laser forming is a new forming technology, which deforms a metal sheet using laser-induced thermal stresses. This paper presents an experimental investigation of pulsed laser forming of stainless steel in water and air. The effects of cooling conditions on bending angle and morphology of the heat affected zone (HAZ) are studied. It is shown that the case of the top surface in air and the bottom surface immersed in water has the greatest bending angle based on the forming mechanism of TGM. The water layer above the sample decreases the coupling energy, leading to a small bending angle. For a thin water thickness (1 mm), the water effects on the HAZ are limited. As water layer thickness increases (5 mm), the concave shape of the HAZ is more remarkable and irregular because the shock waves by high laser energy heating water are fully developed. However, the area and the depth of the HAZ become less significant when water thickness is 10 mm due to the long pathway that laser undergoes.     

Mold-free fs laser shock micro forming and its plastic deformation mechanism

Mold-free micro forming using a fs laser was investigated by producing micro pits on pure aluminum foil. The characteristics of the pit profiles, their forming mechanisms, and the influences of some important parameters on the pit profiles were investigated by measuring the profiles and the surface morphologies of the pits. The microstructures of the shocked aluminum foil were observed through transmission electron microscopy (TEM). Pits obtained through fs laser shock forming are composed of two regions: the directly impacted region and the plastically bending region. Diameters of the former strongly depend on laser beam sizes. The plastically bending region has a negative effect on forming precision. Shorter laser pulse width is beneficial for narrowing the range of the plastically bending region and enhancing the forming precision. Using a single-side clamping mode can also narrow the plastically bending region through buffering the local bending. Fs laser-induced microstructures are characteristic of fragmentary short dislocation lines and parallel slip lines, which are the results of the ultrafast and ultrahigh pressure loading. The localization of the fs laser shock forming induced by ultrafast loading can enhance the precision of mold-free forming.     

Study of the buckling mechanism in laser tube forming

The buckling mechanism of a thin metal tube during laser forming was investigated numerically and experimentally in this study. Metal tubes made of 304 stainless steel were heated by a CO₂ Gaussian laser beam, which induced the buckling phenomenon on the tube surface due to elastic–plastic deformation. This uncoupled thermal–mechanical problem was solved using a three-dimensional finite element method and was subsequently satisfactorily verified with displacement measurements. The transient bending angle and residual stress of the thin metal tube under specific operation conditions were also studied.     

Effect of laser energy on the deformation behavior in microscale laser bulge forming

Microscale laser bulge forming is a high strain rate microforming method using high-amplitude shock wave pressure induced by pulsed laser irradiation. The process can serve as a rapidly established and high precision technique to impress microfeatures on thin sheet metals and holds promise of manufacturing complex miniaturized devices. The present paper investigated the forming process using both numerical and experimental methods. The effect of laser energy on microformability of pure copper was discussed in detail. A 3D measuring laser microscope was adopted to measure deformed regions under different laser energy levels. The deformation measurements showed that the experimental and numerical results were in good agreement. With the verified simulation model, the residual stress distribution at different laser energy was predicted and analyzed. The springback was found as a key factor to determine the distribution and magnitude of the compressive residual stress. In addition, the absorbent coating and the surface morphology of the formed samples were observed through the scanning electron microscope. The observation confirmed that the shock forming process was non-thermal attributed to the protection of the absorbent coating.     

Mechanics and energy analysis on molten pool spreading during laser solid forming

This paper studies the issue that the molten pool width gradually increases under some conditions during laser solid forming (LSF), which can decrease the shape and dimension accuracy of LSFed component to a large extent. By using the statics analysis method and calculating the interfacial tensions at the solid-liquid-gas triple point of molten pool, the proposed two-dimensional (2D) cross-sectional model of single deposition layer illustrates qualitatively that the deposition width would increase with the increasing pool temperature at a certain powder feeding rate, which we called the pool spread behavior here. Meanwhile, by calculating the maximum equilibrium contact angle for keeping solid-liquid-gas triple point balance, it is found that the molten pool is solidified during non-equilibrium state. Furthermore, in order to control the pool temperature and decrease pool spread amount, the optimal match of pool energy and mass inputs is determined for obtaining an optimum balance between the energy input and deposition efficiencies.     

A conduction model for deep penetration laser welding based on an actual keyhole

Using a specially designed experimental setup and properly choosing the sample material and the process parameters, we obtained a clear stable keyhole with a high-speed camera. On the basis of the actual keyhole profile, a conduction model with a cylindrical surface heat source has been developed under the assumption of the keyhole per thin layer being cylindrical. The model is numerically solved by the finite-difference method, the temperature field around the keyhole and the heat flux lost on the keyhole wall can be obtained. The effects of such factors as the shape and the size of the keyhole, the welding speed on the shape of the melt pool are studied. By comparing the laser intensity absorbed on the keyhole walls with the heat flux lost there, the mechanism of energy balance on the keyhole walls was investigated.     

Research on the temperature field in laser hardening

Temperature field in the laser hardening process was numerically simulated by MSC.Marc software. The influence of energy density on laser hardening effect is analyzed. Simulation result is verified through the thermocouple temperature transducer measuring the specimen surface temperature under the laser irradiation. Experimental curves of temperature versus time are in agreement with simulation results. The simulation results can be regarded as a basis for choosing laser technological parameters.     

An improved lesion detection approach based on similarity measurement between fuzzy intensity segmentation and spatial probability maps

The application of automatic segmentation methods in lesion detection is desirable. However, such methods are restricted by intensity similarities between lesioned and healthy brain tissue. Using multi-spectral magnetic resonance imaging (MRI) modalities may overcome this problem but it is not always practicable. In this article, a lesion detection approach requiring a single MRI modality is presented, which is an improved method based on a recent publication. This new method assumes that a low similarity should be found in the regions of lesions when the likeness between an intensity based fuzzy segmentation and a location based tissue probabilities is measured. The usage of a normalized similarity measurement enables the current method to fine-tune the threshold for lesion detection, thus maximizing the possibility of reaching high detection accuracy. Importantly, an extra cleaning step is included in the current approach which removes enlarged ventricles from detected lesions. The performance investigation using simulated lesions demonstrated that not only the majority of lesions were well detected but also normal tissues were identified effectively. Tests on images acquired in stroke patients further confirmed the strength of the method in lesion detection. When compared with the previous version, the current approach showed a higher sensitivity in detecting small lesions and had less false positives around the ventricle and the edge of the brain.