Laser Piercing Research Articles

Laser cutting path optimization with minimum heat accumulation

A new approach for minimizing both cutting path and heat accumulation in laser cutting process is presented in this paper. The proposed algorithm was based on a memetic algorithm combining a powerful genetic algorithm with an adaptive large neighborhood search. The cutting path problem was modeled and solved in accordance with generalized traveling salesman problem. A heat conduction model was incorporated into the cutting path optimization by assigning a critical radius of heat-affected zone as a constraint. A penalty was given to the cutting paths overlapping the heat-affected zone to minimize the heat accumulation in workpiece. A heat map indicating thermal gradient around the laser piercing points was also presented together with the optimum cutting path to visualize the level of heat accumulation. By comparing to the approach without the heat constraint, the algorithm incorporating with the heat effect was capable of determining the laser cutting path with minimum travel distance and heat accumulation in workpiece. The presented method could further be applied for other thermal-related processes such as surface hardening, welding, and additive manufacturing, whose processing distance and heat issue need to be optimized for gaining their productivity and final product quality.

Influence of Process Parameters in Laser Piercing

This work addresses the analysis of the influence of process parameters in laser piercing for mild steel, stainless steel, and aluminum thick sheets, carried out by the monitorization of the signal with a photodiode installed coaxially in the cutting head. The sensor captures the infrared or visible signal emitted during the piercing. The relationship between the intensity of the signal and the parameter values was analyzed, distinguishing between soft and fast piercings. The results permit the optimization of the piercing strategy with a reduction of 25% in time and the possibility of establishing a threshold to control the piercing process. This study reveals the importance of knowing parameter dependencies with the process results and highlights the potential of monitorization systems in laser cutting to improve the piercing duration and avoid wasting time during production.

Dynamic laser piercing of thick section metals

Before a contour can be laser cut the laser first needs to pierce the material. The time taken to achieve piercing should be minimised to optimise productivity. One important aspect of laser piercing is the reliability of the process because industrial laser cutting machines are programmed for the minimum reliable pierce time. In this work piercing experiments were carried out in 15 mm thick stainless steel sheets, comparing a stationary laser and a laser which moves along a circular trajectory with varying processing speeds. Results show that circular piercing can decrease the pierce duration by almost half compared to stationary piercing. High speed imaging (HSI) was employed during the piercing process to understand melt behaviour inside the pierce hole. HSI videos show that circular rotation of the laser beam forces melt to eject in opposite direction of the beam movement, while in stationary piercing the melt ejects less efficiently in random directions out of the hole.

Investigation of the piercing process in laser cutting of stainless steel

This paper investigates the laser piercing process which precedes nearly every laser cutting operation. The two most important aspects of the piercing process are: (a) How long does it take to pierce the material? and (b) How wide is the pierced hole? If the hole is no wider than the cut line, the material can be pierced on the line to be cut. In this paper, 10 mm thick stainless steel was pierced by a multikilowatt fiber laser to compare efficiency and quality when piercing with a continuous wave output and a selected range of power modulation parameters. The different processes were observed by high speed imaging and subsequently examined by visual observation. High speed imaging is used to time the penetration event and to study the laser-material interactions involved in drilling the pierced holes. The results show that appropriate laser power modulation settings can considerably reduce both the piercing time and the required energy to generate any piercing hole required for the subsequent cutting process. This pulse-pierce technique and the differences between piercing with a continuous and a power modulated laser beam are further explained and discussed. Also, the effect on the size of the entrance to the pierced hole depending on power modulation regimes was investigated in this paper.

Development of concurrent stamping-piercing trimless hot stamping process

After a material undergoes hot stamping, post-processes are required to ensure that the final product has the expected quality. Typical post-processes are mechanical or laser trimming and piercing. These post-processes are expensive and time consuming because when the material undergoing hot stamping cools, its strength becomes very high. The purpose of this study was to develop a concurrent stamping–piercing trimless hot stamping process in which the hot stamping and hot piercing processes could be carried out in one operation for manufacturing automotive side sills. To achieve this purpose, first, a trimless blank was designed by carrying out an integrated numerical analysis including computational fluid dynamics and finite element analyses and employing an optimization technique. The process conditions for the hot piercing process were set based on the results of the finite element analysis of hot piercing process. Finally, the effectiveness of the developed hot stamping process was validated by performing a hot stamping experiment.

Fibre laser piercing of mild steel – The effects of power intensity, gas type and pressure

Laser piercing is used to generate a starting point for laser cutting. The pierced hole is normally larger than the kerf width, which means that it cannot lie on the cut line. An experimental programme investigating the piercing process as a function of laser and assist gas parameters is presented. An Nd:YAG fibre laser with a maximum power of 2kW was used in continuous wave mode to pierce holes in 2mm thick mild steel. Oxygen and nitrogen were used as assist gases, with pressures ranging from 0.3 to 12bar. The sizes, geometries and piercing time of the holes produced have been analysed. The pierced hole size decreases with increasing gas pressure and increasing laser power. Oxygen assist gas produced larger diameter holes than nitrogen. A new technique is presented which produces pierced holes no larger than the kerf with and would allow the pierced hole to lie on the cut line of the finished product – allowing better material usage. This uses an inclined jet of nitrogen when piercing prior to oxygen assisted cutting.

Development of a Lens Driving Maglev Actuator for Laser Beam Off-Axis Cutting and Deep Piercing

In laser beam cutting and laser piercing process, the machining speed and quality are very sensitive to the flow of assist gas and laser beam focusing position. In order not only to improve the cutting speed and the removal capability of the molten material and to save the consumption of the gas flow in laser beam cutting, but also to realize high speed piercing of high aspect-ratio holes, a magnetic-levitated (maglev) lens driving actuator was proposed and fabricated. The actuator can drive the lens to achieve real-time positioning control of the relative radial displacement between the lens axis and the assist gas jet nozzle axis (off-axis control) in radial directions in a range of ±1mm within 1.5 μm of tracking error and bandwidths more than 150Hz, and to achieve real-time positioning control of laser beam focusing point in axial direction in a range of ±5mm within 3 μm of tracking error and bandwidth more than 100Hz.

A power ramped pulsed mode laser piercing technique for improved CO 2 laser profile cutting

Laser piercing is one of the inevitable requirements of laser profile cutting process and it has a direct bearing on the quality of the laser cut profiles. We have developed a novel power ramped pulsed mode (PRPM) laser piercing technique to produce much finer pierced holes and to achieve a better control on the process parameters compared to the existing methodology based on normal pulsed mode (NPM). Experiments were carried out with both PRPM and NPM laser piercing on 1.5-mm-thick mild steel using an in-house developed high-power transverse flow continuous wave (CW)-CO 2 laser. Significant improvements in the spatter, circularity of the pierced hole and reproducibility were achieved through the PRPM technique. We studied, in detail, the dynamics of processes involved in PRPM laser piercing and compared that with those of the NPM piercing.

Monitoring of laser processing using induced current under applied electric field on laser produced-plasma

A new electrode circuit for measuring the induced current from laser plasma produced during laser material processing has been devised. The new electrode circuit has greatly increased the possible separation between the electrodes and the workable probing distance of the electrodes from the processed sample surface, namely 38 and 22 mm, respectively, sufficing the requirements of actual industrial laser material processing, avoiding direct contact of the electrodes with the big plasmas and profound sputtering. Several possible electrode configurations were investigated to accommodate the specific needs of the actual laser processing. It was shown that this technique is a very effective tool for accurately determining the moment of completion of penetration, as required in the laser piercing process. In contrast to this, the optical method currently in use cannot accurately determine the piercing completion moment due mainly to the disturbance from smoke arising at the processing point. A preliminary work using a CW CO 2 laser normally used in the actual industrial material processing was also made, resulting in a strong induced current signal corresponding to the laser pulse duration and frequency. The signal was successfully used for monitoring the penetration completion in a stainless steel plate drilling process. This reveals that this technique can be used for monitoring actual industrial material processing using high power lasers.

Laser microfabrication of L605 cobalt-chromium cardiovascular stent implants with modulated pulsed Nd:YAG laser

Photonics modulation techniques can be effectively used for the laser microfabrication of biomedical implants to achieve better flexibility and control of the available optical power. These enable the processing and manufacturing of biomedical implants like cardiovascular stents and micro-metallic components with reduced heat affected zone (HAZ) and extremely precise edge cutting. Our investigation deals with the study of acousto-optic modulation of a pulsed Nd:YAG laser beam and further profile cutting of thin cobalt-chromium tubes for cardiovascular stent manufacturing. The acousto-optic modulator used in the investigations is a Bragg diffraction device. During one piercing process, the focused laser power is gradually varied from a low value and reaches the maximum as the beam goes deeper into the material. The optimum value for dwell time during which the laser piercing is occurring is found as 58 ms. A 14-step staircase modulating voltage with a maximum AC component of 900 mV was used, and the laser power was 4.4 W in TEM00 operation. Pulsed Nd:YAG laser modulated using an acousto-optic device in the Bragg regime is capable of cutting extremely complex geometries of stents on L605 tubing. Laser cutting resulted in a kerf width of 22.5±0.5 µm. Precise strut dimensions of 112±15 µm for the subsidiary strut, 140±15 µm for the main strut, and 140±15 µm for the link were also obtained. Scanning electron microscopy (SEM) images of L605 stents illustrate the higher strength Co-Cr alloy L605 with the same ductility, its ability to be translated into thinner struts, and improved deliverability. L605 also exhibits enhanced radio visibility thanks to its tungsten content and exceptional work hardening rate for a uniform expansion with minimal recoil.

Micromachining of 316LVM stainless steel tubes using periodic acousto-optic modulation of pulsed Nd:YAG lasers for cardiovascular stent applications

Optoelectronic techniques can be used in laser applications in biomedical instrumentation to achieve better flexibility and control of the available optical power. These techniques enable the processing and manufacturing of biomedical implants like cardiovascular stents and micrometallic components with reduced heat-affected zone (HAZ) and extremely precise edge cutting. The present investigations deal with the study of laser piercing routines and further profile cutting of thin stainless steel sheet tubes for biomedical implant manufacturing. The process is performed using an acousto-optic (AO) modulator-based pulsed Nd:YAG laser. The AO modulator used in the experiments is a Bragg diffraction device. During the piercing of holes, the focused laser power is gradually increased from a low value and finally reaches the maximum as the beam goes deeper into the material. The most suitable value of dwell time is found to be 50 ms. A nine step staircase modulating voltage with a maximum ac component of 800 mV is used, and the laser power is 4 W in TEM00 operation. The acousto-optic modulator-based pulsed Nd:YAG laser is capable of cutting extremely complex geometries of stents on 316LVM tubing. Laser cutting results in a kerf width of 20.5±0.5 µm. Precise strut dimensions of 115±15 µm for subsidiary strut, 150±15 µm for main strut, and 150±15 µm for the link are also obtained. With the piercing routine, extremely fine holes with reduced heat-affected zones are produced, and this quality is reflected during consequent machining of required profiles for implants.

Inert gas cutting of titanium sheet with pulsed mode CO 2 laser

The present work aimed at studying the dynamic behavior of melt ejection in laser cutting of 1 mm thick titanium sheet and to obtain dross-free cuts with minimum heat affected zone (HAZ). CO 2 laser cutting of titanium sheet was carried out with continuous wave (CW) and pulsed mode laser operation with different shear gases namely argon, helium and nitrogen. Laser cutting with high frequency and low-duty cycle pulse mode operation produced dross-free cuts with no noticeable HAZ. Helium, because of its high heat convection and ability to generate high shear stress, produced laser-cuts with narrow HAZ and low dross, as compared to those produced with argon as the shear gas. Microscopic features of laser cut surfaces were analyzed and correlated with dynamic mechanism involved in laser cutting process. Process parameters for laser piercing, required for the initiation of fusion cut within the sheet, were also studied. Laser piercing requires either CW or high-duty cycle (>80%) pulse mode operation.

レーザ穿孔難易性に及ぼす工作物材料物性値の影響

This paper describes the influence of physical properties of workpiece material on the difficulty in laser piercing through measurement of the absorption factor change with extension of the depth of hole pierced by a YAG laser. To measure the absorption factor, which is defined as the ratio of the power absorbed by the workpiece to the incident laser power, the inverse problem solution was utilized. Regardless of material properties, the absorption factor increases with the increase of the depth of the keyhole, and after reaching about 100%, the increase is stagnated and the absorption factor is saturated at 100% followed by a rapid decrease after the penetration. Thus it was concluded that the absorption factor in laser piercing is determined not by the intrinsic reflectivity of workpiece material but by the depth of the hole being processed. Consequently, difficulty in cutting copper workpiece by laser beam results from the high thermal diffusivity of copper rather than its reflectivity.