Bert Huis in 't Veld

Toward 3D Printing of Pure Metals by Laser‐Induced Forward Transfer

3D printing of common metals is highly challenging because metals are generally solid at room conditions. Copper and gold pillars are manufactured with a resolution below 5 μm and a height up to 2 mm, using laser-induced forward transfer to create and eject liquid metal droplets. The solidified drops shape is crucial for 3D printing and is discussed as a function of the laser energy.

Closed Loop Control of Penetration Depth during CO2 Laser Lap Welding Processes

In this paper we describe a novel spectroscopic closed loop control system capable of stabilizing the penetration depth during laser welding processes by controlling the laser power. Our novel approach is to analyze the optical emission from the laser generated plasma plume above the keyhole, to calculate its electron temperature as a process-monitoring signal. Laser power has been controlled by using a quantitative relationship between the penetration depth and the plasma electron temperature. The sensor is able to correlate in real time the difference between the measured electron temperature and its reference value for the requested penetration depth. Accordingly the closed loop system adjusts the power, thus maintaining the penetration depth.

Additive interconnect fabrication by picosecond Laser Induced Forward Transfer

Laser Induced Forward Transfer (LIFT) is a single step, dry deposition process which shows great potential for interconnect fabrication. TNO, in cooperation with ALSI and University of Twente have studied the feature size and resistivity of copper structures deposited using picosecond (ps) LIFT. Small droplets are generated in response to single pulses at high repetition rate (400kHz). It has been shown that with a relatively simple setup, very promising features could be realized. These first results justify further development of the process towards industrial application.

In-plane laser forming for high precision alignment

Abstract. Laser microforming is extensively used to align components with submicrometer accuracy, often after assembly. While laser-bending sheet metal is the most common laser-forming mechanism, the in-plane upsetting mechanism is preferred when a high actuator stiffness is required. A three-bridge planar actuator made out of Invar 36 sheet was used to characterize this mechanism by experiments, finite element method modeling, and a fast-reduced model. The predictions of the thermal models agree well with the temperature measurements, while the final simulated displacement after 15 pulses deviates up to a factor of 2 from the measurement, using standard isotropic hardening models. Furthermore, it was found from the experiments and models that a small bridge width and a large bridge thickness are beneficial to decrease the sensitivity to disturbances in the process. The experiments have shown a step size as small as 0.1  μm, but with a spread of 20%. This spread is attributed to scattering in surface morphology, which affects the absorbed laser power. To decrease the spread and increase the positioning accuracy, an adapted closed-loop learning algorithm is proposed. Simulations using the reduced model showed that 78% of the alignment trials were within the required accuracy of ±0.1  μm.

Spectral analysis of the process emission during laser welding of AISI 304 stainless steel with disk and Nd:YAG laser

Optical emissions from the laser welding process can be obtained relatively easy in real-time. Such emissions come from the melt pool, keyhole, or plume during welding. Therefore it is very beneficial to establish a clear relation between characteristics of these emissions and the resulting weld quality or the ocurrence of welding defects for any of the laser ype currently used in industry. Most commonly used in continuous wave (CW) laser welding applications are the CO2 laser, Nd:YAG laser, diode, disk and fiber lasers. The disk lasers are relatively new and need to be characterized with respect to the nature of their plasma or plume generation. In this paper, the process emissions during laser welding of AISI 304 stainless steel with disk laser and Nd:YAG laser are compared. Broadband spectral analysis between 200 nm and 1100 nm reveals spectral lines as well as the black body radiation emitted from bead-on-plate welding configuration. The relation between the process emission and weld quality is discussed.

High precision micro actuators using in-plane laser forming

For assembly of micro-devices, such as photonic devices, the precision alignment of components is often critical for their performance. Laser forming, also known as laser-adjusting, can be used to create an integrated microactuator to align the components with sub-micron precision after bonding. In this paper a so-called three-bridge planar manipulator was used to study the laser-material interaction and thermal and mechanical behavior of the laser forming mechanism. A 3-D Finite Element Method (FEM) model and experiments are used to identify the optimal parameter settings for a high precision actuator. The goal in this paper is to investigate how precise the maximum occurring temperature and the resulting displacement are predicted by a 3-D FEM model by comparing with experimental results. A secondary goal is to investigate the resolution of the mechanism and the range of motion. With the experimental setup we measure the displacement and surface temperature in real-time. The time-dependent heat transfer FEM models match closely with experimental results, however the structural model can deviate more than 100% in absolute displacement. Experimentally, a positioning resolution of 0.1μm was achieved, with a total stroke exceeding 20μm. A spread of 10% in the temperature cycles between several experiments was found, which was attributed to a spread in the surface absorptivity. Combined with geometric tolerances, the spread in displacement can be as large as 20%. This implies that feedback control of the laser power, in combination with iterative learning during positioning, is required for high precision alignment. Even though the FEM models deviate substantially from the experiments, the 3-D FEM model predicts the trend in deformation sufficiently accurate to use it for design optimization of high precision 3-D actuators using laser adjusting.

High precision laser forming for micro actuation

For assembly of micro-devices, such as photonic devices, the precision alignment of components is often critical for their performance. Laser forming, also known as laser-adjusting, can be used to create an integrated microactuator to align the components with sub-micron precision after bonding. In this paper a so-called three-bridge planar manipulator was used to study the laser-material interaction and thermal and mechanical behavior of the laser forming mechanism. A 3-D Finite Element Method (FEM) model and experiments are used to identify the optimal parameter settings for a high precision actuator. The goal in this paper is to investigate how precise the maximum occurring temperature and the resulting displacement are predicted by a 3-D FEM model by comparing with experimental results. A secondary goal is to investigate the resolution of the mechanism and the range of motion. With the experimental setup we measure the displacement and surface temperature in real-time. The time-dependent heat transfer FEM models match closely with experimental results, however the structural model can deviate more than 100% in absolute displacement. Experimentally, a positioning resolution of 0.1μm was achieved, with a total stroke exceeding 20μm. A spread of 10% in the temperature cycles between several experiments was found, which was attributed to a spread in the surface absorptivity. Combined with geometric tolerances, the spread in displacement can be as large as 20%. This implies that feedback control of the laser power, in combination with iterative learning during positioning, is required for high precision alignment. Even though the FEM models deviate substantially from the experiments, the 3-D FEM model predicts the trend in deformation sufficiently accurate to use it for design optimization of high precision 3-D actuators using laser adjusting.

Spectroscopic closed loop control of penetration depth in laser beam welding process

In-process monitoring and feedback control are fundamental actions for stable and good quality laser welding process. In particular, penetration depth is one of the most critical features to be monitored. In this research, overlap welding of stainless steel is investigated to stably reproduce a fixed penetration depth using both CO2 and Nd:YAG lasers. Plasma electron temperatures of Fe(I) and Cr(I) are evaluated as in process monitoring using the measurement of intensities of emission lines with fast spectrometers. The sensor system is calibrated using a quantitative relationship between electron temperature and penetration depth in different welding conditions. Finally closed loop control of the weld penetration depth is implemented by acquiring the electron temperature value and by adjusting the laser power to maintain a pre-set penetration depth. A PI controller is successfully used to stabilize the electron temperature around the set point corresponding to the right penetration depth starting from a wrong value of any initial laser power different than the set point. Optical inspection of the weld surface and macroscopic analyses of cross sections verify the results obtained with the proposed closed-loop system based on a spectroscopic controller and confirms the reliability of our system.

Closed loop control of laser welding using an optical spectroscopic sensor for Nd:YAG and CO2 lasers

Recent developments in laser joining show the applicability of spectral analysis of the plasma plume emission to monitor and control the quality of weld. The analysis of the complete spectra makes it possible to measure specific emission lines which reveal information about the welding process. The subsequent estimation of the electron temperature can be correlated with the quality of the corresponding weld seam. A typical quality parameter, for laser welds of stainless steel, is the achieved penetration depth of the weld. Furthermore adequate gas shielding of the welds has to be provided to avoid seam oxidation. In this paper monitoring and real-time control of the penetration depth during laser welding is demonstrated. Optical emissions in the range of 400nm and 560nm are collected by a fast spectrometer. The sensor data are used to determinethe weld quality of overlap welds in AISI 304 stainless steel sheets performed both with CW Nd:YAG and CO2 lasers. A PI-controller adjusts the laser power aiming at a constant penetration. Optical inspection of the weld surface and microscopic analysis of weld cross sections were used to verify the results obtained with the proposed closed-loop system of spectroscopic sensor and controller.

Ultra short laser pulse generation of anti-ice surfaces

Wetting properties of many materials can be changed by accurate laser micromachining with ultra short laser pulses (USLP). USLP machining provides a method for creating a surface topography with the specific micro and sub-micrometer scale features which are required for creating water-repellent surfaces. These structures with a high water contact angle and low contact angle hysteresis remain dry, and can be used as anti-ice coatings because of its reduced ice-adhesion strength and easy water roll off. This paper investigates the potential anti-ice properties of laser machined designed patterns with changing geometries, in combination with a CVD deposited hydrophobic coating. The result of a highly adjustable surface topography and a reduced surface energy yields extraordinary water repellent properties.Wetting properties of many materials can be changed by accurate laser micromachining with ultra short laser pulses (USLP). USLP machining provides a method for creating a surface topography with the specific micro and sub-micrometer scale features which are required for creating water-repellent surfaces. These structures with a high water contact angle and low contact angle hysteresis remain dry, and can be used as anti-ice coatings because of its reduced ice-adhesion strength and easy water roll off. This paper investigates the potential anti-ice properties of laser machined designed patterns with changing geometries, in combination with a CVD deposited hydrophobic coating. The result of a highly adjustable surface topography and a reduced surface energy yields extraordinary water repellent properties.