Haihong Zhu

Microstructural evolution in direct laser sintering of Cu‐based metal powder

Purpose – To identify the effects of laser scan speed and scan spacing on surface morphology, microstructure and structure evolution in direct laser sintering of Cu‐based metal powder.Design/methodology/approach – Scanning electron microscope, differential thermal analyser (DTA) and X‐ray diffractometer were used to examine the microstructure of the sintered parts.Findings – It was found that the decrease of the scan speed and scan spacing could lead to densification due to solute‐reprecipitation mechanism. The formation of oxide Cu2O is sensitive to the scan spacing due to the lack of Cu3P protection under the re‐heating condition if using small scan spacing. Furthermore, the result shows that there exist two mechanisms in determining the phosphor distribution. During the laser sintering, concentration diffusion acts as the main mechanism at a fast scan speed and a large scan spacing while solute‐reprecipitation acts as the main mechanism at a low scan speed and small scan spacing.Originality/value – Thi...

Effect of the gap on discharge characteristics of the three-electrode trigatron switch

A mathematical model was proposed to calculate the breakdown time based on the theoretical analysis of the discharging mechanism for three-electrode trigatron switch with the trigger electrode located inside the main electrode. The simulation result demonstrates that the breakdown time decreases with operating voltage increasing. The dependence on voltage is weak for the high voltages. A threshold voltage exists and threshold voltage increases with the main gap. If the charging voltage is enough high, the breakdown time is about same for both main gaps. Experimental result agrees with the simulation result very well.

Investigation on Selective Laser Melting AlSi10Mg Cellular Lattice Strut: Molten Pool Morphology, Surface Roughness and Dimensional Accuracy

AlSi10Mg inclined struts with angle of 45° were fabricated by selective laser melting (SLM) using different scanning speed and hatch spacing to gain insight into the evolution of the molten pool morphology, surface roughness, and dimensional accuracy. The results show that the average width and depth of the molten pool, the lower surface roughness and dimensional deviation decrease with the increase of scanning speed and hatch spacing. The upper surface roughness is found to be almost constant under different processing parameters. The width and depth of the molten pool on powder-supported zone are larger than that of the molten pool on the solid-supported zone, while the width changes more significantly than that of depth. However, if the scanning speed is high enough, the width and depth of the molten pool and the lower surface roughness almost keep constant as the density is still high. Therefore, high dimensional accuracy and density as well as good surface quality can be achieved simultaneously by using high scanning speed during SLMed cellular lattice strut.

Heat Transfer and Thermal Deformation Characteristics of Liquid-Cooled Laser Mirror

A coupled finite volume-element method is developed to simulate the transient thermal deformation of water-cooled mirror by considering fluid flow and convective heat transfer. The simulation process consists of two steps: the 3D finite volume models of fluid flow and heat transfer equation are solved to obtain the time-dependent temperature field by using CFD; then, the obtained temperature field used as final temperature field is unidirectionally coupled to the finite element model for solving the thermoplastic equation. It is concluded that fluid flow not only affects the magnitude of temperature rise and thermal deformation, but also affects the distribution of temperature and thermal deformation. The temperature gradient in the thickness direction (z direction) is found to be much larger than that in transverse direction. It is found that the temperature and the consequent deformation of water-cooled mirror increase significantly in the first seconds and gradually become steady state in the subsequent time. Experiments are conducted to estimate the precision of numerical models, and the experimental results agree well with the simulated results.

Fabrication of water-cooled laser silicon mirror by direct laser sintering

Coppery heat sink with micro pores and Sub-millimeter channel has been fabricated by direct laser sintering on the back of the silicon mirror. To verify the heat dissipation capability of the fabricated heat sink, a Twyman-Green interferometer was employed to measure the thermal deformation of the silicon mirror radiated by a high power laser. It is shown that the thermal deformation of the mirror increases with the irradiating time and laser intensity. The heat balance can be achieved after several seconds of laser irradiation even when the net absorbed laser power density is up to 5.3 × 10(5) W/m(2). The time for reaching the heat balance also increases with the laser intensity. The maximum thermal deformation of the mirror is 0.65 µm if the net absorbed laser power density is 5.3 × 10(5) W/m(2).

Fabrication and heat treatment of high strength Al-Cu-Mg alloy processed using selective laser melting

The proposed paper illustrates the fabrication and heat treatment of high strength Al-Cu-Mg alloy produced by selective laser melting (SLM) process. Al-Cu-Mg alloy is one of the heat treatable aluminum alloys regarded as difficult to fusion weld. SLM is an additive manufacturing technique through which components are built by selectively melting powder layers with a focused laser beam. The process is characterized by short laser-powder interaction times and localized high heat input, which leads to steep thermal gradients, rapid solidification and fast cooling. In this research, 3D Al-Cu-Mg parts with relative high density of 99.8% are produced by SLM from gas atomized powders. Room temperature tensile tests reveal a remarkable mechanical behavior: the samples show yield and tensile strengths of about 276 MPa and 402 MPa, respectively, along with fracture strain of 6%. The effect of solution treatment on microstructure and related tensile properties is examined and the results demonstrate that the mechanical behavior of the SLMed Al-Cu-Mg samples can be greatly enhanced through proper heat treatment. After T4 solution treatment at 540°C, under the effect of precipitation strengthening, the tensile strength and the yield strength increase to 532 MPa and 338 MPa, respectively, and the elongation increases to 13%.

Effects of peak laser power on laser micro sintering of nickel powder by pulsed Nd:YAG laser

Purpose – The purpose of this paper is to report the influence of the peak laser power on laser micro sintering 4-μm nickel powder using Q-switched 1064-nm Nd:YAG laser. Design/methodology/approach – Experimental study has been performed. Nickel powder with grain size of 4 μm has been utilized. A Q-switching duration of 20-25 μs and rate of 20-40 kHz have been used. Findings – The peak power intensity is so high that the metal particles and molten pool are blown away, leading to laser micro sintering not being successfully proceeded. The scanning line obtained by continuous-wave (CW) laser looks like a rod owing to balling effect. Using a suitable peak power intensity, a good-shaped sintering line can be obtained because the plasma can protect the molten metal from oxidation, and improve the wettability of the system. In addition, the plasma flattening effect may also contribute to the form of the good-shaped sintering line in pulsed laser sintering regime. Originality/value – The role of plasma induced b...

Study of Deformation of Water-Cooled Laser Mirror Used in High Power Laser System

The transient thermal deformations of laser water-cooled mirror surface with minichannels have been numerically studied. The theoretical results show that fluid flow has a great influence on temperature distribution and thermal deformation. The maximum of temperature and thermal deformation appear in the downstream area, which is different from the previous research results. Meanwhile, experiments with different working conditions are conducted to validate the numerical simulation, and the numerical results agree well with the experimental data. The emulation method of thermal deformation of water-cooled mirror used in this study is proved to be effective research method.© 2013 ASME

Effect of the Q‐switch parameters on the sintering behavior of laser micro sintering Cu‐based metal powder using Q‐switched Nd‐YAG laser

Purpose – The purpose of this paper is to investigate the effect of the Q‐switching parameters on the sintering behavior of laser micro sintering Cu‐based metal powder, using Q‐switched 1064u2009nm Nd‐YAG laser.Design/methodology/approach – An experimental study has been performed. Metal powder mixture with Cu and Cu‐P alloy powders has been utilized. Q‐switching duration of 15u2009μs∼25u2009μs, rate of 25u2009kHz∼45u2009kHz have been used.Findings – The results show that as the Q‐switching rate and duration increases, the peak laser power decreases and the densification enhances. However, an optimal peak laser power exists and if the peak laser power is too low, the density of the sample is also low. The densification regime of laser micro‐sintering is not only caused by the liquid phase filling the pores, but is also caused by the Cu powder migrating and by coalescence, e.g. including initial stage and intermediate stage of the traditional furnace liquid phase sintering. However, the degree of these stages depends on the p...

Emission properties of tin droplets laser-produced-plasma light sources

Laser produced plasma (LPP) light sources for extreme ultraviolet (EUV) lithography currently has been extensively studied. Most of the studies are based on CO2 laser induced plasma from mass limited tin targets. In this work, a droplet dispenser that produces uniform droplets size of about 150μm was established. A pulsed TEA-CO2 laser and a Nd: YAG laser irradiated the droplets producing plasma respectively to get EUV emission. An X-ray Spectrometer and EUV photodiodes were used to collect the spectra and EUV radiation. The different EUV spectral composition and angular distribution of EUV emission from plasmas induced by the CO2 and Nd: YAG laser were studied.