Energy-Beam redistribution and absorption in a drilling or welding cavity

Energy redistributed and absorbed in the cavity produced by a high-intensity beam during drilling and welding is systematically and quantitatively investigated. The incident energy flux is assumed to be a realistic Gaussian distribution, and the cavity is idealized by a paraboloid of revolution having both specular and diffuse reflectivities. By using a Monte Carlo method, the results show that the energy absorbed by the cavity wall deviates markedly from the Gaussian distribution. Effects of the cavity depth-to-opening radius ratio, absorptivity, and specular reflectivity on the absorbed energy are presented. A comparison of incident energy fluxes of uniform and Gaussian distributions is also made.     

激光焊接速度对焊缝组织和硬度分布的影响

利用激光焊接工艺焊接核级不锈钢,通过改变激光焊接速度得到不同的焊接接头.采用金相显微镜、扫描电子显微镜等手段研究了熔深比、δ/γ比、组织形貌以及焊缝中各相的成分.通过显微硬度测试了焊缝接头上硬度分布.随着焊接速度的增加,焊缝熔深比线性增加,焊缝中δ-铁素体含量增加,岛状组织增多,板条状组织增多,蠕虫状组织减少,焊缝区的平均硬度值增加.     

CO2 laser beam welding of 6061-T6 aluminum alloy thin plate

Laser beam welding is an attractive welding process for age-hardened aluminum alloys, because its low heat input minimizes the width of weld fusion and heat-affected zones (HAZs). In the present work, 1-mm-thick age-hardened Al-Mg-Si alloy, 6061-T6, plates were welded with full penetration using a 2.5-kW CO₂ laser. Fractions of porosity in the fusion zones were less than 0.05 pct in bead-on-plate welding and less than 0.2 pct in butt welding with polishing the groove surface before welding. The width of a softened region in the-laser beam welds was less than 1/4 times that of a tungsten inert gas (TIG) weld. The softened region is caused by reversion of strengthening β″ (Mg₂Si) precipitates due to weld heat input. The hardness values of the softened region in the laser beam welds were almost fully recovered to that of the base metal after an artificial aging treatment at 448 K for 28.8 ks without solution annealing, whereas those in the TIG weld were not recovered in a partly reverted region. Both the bead-on-plate weld and the butt weld after the postweld artificial aging treatment had almost equivalent tensile strengths to that of the base plate.     

Microstructural modification of plain carbon steels irradiated by high-energy electron beam

The main objective of the present study is to analyze the microstructural modification of the surface hardened by the irradiation of high-energy electron beam in 0.18 pct C and 0.38 pct C plain carbon steels. Steel samples were irradiated using an electron accelerator (1.4 MeV), and the detailed microstructures of the irradiated surface were examined. Upon irradiation, the ferrite-pearlite structure near the sample surface was changed to the dual-phase structure, i.e., ferrite and martensite, and fine particles or needlelike lamellae were observed in the ferrite/martensite interface. In order to investigate these complex microstructures as well as the martensitic transformation mechanism, the simulation test, including thermal cycles of abrupt heating and quenching, was carried out. The test results indicated that the irradiated surface was heated up to about 1100°C and then quenched to room temperature, which was enough to obtain the surface hardening through martensitic transformation. Thermal analysis of the irradiated surface was also carried out for systematic understanding of the microstructural modification in terms of the irradiation parameters such as beam travel speed.     

Electron and laser beam welding of high strain rate superplastic Al-6061/SiC composites

The welding characteristics of a fine-grained 6061 Al and three 6061/1, 5, and 20 pct SiC composites under high energy electron beam welding (EBW) and laser beam welding (LBW) were examined. The three composites exhibited high strain rate superplasticity (HSRS). The 6061 Al was more readily welded by EBW than by LBW, and the situation was reversed in the reinforced composites. In the reinforced composites, the fusion zone contained the once fully melted matrix and fully reacted SiC, and the heat affected zone (HAZ) contained the partially melted matrix and nearly unreacted SiC. This effect was particularly apparent in the 20 pct SiC composite. With increasing SiC content from 0 to 20 pct, the reflection of the laser beam decreased, and the melt viscosity increased due to the increasing amount of Al₄C₃ compounds. For the HSRS fine-grained 6061/20 pct SiC composite, there formed a sharp V-notch under EBW. The high viscosity or low fluidity of the melt inside the fusion zone of 6061/20 pct SiC resulted in incomplete backfill and notch formation. The postweld mechanical performance and joint efficiency both became seriously degraded. The original fine structures in the HSRS composites could not be restored after welding.     

Investigation of the temperature field developed by a spinning beam in laser processing

Various laser treatments have been developed for metallic surface modification. In these processes, rapid heating of a specific area on the surface by a laser is the critical feature employed to produce a different phase or layer on the surface. The laser beam mode, such as a Gaussian, rectangular, or annular beam in a stationary or spinning state, has been found to have a very important effect in the laser processing. Many significant models have been established to estimate the temperature field developed by a laser and therefore to predict the optimum conditions in the process, but these models are mainly applicable to a stationary beam. Previous work has shown the advantages in some applications of using a spinning beam. Therefore, modeling work for a spinning beam is necessary. The present article reworked our previous model on a spinning beam mode for a continuous CO₂ laser, to calculate a two-dimensional temperature profile by using a line source and superposition of a number of Gaussian sources. An excellent agreement with experimental work for a nitrided Ti-6Al-4V alloy (IMI 318) for a situation of a small (50-μm) melt pool was achieved. A relationship was derived between the normalized laser power and specimen speed to produce a uniformly thick surface layer.     

Study of dental materials by laser beam scanning

Several properties of dental materials are characterized by dimensional variation induced by different parameters. In this work a non-contact method used for measurement of linear dimensional variation is shown. The accuracy is +/- 1 micron for sample length ranging from 1 to 70 mm.