Metal solid freeform fabrication using semi-solid slurries

A new process for the direct solid freeform fabrication (SFF) of metallic prototypes and components offers a significant advantage over most other metal-SFF processes: it does not involve the use of powders, thus minimizing porosity and shrinkage distortion. This process utilizes the unique rheological and thermophysical properties of semi-solid-metal (SSM) slurries to build a near-netshape metallic component in one step, without the need of sintering, molds, roughmachining, or post-processing operations. A stream of semi-solid is deposited over a moving substrate that follows a three-dimensional pattern. The high viscosity of semi-solid slurries and their particular rheology allows the stream to be deposited over previous layers in a controlled fashion, without traces of an interface. Because the rate of deposition is an order of magnitude faster than in other SFF processes, manufacturing is also faster. In addition, distortion problems characteristic of other processes involving fully molten metal are significantly reduced because the material deposited is already partially solid. In this paper, the first implementation of this technology is presented in detail. Eventually, this process could be useful in the production of a small series of large metallic components that would otherwise be produced by casting or machining. Those processes cost more and result in lower-quality components. For more information, contact P.F. Mendez, Massachusetts Institute of Technology, Department of Materials Science and Engineering, Room 4-133, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139; (617) 253-3204; fax (801) 729-6929; e-mail pat@mit.edu.     

The role of process variables in laser-based direct metal solid freeform fabrication

A fundamental understanding of how process variables relate to deposit characteristics is essential for solid freeform fabrication (SFF) process control. In this article, recent research into modeling and understanding solidification-related phenomena in laser-based direct metal SFF processes is summarized. A “process map” approach is outlined, which provides nonlinear thermomechanical model results for key deposit characteristics over the full range of relevant process variables. Process maps for predicting melt pool size, thermal gradients and maximum residual stress in thin-walled structures are presented, and process-related insights from them are described. Work underway to develop additional process maps, including those for microstructural control, is also summarized. For more information, contact Jack Beuth, Carnegie Mellon University, Department of Mechanical Engineering, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213-3890; (412) 268-3873; fax (412) 268-3348; e-mail beuth@andrew.cmu.edu.     

Surface finish in laser solid freeform fabrication of an AISI 303L stainless steel thin wall

Laser solid freeform fabrication is a flexible manufacturing technique that can be used to produce a near net shape component by consolidation of successive deposited layers of additive materials. This study investigates the effects of the main process parameters, the laser power and scanning speed, on the surface finish of AISI 303L thin walls through reducing the thickness of the deposited layers. To study the effects of the variations of the operating parameters, the process is simulated. The numerical results showed that by increasing the scanning speed the laser power must be adjusted in order to maintain the melt pool maximum temperature within a pre-defined value. In this way, the surface finish of the fabricated walls considerably improves due to the decreased thickness of each individual layer without significantly disturbing the melt pool and the main physical characteristics of the build-ups. This was also verified through experimentally fabrication of the walls and their microstructural examination.     

Fabrication of parts and their evaluation using a dual laser in the solid freeform fabrication system

A solid freeform fabrication (SFF) system using selective laser sintering (SLS) is currently recognized as a leading process for SLS extended application to machinery and automobiles, whose production involves the use of various materials. For the fabrication of a large surface using the SFF system, the dual-laser approach has been introduced in this paper. Since the building room is divided into two regions, each scan path for the dual-laser system is generated based on a single-laser scan path. The scan paths for both lasers have to be synchronized, and the mechanical strength of the large surface must be considered to prevent fracture at the interface region. This paper addresses the generation of a single-laser scan path for special cases such as those involving unnecessary scan points, and the generation of a dual-laser scan path according to various divided regions so to enhance the mechanical strength of the large surface. Moreover, the tensile strength of the specimen manufactured through single-laser scan path system has been compared with the specimen manufacture through dual-laser scan path and the effect of overlapping region with respect to tensile strength is also highlighted.     

电子束熔丝沉积快速制造技术研究现状

电子束熔丝沉积快速制造技术具有制造效率高、真空环境有利于零件保护、内部质量好和可实现多功能加工等优势,非常适合大型金属材料结构的快速制造,近年来成为高能束快速制造技术的研究热点。在对比分析典型高能束快速制造技术的基础上,总结了电子束熔丝沉积快速制造技术的特点,综述了国内外电子束熔丝沉积快速制造技术的研究进展,并展望了电子束熔丝沉积快速制造技术的发展前景。     

国内外电子束熔丝沉积增材制造技术发展现状

随着增材制造技术的不断发展,各种增材制造技术,如电弧增材制造、激光增材制造和电子束增材制造等,在其相应的领域内展开了广泛的研究.文中总结了电子束熔丝沉积增材制造技术的特点.重点介绍了国内外对电子束熔丝沉积技术开展的研究工作,简要介绍了国内外学者在电子束熔丝沉积技术设备和工艺方面取得的最新研究成果.分析了电子束熔丝沉积技术目前亟需展开的研究工作,并展望了该技术应向活泼难熔金属、复合材料、梯度材料制备与大型复杂构件的增材制造等方向发展.     

Cold gas dynamic manufacturing: A non-thermal approach to freeform fabrication

This paper reports on the development of a novel freeform fabrication technique using a cold spray (CS) system. In the CS process, metallic powder particles are accelerated in a supersonic gas jet and impacted with a substrate at speeds in excess of 600 m/s. The non-melting nature of its deposition mechanism ensures that the sprayed material is free from thermally induced tensile stresses, while the underlying substrate remains unchanged. The process is seen as a viable method for additive manufacturing because of its high deposition rates and controllable spray jet. A process was developed to investigate the potential of non-thermal freeform fabrication and was coined Cold Gas Dynamic Manufacturing (CGDM). Here, additive and subtractive techniques were combined to enable the production of complex geometries. Whereas most CS facilities concentrate on the application of wear or corrosion-resistant coatings, CGDM is dedicated to the production of freeform components, whilst still retaining an inherent coating ability. The process can produce functional forms using novel manufacturing strategies that are unique to CS. This paper presents information on the process, and details the various strategies employed during component fabrication. It was possible to construct components from many materials, including titanium, which exhibited freeform surfaces, internal channels and embedded devices. A breakdown of the process economics is also provided, with and without helium recycling.     

Research on lateral instability of the uniform-charged droplet stream during droplet-based freeform fabrication

A droplet initial displacement assumption was made and a dynamic model of the charged droplets was developed to understand the lateral instability of the uniform-charged droplet stream for freeform fabrication. The distribution of charged droplets depending on deposition distance was obtained based on this dynamic model after the droplet charge was calculated by the FEM. Then, a droplet charging and depositing apparatus is set up to generate the uniform-charged droplet stream. The droplet charge was measured and the droplet distributions at different distances were collected in the experiment. The results show that the theoretical radius of droplet distribution area agrees well with the mean experimental value at the deposition distance of 50 cm. The theoretical trend of droplet stream dispersion fits well with experimental result. The maximum accurate deposition distance within which the radius of distribution area is less than one droplet diameter is about 27 cm according to the prediction of droplet distribution, which provides the basis for further accurate metal droplet deposition.     

固体氚增殖材料Li2TiO3制备研究进展

富含6Li的锂基陶瓷Li2TiO3以其可观的锂原子密度、低活化性、优异的化学稳定性、与结构材料良好的相容性以及良好的氚低温释放性能,被公认为是一种综合性能优良、最具前途的固体氚增殖剂之一。详细评述国内外固相反应法及各种湿化学法合成Li2TiO3粉末的研究现状,以及固相法、间接湿法和直接湿法制备Li2TiO3陶瓷小球的工艺现状及研究进展,并对其优缺点进行比较,探讨Li2TiO3陶瓷小球制备的未来发展方向。     

Freeze-form extrusion fabrication of functionally graded materials

Presented in this paper is a novel additive manufacturing technology for making three-dimensional parts with functionally graded materials (FGMs), called Freeze-form Extrusion Fabrication (FEF). The system development included a triple-extruder mechanism, extruder modeling and control, pastes extrusion planning for desired composition gradients, and software coding for motion and extrusion control. The effectiveness of the developed FEF system was demonstrated first by fabricating limestone (CaCO₃) parts with graded colors and then by fabricating ‘green’ parts with graded compositions between alumina (Al₂O₃) and zirconia (ZrO₂). The fabricated part went through post-processing, and the sintered part was analyzed using energy dispersive spectroscopy (EDS) to determine its material compositions.     

Acoustic microscopy of solid materials

Conventional ultrasonic imaging uses frequencies up to 10 MHz or perhaps a little higher. Scanning acoustic microscopy extends the frequencies used to the GHz region, with a corresponding improvement in resolution. The technique is now beginning to be applied to problems in the study of materials. The advantages over other kinds of microscopy lie in the unique ways that ultrasonic waves interact with elastic solids. The development of diffusion bonds and plastic deformation at a stressed notch have been studied using transmission. In reflection, the contrast must be understood in terms of its variation with defocus. This enables images of grain structure in polycrystalline materials to be accounted for. The depth sampled corresponds to a Rayleigh wave profile. Damping mechanisms in the material, due, for example, to dislocations, can affect the contrast. Enhanced contrast is found from surface-breaking discontinuities, such as fine microcracks.     

3D welding and milling: Part I–a direct approach for freeform fabrication of metallic prototypes

Solid Freeform Fabrication (SFF) gives engineers a new freedom to build parts that have thus far proved difficult to manufacture using conventional machining. However, the surface finish and accuracy of SFF parts are lower than those of conventionally machined parts. A process combination of additive and subtractive techniques is currently being developed in order to overcome this problem. A novel hybrid approach of our group called ‘3D welding and milling’ uses gas metal arc welding as an additive and milling as a subtractive technique, thereby exploiting the advantages of both processes. Compared to other deposition processes, gas metal arc welding is the most economic way of depositing metals. In this paper, the initial results of the process development and the characterization of the parts fabricated by this process are reported.     

服役条件下材料制备与加工的思路设想

为了获得性能优异的材料,人们采用仿生的方法,制造显微组织与生物系统材料相似的材料,在复合材料制备等方面获得了成功.但可以看出,目前的仿生方法主要是仿制生物系统材料的显微组织和微观结构,还未涉及材料在应用时的具体条件,而这些具体条件对特定的组织形成往往具有极其重要的作用.     

Dip coating technique in fabrication of cone-shaped anode-supported solid oxide fuel cells

A simple and cost-effective dip coating technique was successfully developed to fabricate NiO-YSZ anode substrates for cone-shaped anode-supported solid oxide fuel cells. A single cell, NiO-YSZ/YSZ/LSM-YSZ, was assembled and tested to demonstrate the feasibility of the technique applied. Using humidified hydrogen (75 ml/min) as fuel and ambient air as oxidant, the maximum power density of the cell was 0.78 and 1.0 W/cm² at 800 and 850 °C, respectively. The observed open-circuit voltages (OCV) was closed to the theoretical value and the scanning electron microscope (SEM) results revealed that the microstructures of the anode substrate and the cathode layer are porous and the electrolyte film is dense.     

Metrology of freeform shaped parts

The scope of this keynote paper is to present the state of the art in the metrology of freeform shapes with focus on the freeform capabilities of the most important measuring techniques and on related metrological issues. Some examples of products are presented, for which the metrology of freeform shapes is important to guarantee the desired functional performance of the product. A classification of freeform measuring tasks and the corresponding metrological requirements are presented. A review of the most important measuring techniques is presented along with their capabilities for freeform measuring tasks. Specification and verification of freeform surfaces, including data evaluation and comparison to specifications are discussed, along with the measurement uncertainty and traceability of freeform measurements.     

Integrated fabrication process for solid oxide fuel cells in a triple torch plasma reactor

This paper describes an approach for an integrated manufacturing process for solid oxide fuel cells. The approach is based on successively depositing the different layers of the cell using plasma deposition processes in a controlled-atmosphere chamber. Cells have been manufactured following this approach with minimal changes in process conditions for the different processes. The cells have been evaluated with regard to their materials characteristics and with regard to their electrical performance. The cell performance has been acceptable, with open circuit voltages of about 1 V and power densities between 325 and 460 mW/cm². Process modifications to improve the performance further are possible. The described process has the potential for being easily automated.     

石墨固体自润滑材料研究现状

石墨材料的结构决定其自润滑的性能表现，并且在复杂环境下比在真空环境下表现出更好的润滑性能，石墨复合材料也表现出良好的润滑性能。但石墨材料的润滑性能并不与石墨含量成正比关系，同时石墨的粒度和结构形态都对其润滑性能有着重要的影响。固体自润滑材料大体分两类：整体自润滑材料和表层自润滑材料，而常见的整体固体自润滑复合材料的工艺方法有半固态铸造和粉末冶金两种。表层固体自润滑材料大多数是采用铸渗工艺。表层自润滑复合材料可以在保证基体材料本身具有优异力学性能的基础上具有自润滑、耐磨损等特殊性能，而且利用表层润滑大大节约了原材料和生产时间，因此表层固体自润滑材料成为将来研究发展的趋势。     

Curvature-adaptive multi-jet polishing of freeform surfaces

This paper presents a curvature-adaptive multi-jet polishing (CAMJP) method that can achieve high efficiency and cater for adaptation of the variation of the material removal rate (MRR) to the curvature of freeform surfaces. CAMJP makes use of a purposely designed multi-jet nozzle incorporated with a pressure control system. The effect of surface curvature on the MRR is analysed by computational fluid dynamic modelling. The fluid pressure of each jet is controlled independently to vary the MRR according to the curvature of freeform surfaces. Experimental results show that CAMJP is effective in improving the accuracy of polishing freeform surfaces.