A new hybrid electrochemical-mechanical process (PEMEC) for polishing complex and rough parts

A new polishing process for metallic parts has been developed by simultaneously coupling drag finishing and electrochemical polishing. This new hybrid process, called PEMEC, enables an improvement in the surface roughness within some minutes, and at the same time, preserves the shape of the edges. This hybrid process is based on a synergistic effect between abrasive and chemical mechanisms. It has confirmed its suitability to be applied on complex and rough surfaces produced by additive manufacturing SLM parts.     

Interfacial Characteristics and Mechanical Behavior of Hybrid Manufactured AlSi10Mg-Al6061 Bimetal via Selective Laser Melting and Forging

Hybrid manufacturing (HM) uses additive manufacturing (AM) techniques to prepare complex or fine structures on traditional processing parts, which can fully utilize the advantages of AM to form complex parts, and the high efficiency and low cost of conventional processing methods in manufacturing regular components. A bimetal material was produced by selective laser melting (SLM) of AlSi10Mg on the deformed Al6061 alloy substrate in this study. Firstly, the interface characteristics of the two alloys were studied. The results show that a metallurgical interface with a thickness of 100-200 μm was formed as the result of the Marangoni convection during SLM preparation. In detail, the circular flow in the melt pool at the interface led to the dilution of alloying elements and the change of microstructure. Moreover, a suitable first-layer thickness can effectively suppress the hot cracks at the interfacial region. Based on the optimized results, the hybrid manufactured samples with different SLM processed volume ratios (SLM-part) to the substrate were prepared to study the mechanical properties and the deformation behavior of hybrid parts. The results show that the volume ratio of the SLM-part to the substrate had an influence on the strength and the elasto-plastic behavior by affecting the distribution of plastic deformation.     

Research and Development in NiTi Shape Memory Alloys Fabricated by Selective Laser MeltingEI北大核心CSCD

由于NiTi形状记忆合金(SMAs)具有高反应敏感性和低热导率等物性,导致其初步成形件的后续加工十分困难,作为一种典型的金属增材制造技术,选区激光熔化(SLM)在近净成形复杂几何形状的金属构件方面具有显著优越性,能够有效解决NiTiSMAs冷加工难、加工成本高的问题。为实现SLMNiTiSMAs的工程应用,需厘清其工艺参数-微观结构-功能特性的内在联系,揭示其相转变行为与功能特性变化的机理,建立坚实的理论基础。基于此,本文重点对SLMNiTi SMAs的成形性、相转变行为、微观结构、力学性能和热机械性能的相关研究结果进行了分析与总结。同时,对近来SLM多孔NiTiSMAs的设计及其生物相容性的探索研究进行了阐述。最后,本文展望了SLMNiTiSMAs研究过程中需要重点突破的问题。     

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.     

Microstructures and Mechanical Properties of Ti6Al4V Parts Fabricated by Selective Laser Melting and Electron Beam Melting

This work compares two metal additive manufacturing processes, selective laser melting (SLM) and electron beam melting (EBM), based on microstructural and mechanical property evaluation of Ti6Al4V parts produced by these two processes. Tensile and fatigue bars conforming to ASTM standards were fabricated using Ti6Al4V ELI grade material. Microstructural evolution was studied using optical and scanning electron microscopy. Tensile and fatigue tests were carried out to understand mechanical properties and to correlate them with the corresponding microstructure. The results show differences in microstructural evolution between SLM and EBM processed Ti6Al4V and their influence on mechanical properties. The microstructure of SLM processed parts were composed of an α′ martensitic phase, whereas the EBM processed parts contain primarily α and a small amount of β phase. Consequently, there are differences in tensile and fatigue properties between SLM- and EBM-produced Ti6Al4V parts. The differences are related to the cooling rates experienced as a consequence of the processing conditions associated with SLM and EBM processes.     

Experimental investigations into rapid prototyping of composites by novel hybrid deposition process

Solid Freeform Fabrication (SFF) gives engineers a new freedom to build parts that have been impossible to manufacture using conventional techniques. However, the surface finish and accuracy of SFF parts remain lower than those of parts that have been machined in conventional methods such as milling. A process combination of additive and subtractive techniques is currently being developed by our group at KIST to overcome this problem. The hybrid approach called “3D welding and milling” uses welding as an additive, and conventional milling as a subtractive technique. As part of this process development, two different building strategies have been developed to build multi-material parts directly. The results prove the applicability of the 3D welding and milling process for rapid prototyping of bimetallic parts. A significant potential application is for the rapid prototyping of injection mold inserts.     

Selective Laser Melting Process of Fe-Ni Metal Powder

The density of the SLM forming parts is investigated to determine the good technological parameters of SLM at the same time obtain the dense parts. In the SLM experiment, material used is Fe-Ni metal powder, the good technological parameters of SLM are determined by analyzing the effect of the laser electric current, the laser pulse width, the pulse of laser light frequency, the scan speed, the scan interval, push powder thickness and the scanning way on the single channel scanning, the single-layer scannin...     

基于混合推理的切削参数优选系统开发

针对现有航空发动机结构件切削加工参数数据库数据分散、针对性和准确性不强等问题,通过分析航空发动机结构件的加工工艺,提炼出影响切削加工参数选择的主要因素;据此,建立适用于航空发动机结构件加工的切削参数数据库系统,提出一种基于规则混合推理的加工参数智能优选方法;进而利用NX二次开发技术,开发出基于混合推理的切削参数优选系统,实现了切削参数的自动加载,从而有效减轻工艺人员设计负担,提高零件编程效率。最后,在航空发动机典型结构件上验证了所开发系统的有效性。     

A pragmatic model for selective laser melting with evaporation

Selective laser melting (SLM) is a Rapid Manufacturing technique in which parts of complex shape are produced by selectively melting layers of powder. A thorough understanding of the process is crucial for a good control of the properties of the produced parts. In this paper we present a pragmatic engineering model to study aspects of the SLM process using an enthalpy formulation and accounting for shrinkage and laser light penetration. We investigate the importance of evaporation for a set of process parameters relevant to production and find that evaporation is a phenomenon that cannot be neglected at realistic power inputs. However, phenomena such as Marangoni convection and wetting due to surface tension effects need to be considered in the future.     

Development of a new tool to generate compressive residual stress within a machined surface

Generally, critical machined parts such as aircraft parts require high fatigue strength and resistance to stress corrosion cracking. These machined parts almost all usually have tensile residual stress within the machined surface after milling. However, if the compressive residual stress within the machined surface can be obtained by a milling process alone, it is expected that fatigue strength and resistance to stress corrosion cracking of the machined components will be improved. The purpose of this study is to develop a new tool that can generate compressive residual stress within the machined surface concurrently with the milling process. This tool has cutting edges for material removal and a projection pin for a burnishing-like process. It was shown that the proposed cutter could generate effective compressive residual stress within the machined surface during the milling process. Residual stress levels were in the region of −100 to −200 MPa on the machined surface, and −300 to −400 MPa at 0.05 mm beneath the surface. These levels are almost comparable with those obtained by the shot peening process.     

Basic study on the process combination of deposition welding and subsequent hot bulk forming

Today most technical parts and components are made of monolithic materials. Nevertheless, the previously used monolithic materials reach their technological and constructive limits, so that an improvement of the component properties can be realized by hybrid parts. Forging of previously joined semi-finished products to net shape hybrid components is a promising method to produce functional adapted parts in a few process steps. This new process chain offers a number of advantages compared to other manufacturing technologies. Examples are the production of specific load-adapted forging parts with a high level of material utilization, an improvement of the joining zone caused by the followed forming process and an easy to implement joining process because of the simple geometries of the semi-finished products. This paper describes the production process of hybrid steel parts, which are produced by a combination of a deposition welding process with a subsequent hot forging (upsetting) or cross-wedge-rolling. It could be shown that the innovative process chain enables the production of hybrid parts whereby the forging processes lead to an improvement of the mechanical properties of the laser deposited material.     

体能量密度对SLM成形316L不锈钢耐腐蚀性的影响

为了探究不同体能量密度对SLM成形316L不锈钢耐腐蚀性的影响,采用正交试验法制备不同激光功率、扫描间距和扫描速度下的SLM 316L不锈钢成形件,利用扫描电镜和电化学试验对其微观组织和自腐蚀电位进行观察和测量。结果表明,体能量密度过大或过小时,成形件表面的气孔和孔洞等缺陷较多,自腐蚀电位减小,耐腐蚀性变差。体能量密度为44.64 J/mm^-3时,SLM 316L不锈钢成形件的自腐蚀电位最高,组织表面的气孔等缺陷相对较少,耐腐蚀性最好。激光功率、扫描间距和扫描速度对SLM 316L不锈钢成形件的耐腐蚀性影响的次序为:激光功率＞扫描间距和扫描速度,最佳的工艺参数组合为激光功率250 W,扫描间距0.14 mm,扫描速度800 mm/s。     

车加工带凸台直管夹具设计

设计夹具,变不能加工部位为可加工部位,扩大车床加工范围,提高加工效率。该设计采用6点定位,V形块支撑和夹紧,夹具操作简单,使用方便。     

Flexible manufacturing of metallic products by selective laser melting of powder

In order to produce metallic parts directly from powder material using CAD data, the selective laser melting (SLM) process has been developed. From a series of material tests, nickel-based alloy, Fe alloy and pure titanium powders are found to be feasible for fabrication of metallic models by SLM. Finite element simulation shows stress distribution within the solid single layer formed on the powder bed during forming and some methods for avoiding defects in the products are suggested. The die for metal forming from the nickel-based alloy and the pure titanium models of bone and dental crown are demonstrated. The density of the model made by SLM is higher than 90% of the solid model. The mechanical properties of the formed model can be improved to those of the solid by post-processing.     

On Optimization of Surface Roughness of Selective Laser Melted Stainless Steel Parts: A Statistical Study

In this work, the effects of re-melting parameters for postprocessing the surface texture of Additively Manufactured parts using a statistical approach are investigated. This paper focuses on improving the final surface texture of stainless steel (316L) parts, built using a Renishaw SLM 125 machine. This machine employs a fiber laser to fuse fine powder on a layer-by-layer basis to generate three-dimensional parts. The samples were produced using varying angles of inclination in order to generate range of surface roughness between 8 and 20 µm. Laser re-melting (LR) as post-processing was performed in order to investigate surface roughness through optimization of parameters. The re-melting process was carried out using a custom-made hybrid laser re-cladding machine, which uses a 200 W fiber laser. Optimized processing parameters were based on statistical analysis within a Design of Experiment framework, from which a model was then constructed. The results indicate that the best obtainable final surface roughness is about 1.4 µm ± 10%. This figure was obtained when laser power of about 180 W was used, to give energy density between 2200 and 2700 J/cm² for the re-melting process. Overall, the obtained results indicate LR as a post-build process has the capacity to improve surface finishing of SLM components up to 80%, compared with the initial manufactured surface.     

Surface roughness analysis,modelling and prediction in selective laser melting

Selective Laser Melting (SLM) is an increasingly employed additive manufacturing process for the production of medical, aerospace, and automotive parts. Despite progresses in material flexibility and mechanical performances, relatively poor surface finish still presents a major limitation in the SLM process.In this study an investigation of surface roughness and morphology is presented for Steel 316L alloy parts made by SLM. In order to characterise the actual surfaces at different sloping angles, truncheon samples have been produced and an analysis has been conducted at different scales, by surface profilometer and scanning electron microscope (SEM). The surface analysis has showed an increasing density of spare particles positioned along the step edges, as the surface sloping angle increases. When layer thickness is comparable to particle diameter, the particles stuck along step edges can fill the gaps between consecutive layers, thus affecting the actual surface roughness.Classic models for roughness prediction, based on purely geometrical consideration of the stair step profile, fail to describe the observed trend of the experimental data. A new mathematical model is developed to include the presence of particles on top surfaces, in addition to the stair step effect, for the accurate prediction of surface roughness. Results show that surface roughness predicted by this model has a good agreement with the experimentally observed roughness. The paper investigates the key contributing factors influencing surface morphology, and a theoretical model for roughness prediction that provides valuable information to improve the surface quality of SLM parts, thus minimising the need of surface finishing.     

磨料流加工中夹具设计的研究

磨料流加工(AFM)是光整加工领域的一项新技术。磨料流加工由磨料流加工机床,夹具和流体磨料三部分组成。当用磨料流加工形状较复杂的零件时,夹具设计的好坏对表面加工质量和加工效率有重要的影响。本文分析了磨料通过工件孔道的特性。介绍了用于研究材料去除量与加工距离和形状关系的实验。材料去除量在通道两端比中间小是磨料流加工的一基本特性。本文讨论了如何避免因此而产生的工件变形以及如何利用这一特性来获取某种特殊效果。最后,给出了一个用于加工某不锈钢三通体的夹具。     

新型飞机用铝合金薄壁件的选区激光熔化成形研究

高效率、短流程、一体化成形复杂金属件对新型飞机结构优化、成本降低具有重要意义。针对飞机用复杂薄壁零件开展选区激光熔化(Selective Laser Melting,SLM)成形AlSi10Mg合金设计优化和工艺研究,讨论了激光功率、扫描间距、保护气氛和热处理温度对成形质量的影响。结果表明,在较优工艺参数下,SLM成形AlSi10Mg合金组织致密,孔洞少,沉积态室温拉伸性能相较于铸件有明显提高;零件内部与表面质量良好,热处理后零件侧壁各方向尺寸与设计值正负偏差在0.2 mm以内,满足新型飞机复杂零件装配设计要求。     

Investigation of the coating thickness of plasma-transferred arc deposition welded and cross wedge rolled hybrid parts

Most of today’s technical parts and components are made of monolithic materials. These mono-material components produced in established production processes reach their limits due to their respective material characteristics. Thus, a significant increase in production quality and efficiency can only be achieved by combining different materials in one part. Bulk forming of previously joined semi-finished products to net shape hybrid components that consist of two different materials is a promising method to produce parts with locally optimized characteristics. This new production process chain offers a number of advantages compared to conventional manufacturing technologies. Examples are the production of specific load-adapted forged parts with a high level of material utilization, an improvement of the joining zone caused by the following forming process and an easy to implement joining process due to the simple geometries of the semi-finished products. This paper describes the production process of hybrid steel parts, produced by combining a plasma-transferred arc deposition welding process with a subsequent cross wedge rolling process. This innovative process chain enables the production of hybrid parts. To evaluate the developed process chain, coating thickness of the billet is analysed before and after cross wedge rolling. It could be shown, that the forming process leads to an improvement of the coating, meaning a more homogeneous distribution along the main axis.     

Selective laser sintering/melting (SLS/SLM) of pure Al,Al–Mg,and Al–Si powders: Effect of processing conditions and powder properties

Selective laser sintering/melting (SLS/SLM) processing difficulties of aluminium powders had been attributed to issues associated with laser–materials interaction only while neglecting the role of powder properties. This study provides a wholistic understanding of factors that influence the development of SLS/SLM processing window, densification, and microstructure of pure Al, Al–Mg, and Al–Si powders, fabricated in single and multiple layer parts by exploring the roles of processing and material parameters. It was demonstrated that similarities existing in the SLS/SLM processing maps of the powders could be attributed to similarities in their packing densities with the alloying addition of magnesium and silicon having no predominant effect on their processing maps’ boundaries. Rather, alloying addition has significant effect on the nature of the evolved surface morphology of SLS/SLM processed aluminium powders in their processing windows. In addition, the flow and solidification behaviour of the melt pool of the powders during single layer scan was strongly influenced by the particle morphology and oxygen content of the powders as well as applied energy density. The energy density in the range of 12–16 J/mm² was found to be the threshold below which SLS was predominant and above which SLM occurred for the investigated powders. Moreover, successful oxide disruption phenomena which is necessary for inter-particulate coalescence in multi-layered SLS/SLM processed aluminium powders are found to be mainly controlled by the amount of oxide in the as-received powder, the degree of the uniformity of the distribution of the surface oxide film covering the aluminium particles, the nature of thermal mismatch existing between the oxide film and the parent aluminium particle which was dependent on the phase present in the oxide film. Al–12 wt% Si powder is hereby affirmed as a suitable candidate material for SLS/SLM process due to its low thermal expansion and uniform distribution of its surface oxide films as well as the mullite phase in its oxide film.