Potential fatigue strength improvement of AA 5083-H111 notched parts by wire brush hammering: Experimental analysis and numerical simulation

The effects of milling as machining process and a post-machining treatment by wire-brush hammering, on the near surface layer characteristics of AA 5083-H111 were investigated. Surface texture, work-hardening and residual stress profiles were determined by roughness measurement, scanning electron microscope (SEM) examinations, microhardness and X-ray diffraction (XRD) measurements. The effects of surface preparation on the fatigue strength were assessed by bending fatigue tests performed on notched samples for two loading stress ratios R0.1 and R0.5. It is found that the bending fatigue limit at R0.1 and 107 cycles is 20% increased, with respect to the machined surface, by wire-brush hammering. This improvement was discussed on the basis of the role of surface topography, stabilized residual stress and work-hardening on the fatigue-crack network nucleation and growth. The effects biaxial residual stress field and surface work-hardening were taken into account in the finite element model. A multi-axial fatigue criterion was proposed to predict the fatigue strength of aluminum alloy notched parts for both machined and treated states

An Evaluation of Ultrasonic Shot Peening and Abrasive Flow Machining As Surface Finishing Processes for Selective Laser Melted 316L

Additive Manufacturing, and specifically powder bed fusion processes, have advanced rapidly in recent years. Selective Laser Melting in particular has been adopted in a variety of industries from biomedical to aerospace because of its capability to produce complex components with numerous alloys, including stainless steels, nickel superalloys, and titanium alloys. Post-processing is required to treat or solve metallurgical issues such as porosity, residual stresses, and surface roughness. Because of the geometric complexity of SLM produced parts, the reduction of surface roughness with conventional processing has proven especially challenging. In this Thesis, two processes, abrasive flow machining and ultrasonic shot peening, are evaluated as surface finishing processes for selective laser melted 316L. Results of these experiments indicate that AFM can reliably polish as-built internal passages to 1 µm Ra or better but is unsuitable for passages with rapidly expanding or contracting cross-sections. AFM can also polish relatively small passages, but lattice components may prove too complex for effective processing. USP cannot achieve such low surface roughness, but it is a versatile process with multiple advantages. Exterior surfaces were consistently processed to 1.7 to 2.5 µm Ra. Interior surfaces experienced only partial processing and demonstrated high geometric dependence. USP significantly hardened the surface, but steel media hardened the surface better than ceramic media did. Both AFM and USP are recommended processes for the surface finishing of SLM manufactured parts. Good engineering judgement is necessary to determine when to use these processes and how to design for post-processing

Microstructural features controlling very high cycle fatigue of nitrided maraging steel

Maraging steels belong to the group of ultra-high strength materials and are often used in critical aerospace, automotive and tooling components. By applying a surface treatment such as nitriding, the fatigue and wear resistance can be improved. The microstructural features that influence the (very) high cycle fatigue response of nitrided maraging steels are studied in this work. Although the used steel has practically no inclusions, it was found that small surface imperfections, introduced during processing, may form potential fatigue initiation points. The samples are nitrided during aging in order to form nitrided layers with various thicknesses, microstructures and hardness profiles without formation of a continuous (compound) iron nitride layer. Data from microhardness tests, scanning electron microscopy, electron backscatter diffraction, x-ray diffraction and transmission electron microscopy were used to characterize the microstructure of the layers. Bending fatigue tests were employed to evaluate the fatigue response of the steel. It was found that the best fatigue behavior is obtained in samples with a thin diffusion zone with a narrow constant hardness region. In this zone, coherent disc-shaped nitride precipitates are detected with TEM

A Study of Ultrasonic Nanocrystal Surface Modification on the Materials Fabricated via Direct Energy Deposition

The laser metal deposition(LMD) technology has garnered much attention for being able to realize complex shapes and producing little waste material, thereby addressing the limits of existing metal product processing. Direct energy deposition (DED), a branch of laser melting deposition technology, can realize precise structures and excellent mechanical properties, and thus, it is used for various applications such as strengthening the surface of products(hardfacing) or remodeling and repairing the damaged components. Howerver, surfaces that have been deposited by DED exhibit poor quality because the deposition bead layer shape and spatter remain on the surfaces. The surface characteristics affect not only the appearance of the product but also its mechanical functions and lifetime, making post-milling processing indispensable for refining the surface after the DED process. Furthermore, In a metal additive manufacturing process, metal powder is melted by a high-power laser before solidification; thus, a rapid temperature change is experienced, which generates tensile residual stress. The tensile residual stress in the DEDed M4 may reduce the fatigue performance and shorten the product life. UNSM technology not only improving surface roughness and refining surface microstructure by applying severe plastic deformation(S2PD) to the material surface, but also induces the compressive residual stress and improves the wear-resistance of the materials. Given that UNSM can improve a material's surface performance, in this study, UNSM technology is applied to DEDed surfaces with the aim of improving the DED product's surface quality. It was found to the UNSM technology has a beneficial effect on reducing the surface roughness and waviness by improving the poor surface of DED products. Also, it was confirmed that the UNSM technology can induce a compressive residual stress to the DED product and improve mechanical performance including wear resistance.List of Tables ⅲ List of Figures ⅳ Nomenclatures ⅸ Abstract ⅹ 1. 서 론 1 1.1 연구 배경 1 1.1.1 레이저 금속 적층된 제품의 표면 특성 2 1.1.2 직접 용융 적층법 2 1.2 연구 동향 4 1.2.1 적층 소재 표면 개선을 위한 연구 사례 4 1.2.2 적층 소재의 기계적 성능 개선 및 표면 처리 4 1.3 연구 목적 10 2. 초음파 나노표면개질 11 2.1 초음파 나노표면개질 11 2.2 기초 실험 15 2.2.1 실험 재료 22 2.2.2 DED 적층 조건 및 UNSM 조건 설정 23 2.3 실험 결과 25 2.3.1 적층 표면 구조 변화 25 2.3.2 표면부 미세조직 변화 30 3. UNSM 표면 개질 요인 효과 분석 35 3.1 반응표면분석 실험 설계 35 3.2 공정 조건에 따른 표면 특성 분석 42 3.2.1 정적 하중의 영향 43 3.2.2 인터벌의 영향 47 3.2.3 이송 속도의 영향 50 3.3 반응표면분석법을 이용한 공정 최적화 55 3.3.1 실험 결과 및 분석 55 3.3.2 UNSM 공정 최적화 67 4. DED 고속도공구강에 대한 UNSM 효과 분석 80 4.1 실험 장비 및 재료 80 4.2 실험 방법 84 4.2.1 표면 거칠기 분석 84 4.2.2 미세조직 관찰 84 4.2.3 표면 잔류응력 측정 85 4.2.4 경도시험 85 4.2.5 스크래치 시험 86 4.2.6 내마모성 시험 86 4.3 실험 결과 88 4.3.1 표면 거칠기 변화 분석 88 4.3.2 미세조직 변화 분석 94 4.3.3 표면 잔류응력 변화 분석 99 4.3.4 경도 변화 분석 103 4.3.5 스크래치 저항성 분석 107 4.3.6 내마모성 분석 112 5. 결론 118 참고문헌 121Maste

Entwicklung und Analyse eines mechanischen Oberflächenbehandlungsverfahrens unter Verwendung des Zerspanungswerkzeuges

Bei der Herstellung hochbeanspruchter metallischer Bauteile werden mechanische Oberflächenbehandlungsverfahren eingesetzt, um geforderte Bauteileigenschaften, wie die Schwingfestigkeit zu erreichen. Dazu werden Randschichtzustände, wie die Topographie, Eigenspannungen, Kaltverfestigung und/oder Mikrostruktur durch eine mechanische Oberflächenbehandlung gezielt beeinflusst. Eine mechanische Oberflächenbehandlung ist ein zusätzlicher Prozessschritt in der Prozesskette und erhöht die Produktionszeit und -kosten. Daher werden verschiedene Hybridverfahren entwickelt, bei denen eine mechanische Oberflächenbehandlung in einen vorhergehenden Prozessschritt integriert wird. Die Prozessstrategie Komplementärzerspanung kombiniert die spanende Bearbeitung mit der mechanischen Oberflächenbehandlung unter Verwendung des Zerspanungswerkzeuges. Nach der Zerspanung erfolgt entgegen zur Zerspanungsrichtung eine mechanische Oberflächenbehandlung. Dadurch wird eine hohe randschichtnahe plastische Verformung induziert, die zu optimierten Randschichtzuständen, wie eine reduzierte Rauheit, hohe Kaltverfestigung und Druckeigenspannungen sowie einer Kornverfeinerung führt. Ziel der vorliegenden Arbeit ist der Aufbau eines Prozessverständnisses über den Einfluss der Prozessstellgrößen auf die resultierenden Prozesskräfte, Temperaturen und Randschichtzustände bei der Komplementärzerspanung von Armco-Eisen und 42CrMo4. Weiterhin wird eine optimierte Schneidkantenmikrogeometrie identifiziert, um den Werkzeugverschleiß zu reduzieren. Schließlich wird die Schwingfestigkeit von zerspanten und komplementär bearbeiteten Proben analysiert und mit etablierten Prozessen zur mechanischen Oberflächenbehandlung verglichen

A Review of Parameters and Mechanisms in Spray Cooling

Miniaturisation in avionics, electronics, and medical appliances has led to demands for rapid heat dissipation techniques. The spray cooling technique has gained importance recently due to its advantage over other cooling methods. Parameters affecting heat transfer mechanisms during spray cooling are contemplated. This review presents different heat transfer parameters and their effect on spray cooling by analysis from past studies. Heat transfer surface modifications and different coolant variations to enhance heat transfer effectiveness are also reviewed. Apart from high heat flux having more applications, low heat flux studies have also grabbed the researchers to find solutions with a temperature range lower than 250˚C. Therefore, the upcoming spray cooling technology will have broad applications that will contribute to the maximum efficiency of the heat removal rate

The prediction model for additively manufacturing of NiTiHf high-temperature shape memory alloy

NiTi-based alloys are one of the most well-known alloys among shape memory alloys having a wide range of applications from biomedical to aerospace areas. Adding a third element to the binary alloys of NiTi changes the thermomechanical properties of the material remarkably. Two unique features of stability and high transformation temperature have turned NiTiHf as a suitable ternary shape memory alloys in various applications. Selective laser melting (SLM) as a layer-based fabrication method addresses the difficulties and limitations of conventional methods. Process parameters of SLM play a prominent role in the properties of the final parts so that by using the different sets of process parameters, different thermomechanical responses can be achieved. In this study, different sets of process parameters (PPs) including laser power, hatch space, and scanning speed were defined to fabricate the NiTiHf samples. Changing the PPs is a powerful tool for tailoring the thermomechanical response of the fabricated parts such as transformation temperature (TTs), density, and mechanical response. In this work, an artificial neural network (ANN) was developed to achieve a prediction tool for finding the effect of the PPs on the TTs and the size deviation of the printed parts

A holistic review on fatigue properties of additively manufactured metals

Additive manufacturing (AM) technology is undergoing rapid development and emerging as an advanced technique that can fabricate complex near-net shaped and light-weight metallic parts with acceptable strength and fatigue performance. A number of studies have indicated that the strength or other mechanical properties of AM metals are comparable or even superior to that of conventionally manufactured metals, but the fatigue performance is still a thorny problem that may hinder the replacement of currently used metallic components by AM counterparts when the cyclic loading and thus fatigue failure dominates. This article reviews the state-of-art published data on the fatigue properties of AM metals, principally including $S$--$N$ data and fatigue crack growth data. The AM techniques utilized to generate samples in this review include powder bed fusion (e.g., EBM, SLM, DMLS) and directed energy deposition (e.g., LENS, WAAM). Further, the fatigue properties of AM metallic materials that involve titanium alloys, aluminum alloys, stainless steel, nickel-based alloys, magnesium alloys, and high entropy alloys, are systematically overviewed. In addition, summary figures or tables for the published data on fatigue properties are presented for the above metals, the AM techniques, and the influencing factors (manufacturing parameters, e.g., built orientation, processing parameter, and post-processing). The effects of build direction, particle, geometry, manufacturing parameters, post-processing, and heat-treatment on fatigue properties, when available, are provided and discussed. The fatigue performance and main factors affecting the fatigue behavior of AM metals are finally compared and critically analyzed, thus potentially providing valuable guidance for improving the fatigue performance of AM metals.Comment: 201 pages, 154 figure