Ingenieurwissenschaften und zugeordnete Tätigkeiten
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Nickel alloys are cost intensive materials and generally classified as difficult-to-cut material. However, machining of these materials is needed especially in case of alloy 36 (1.3912), which is commonly used in mould construction for the production of fibre-reinforced composites. With regard to repair, modification and manufacturing of such components, additive manufacturing offers significant economic advantages. Nevertheless, subsequent machining steps are needed to achieve the final component contour and defined surface conditions. Dependent on the material and machining process conditions, detrimental tensile residual stresses may be the result on the machined surface, having negative impact on the component performance and safety. In this investigation, machining experiments were carried out on wire arc additive manufactured components made of alloy 36, varying the cutting speed and the feed rate. In addition, the conventional milling process (CM) was compared with a modern, hybrid machining process, the ultrasonic-assisted milling (US). The cutting forces and the surface-near residual stresses were analysed using X-ray diffraction. A significant improvement of the machinability as well as the surface integrity by using the ultrasonic assistance was observed, especially at low cutting speeds. The CM induced mainly tensile residual stresses, the US mainly compressive residual stresses.
Alloy 36 (1.3912), also known as “Invar,” is an alloy with 36% nickel. The alloy has a remarkably low thermal expansion coefficient in certain temperature ranges. This peculiarity is called the invar effect, which was discovered in 1896 by the Swiss physicist Charles Édouard Guillaume. Therefore, it is used in applications in which dimensional stability is critical, such as molding tools for composite materials in aerospace, automotive applications, or liquified natural gas (LNG) cargo tanks. Moreover, increasingly complex structures and the optimization of resource efficiency also require additive manufacturing steps for the production or repair of components. Additively manufactured components have a heterogeneous microstructure and anisotropic mechanical properties. In addition, the manufactured components require subsequent machining surface finishing, like finish milling, to achieve their final contour. Nickel iron alloys are difficult to machine. Additionally, inhomogeneous microstructure may lead to unstable cutting forces and conditions. In part I of this investigation, the initial alloy 36 is modified with the elements Ti, Zr, and Hf up to a maximum of 0.33 wt.-%. The influence of the modification elements on the microstructure as well as on the hardness of the AM components is examined. Furthermore, one modification is applied to metal arc welding process and investigated. Part II focuses on the effect of the alloy modifications on machinability as well as on the surface integrity of plasma-transferred-arc-welded (PTA) and finish milled invar components.
Alloy 36 (1.3912) is an alloy with 36% nickel and 64% iron and is generally classified as a difficult-to-cut material. Increasingly complex structures and the optimization of resource efficiency are making additive manufacturing (AM) more and more attractive for the manufacture or repair of components. Subsequent machining of AM components is unavoidable for its final contour. By using modern, hybrid machining processes, e.g., ultrasonic-assisted milling (US), it is possible to improve the cutting situation regarding the resulting surface integrity as well as the cutting force. Part I deals with the influence of the alloying elements Ti, Zr, and Hf on the microstructure and the hardness of the initial alloy 36. Part II focusses on the effect of the alloy modifications and the ultrasonic assistance on machinability as well as on the surface integrity after finish-milling. The results show a highly significant influence of the ultrasonic assistance. The cutting force during the US is reduced by over 50% and the roughness of approx. 50% compared to conventional milling (CM) for all materials investigated. Moreover, the US causes a defect-free surface and induces near-surface compressive residual stresses. CM leads to a near-surface stress state of approx. 0 MPa.
Nickel-Iron-Alloy Modification to Enhance Additively Welded Microstructure for Subsequent Milling
(2022)
The aerospace industry uses nickel–iron alloys, e.g., FeNi36, to create moulding tools for composite materials, since these alloys have a low coefficient of thermal expansion. Nickel–iron alloys are hard-to-cut materials. The moulding tools are large in size and involve complex structures, making them cost-intensive and difficult to manufacture. Thus, the focus is set on additive manufacturing, which can additionally enable the repair of components in order to eliminate local defects. However, the process usually results in a heterogeneous microstructure and anisotropic mechanical properties. As there is a high demand for a precise and exact fit of the precision moulds and the surface quality, the welded components must be subsequently machined. Additionally, inhomogeneous microstructure may lead to unstable cutting forces and conditions. Consequently, a modification of the microstructure morphology is achieved through specific alloy modifications in order to stabilise and improve the subsequent machining process. Therefore, titanium and zirconium are chosen as modification elements with a maximum 1% weight percent and are added to nickel–iron alloy powder. The elements are alloyed, and build-up welded by plasma-transferred-arcwelding. The resulting microstructure morphology of the welded wall structure and the machining properties are then determined. It can be shown that titanium has a significant effect on the structural morphology of the welded layers, as well as on the machining.
Ni alloys are generally classified as difficult-to-cut materials and cost intensive. Additive manufacturing (AM) offers economic advantages. However, machining of these AM components is mandatory to create the final contour or surface. The inhomogeneous and anisotropic microstructure and properties of AM components causes an unstable cutting process. Moreover, undesirable tensile residual stresses are generated due to subsequent machining. In this investigation, the initial alloy 36 is modified with Ti and Nb up to 1.6 wt.-% and build-up welded via gas metal arc welding (GMAW) and plasma-transferred-arc (PTA). Then, finish-milling tests are carried out to investigate the influence of the modification as well as the cutting parameters on the resulting cutting force and the surface integrity. In addition, the conventional milling process (CM) is compared with the ultrasonic-assisted milling process (US), which has a significant influence on the machinability as well as on the surface integrity.