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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.
Claddings of different nickel based alloys were generated on a mild Steel (S235JR) and a stainless steel type 304 by Cold Metal Transfer (CMT). This welding procedure possesses a low degree of mixing between the Substrate and cladding material. Thus it improves the corrosion behaviour of the claddings. The aim of our investigations was to examine the effect of the welding parameters: energy per unit length, shielding gas and finishing surface treatment on the pitting corrosion behaviour of the claddings.
However, under Standard test conditions nickel based alloys are often resistant to pitting corrosion. Moreover these test methods are inaccurate and require a large period of time resulting from the iterative progress to determine the critical pitting temperature (CPT). A potentiostatic test method was modified using a high concentrated CaCI2-solution and a conventional three electrode measuring cell. The pitting corrosion behaviour is calculated from current noise charges. During the test the course of corrosion is detected online by the evaluation of electrochemical noise Signals while a continuous increase of temperature by a defined heating rate is realised.
Thus a critical pitting temperature can be estimated by only one attempt. Furthermore the deviation of the measured CPT is very low compared to Standard tests using gravimetrical and microscopic evaluation. That enables to figure out the influence of the welding parameters in an exact way.