Iron cast foundry workers have a wide range of processes at their disposal to make gray iron castings. Manufacturers select a casting process and materials based on things like part hardness, size, wall thickness, cooling rate, and budget.
Conventional filler metals are designed primarily for joint welding, not additive manufacturing (AM). As a result, the available alloy designs are limited.
The alloy development and properties of chromium martensitic hot-work tool steels can be customized using metal cored wires applied in the wire arc additive manufacturing (WAAM) process. This is possible due to the flexible material transfer that cored wires allow.
The X10CrMo6 steel alloy used in the present study was fabricated as metal-cored wires and WAAM structures were built up by varying the nickel content. The morphology of the resulting microstructures and impact toughness were investigated.
The results show that the addition of nickel decreases the polygonal formation of delta ferrite and shifts its morphology to a preferable fine grain boundary network. Additionally, the nickel addition increases the density of low-angle grain boundaries (LAGB) and high-angle grain boundaries (HAGB), as shown in Fig. 14. Moreover, the fraction of geometrically necessary dislocations (GND) decreases with increasing Ni content. Consequently, the addition of Ni promotes the refinement of the microstructure in X10CrMo6 steel. This effect was verified by analyzing the X-ray diffraction patterns of AM bulk materials (in the ND-TD plane) with a 0 wt%, 0.5 wt% and 1 wt% nickel addition.
Optical emission spectroscopy (OES) is a well-established analytical technique for the rapid elemental analysis of solid metal samples. It is used to evaluate metals and alloys including steel, cast iron, aluminium and other non-ferrous alloys as well as powders and welded structures.
OES analyses a sample by shining a beam of light through it which is split into a spectrum showing the intensity of each of the different emission lines for each element present in the sample. The spectrum is processed and provides a breakdown of the percentage concentrations of each of the elements in the sample.
When the electrical discharge interacts with an atom in the sample, the outer electrons are ejected leaving a hole which emits the characteristic line. This process is repeated for all atoms in the sample and their relative concentrations are calculated. Typically, OES is performed on tubular metal-cored wires used in WAAM as they are the only filler materials with enough softness to respond to spark discharge excitation.
The X-ray diffractometer is used to identify the crystal structure of materials. It does this by directing an intense X-ray beam at the sample and measuring the intensity of the reflected or diffracted X-rays with a detector. By comparing the measured peaks with those of known crystal structures (typically from commercial databases), the crystalline structure of the material can be identified.
Using synchrotron X-ray and neutron diffraction, the residual stresses in a stress frame of gray iron were investigated before and after the vibratory stress relief treatment. Specifically, the tensile and compressive stresses in the stress frame were found to be reduced after the treatment.
Moreover, the higher Ni content decreases delta ferrite formation and increases the geometrically necessary dislocation density (GND), thereby improving the microstructure of the X10CrMo6-3 steel. This results in a lower brittleness, which is essential for machine tools that demand accuracy down to 1 um. It also helps in achieving good mechanical properties such as fatigue resistance, toughness and wear resistance.
Scanning electron microscopy (SEM) is a test process that uses an electron beam to scan the specimen, producing magnified images for inspection. It is commonly used for failure analysis of solid inorganic materials, such as metals and ceramics. Laboratory Testing Inc near Philadelphia, PA offers a complete SEM testing system with Energy Dispersive X-ray Spectroscopy (EDS) capabilities.
Unlike SEM, which detects reflected or knocked-off electrons to create an image, transmission electron microscopy (TEM) uses transmitted electrons to form the image. This allows TEM to offer valuable information on crystal structure, morphology and stress state.
Fig. 6 shows the microscopic results for built-up AM structures of X10CrMoNi6-3 cored wire with varying nickel content. Adding nickel to the filler metal reduces delta ferrite formation, but it does not fully eliminate it. Consequently, the structural characteristics deviate slightly from the theoretical prediction of full martensite. Nevertheless, the results show that the addition of nickel provides flexibility in alloy development and property customization.
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