The ability of the cold spray process to effectively fill holes of various sizes in a casting was studied by drilling holes of different sizes into a weld plate to mimic porosity in castings. These drilled holes were prepared by various surface preparation techniques and cold sprayed. Initial trials on the test block using minimal or no surface preparation before cold spraying resulted in many holes being unfilled; other showed little or no bonding. Deposited material in the smaller holes tended to pop out while 50% of the larger holes did not fill.
Various surface preparation techniques for preparing porosity to be cold sprayed were then examined. These included grinding the hole with a ½” and 1” ball end mill, grit blasting, and roughening with Scotch-Brite™ to increase surface roughness. The edges of some holes were also chamfered. These efforts showed that chamfering and increasing the surface roughness clearly improved the ability to bond. Samples without any preparation, Scotch- Brite™ or grit blasting, failed.
The effect of spray angle also was examined in the current research. Mixed results were seen as a result of varying the spray angle.
Microstructural homogeneity is important for any technique used to salvage casting with surface porosity. Hence, microstructure examination of the cold sprayed weld plate was conducted. Microstructure showed porosity, internal cracks and voids, and some separation at the interface. Average porosity was on the order of 9%.
Similar to microstructural homogeneity, variations in hardness between the substrate and deposition should be minimum. Hardness of the deposition was lower than the substrate, but higher than what is typically expected for the deposited powder, in this case 316L.
Hardness throughout the deposition as well as the substrate was relatively constant as no specific trend near the interface was observed.
From the experiments carried out during the course of this study it is possible to draw the following conclusions:
It appears that the chamfering with a ball end mill to change the substrate hole geometry to semi spherical improves deposition efficiency. Hole geometry seems to be critical parameter as compared to hole size. Grinding out to a larger size is suggested. Increasing the hole size should increase contact area for the deposit, consequently increasing the points of mechanical interlocking resulting in better adhesion.
Removal of sharp edges by chamfering of the hole appears essential for producing a good deposit. Chamfering the edges improves coating adhesion by removing the effect of stress concentration resulting from sharp edges. Minor chamfer was the best prep technique for cold spraying.
The effect of spray angle is not established at this time. This could be because of the effect of other parameters such as stand-off distance and turbulence of the particles in the spray. These parameters are difficult to control in manual cold spray process.
Microporosity should be expected, even in well bonded samples. Some bonded samples had voids inside the holes. Parameters need to be refined to produce denser fills. However, minimum or no cracks are seen in most of the deposits. This is a major improvement as compared to results observed by Ganesan. [8]
Hardness measurements indicate that hardness of the substrate is not affected by the cold spray. The hardness of the deposited SS 316 L is higher than typical SS 316 L hardness
values. This could be because of the strain hardening resulting from the high plastic deformation at interface with the substrate and with adjoining particles.
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