Micro-forging as a means for improving interlayer and surface mechanical properties in additive manufacturing.

Additive manufacturing processes are nowadays widely used for prototyping, as well as for industrial applications, e.g. the production of machine tools. The finished products often suffer from degraded mechanical properties comparatively, as additive manufacturing processes inherently create mechanical property variations among layers. Issues commonly arise due to incomplete fusion, thermal cycling, contamination, anisotropy, porosity and cooling rate variations.

A few different methods have been introduced for both metallic and synthetic materials, which typically include annealing or work hardening. These can be applied to the finished product or during the manufacturing process to improve the interlayer mechanical properties. Regarding work hardening, deep rolling is a common procedure where a spherical indenter is rolled onto the manufactured surface using a controlled force applied perpendicular to the surface. However, this procedure is generally coarse and mostly designated for large surfaces.

We herein introduce micro-forging with the aid of a controlled repetitive impact indenter, to induce precise levels of work hardening onto manufactured surfaces. The force amplitude perpendicular to the treated surface can be adjusted according to the desired level of work hardening. The indenter, made of hard materials like tungsten carbide, hits the processed surface repeatedly at an adjustable rate. The kinematics are completed by either moving the indenter onto the surface or the workpiece underneath the indenter. By adjusting the impact repetition rate and the feed of the surface underneath the indenter, impact overlapping can be achieved and the resulting mechanical properties optimized. The shape of the indenter can also be freely selected for different applications or for creating a texture finish.

Our findings illustrate that interlayer hardness can be drastically increased leading to more robust machine elements. Surface roughness can be adjusted, especially important for further processing like the subsequent application of coatings. Issues arising from increased porosity can be minimized, as the introduced micro-forging process helps to minimize the overall gaps between the material.