By: Masoud Alimardani

The use of multi-material structures has shown a rapid increase in the past decade due to new fabrication technologies. In many different multidisciplinary engineering applications such as biomedical applications inconsistent material properties are required in order to enhance mechanical properties and functionality of an object. Conventional methods cannot properly create objects in which materials change gradually from one to another. Sharp interfaces between different materials cause stress concentrations that ultimately create delamination and cracks between layers of different materials. Another major problem in fabrication of multi-material structures is controlling the variation of different desired materials. Laser Solid Freeform Fabrication (LSFF) technique has great potential for the creation of multi-material structures in which material composition varies from one layer to another or from one point to another point in the same layer.

LSFF as an additive manufacturing technique can be used to manufacture fully functional near-net-shape three dimensional objects directly from their CAD model by successive layer-by-layer deposition of metallic materials. In this process, a laser beam is utilized to melt a thin layer of a moving substrate and powder particles deposited on the process domain, as schematically shown in the following figure, to form a small track. Each track (clad) is created by solidification of the additive materials together with a thin layer of the moving substrate or previously deposited tracks.

Schematic of the LSFF process

In LSFF, in addition to the layer-by-layer material deposition, the process domain undergoes a high cyclic heating and cooling regime as the result of the concentrated moving heat source (i.e., laser beam). This makes this process vulnerable to thermal stresses as the primary source of potential delamination and crack formation, specifically in multi-material objects in which the thermo-physical properties of the materials also vary across their structures. Therefore, although the gradual transition from one material to another reduces stress concentrations between the interfaces of subsequent deposited layers, this variation should be carefully linked with the LSFF operating parameters. To achieve this, understanding the underlying physics and consequently optimizing the process is crucial to enhance the final qualities of the fabricated parts.  This is the main focus of the article in which its authors investigate the effects of the material properties and their variations on the temperature distributions, thermal stresses and their evolutions throughout the LSFF process. In order to study the temperature distributions and thermal stresses in a layer-by-layer fashion and also to have a basis for comparing the results with their counterparts for single material structures, a four-layer thin wall of two Stellite 6 layers and two Ti layers are numerically and experimentally fabricated. Numerical simulations are performed using an experimentally verified 3D coupled modelling approach. The results are used to characterize the process and define an optimum laser power for each layer deposition.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here