Using Wobble Based Laser Scanning Techniques in Additive Manufacturing Applications

In metal powder bed based additive manufacturing, much focus is on relatively abstract scanning trajectories and techniques. While this is a fundamental and complex part of the process that necessitates such focus, scanning technology has a part to play in productivity and part quality. Often machine manufacturers and end users need to add additional parametric information into their trajectory planning obscuring the underlying technology. Therefore, this detail is hard won and understandably often leads to a closed manufacturer specific file formats containing the meta data required to operate the machine. This article describes wobble based laser scanning techniques that may be employed to deliver enhanced part quality and productivity. Productivity of laser based powder bed additive manufacturing machines is subject to much attention at the present time. Numerous strategies are being pursued within the industry to deliver enhanced productivity. These include multiple scanning heads, various scan field overlap strategies and more recently research undertaken by Tsai et al [1] explores the possibility of using multiple spot scanning strategies. In addition, many more novel methods and approaches are being worked on. As the industry continues to transition from prototyping to series production of functional parts, productivity is key to making the part economically. This paper explores one possible approach to improve productivity. A cursory review of literature yields many papers describing scan trajectory patterns. While this paper will summarize a common approach, it does not analyze such patterns in detail. Most scanning techniques used today by additive manufacturing machine builders are written with a level of abstraction. Therefore, this often doesn’t allow full utilization of the scanning technology. This limits the designer’s ability to influence the melt pool and thus the microstructure. Consider the following points:

Thermal Gradients

In Laser Powder Bed Fusion machines the material exposes to a temperature gradient of 5-20K/um and cooling rates of 1-40K/um [2,3]. As the material transitions between states, Powder to Liquid to Solid. The cooling rate plays a significant part in the microstructure development and end part quality.

Scanning Parameters

In [4-6] the influence of various scanning related parameters on the mechanical properties of SLM parts are studied. Key findings from this study demonstrate that laser power and scanning speed play a key part in the formation of the microstructure. Scan speed and laser power, directly influence whether the surface tension of the melt pool is broken, splatter ejected or powder not fully melted. All of which affects the end part quality, and can contaminate the layer.

Melt Pool

Lawrence Livermore National Laboratory (LLNL) [8,9] use high speed cameras (500K frames / sec) and advanced computing platforms to model melt pool dynamics. Their approach has been to develop a model at the particle level, and verify with experiments using high speed cameras. Therefore, in these experiments a number of observations have been published in literature including the sensitivity of the melt pool to gas flow and how scan speed and laser power contributes to the formation of the key hole, ejection of material (splatter) and pore defects.