Diode area melting single-layer parametric analysis of 316L stainless steel powder

Abstract

Diode area melting (DAM) is a novel additive manufacturing process that utilises customised architectural arrays of low power laser diode emitters for high speed parallel processing of metallic powdered feedstock. The laser diodes operate at shorter laser wavelengths (808 nm) than conventional SLM fibre lasers (1064 nm) theoretically enabling more efficient energy absorption for specific materials. This investigation presents a parametric analysis of the DAM process, identifying the effect of powder characteristics, laser beam profile, laser power and scan speed on the porosity of a single-layer sample. Also presented is the effect of process energy density on melt pool depth (irradiated thermal energy penetration capable of achieving melting) on 316L stainless steel powder. An analysis of the density and the melt depth fraction of single layers is presented in order to identify the conditions that lead to the fabrication of fully dense DAM parts. Energy densities in excess of 86 J/mm 3 were theorised as sufficient to enable processing of fully dense layers.

Investigating the melt pool properties and thermal effects of multi-laser diode area melting

“…Lack of fusion was observed in regions between layers as well as limited substrate bonding. An energy density 86 J mm −3 in DAM has been theorised as the minimum required for producing high-density parts with penetration depths similar to SLM [2]. In the present work, a finite element model (FEM) capable of simulating the DAM process is used for analysing the cooling rate and temperature gradient mechanisms that describe the DAM thermal evolution.…”

“…The model takes into account the DAM Laser Beam Profiles (LBPs) and process parameters (i.e. laser power and scan speed) presented in the DAM experimental investigation conducted in [2]. The non-linearity of temperature-dependant material properties and phase changes is also considered.…”

“…The modelling approach used in the present work is based upon the concept of a moving volumetric heat source (typically used in SLM modelling [17]) with enhanced penetration depth, combined with enhanced thermal conductivity through the substrate and the surrounding powder to improve computational efficiency, as described in [7]. The aim of the present work is to simulate the DAM process using optimised conditions that enable process energy density of 86 J mm −3 , which has been theorised as the minimum required for processing fully dense multi-layer parts in the densification analysis conducted in [2]. The cooling rates, thermal distribution and temperature gradients of DAM are contrasted with those of SLM in order to compare the two different heat input mechanisms.…”

“…This approach was used to account for the laser penetration effect into the powder, which according to [18] is 120 μm for stainless steel 316L powder. To make the simulation more efficient, the volumetric heat source was applied to a 260 μm-thick layer of powder, as used in the DAM densification analysis conducted in [2],alongwitha250μm depth into the substrate [7]. A Modified Prismatic laser Heat Flux (MPHF) model, similar to the cylindrical representation model explained in [19,20] ,w a su s e di nt h ep r e s e n tw o r kt or e p r e s e n tt h e variation of radial laser intensity.…”

“…where P is the laser power in W, and b and h are the base and height of the rectangular-shaped multi-beam laser diode irradiation on the powder bed top surface. From the DAM laser beam profiles (LBPs) described in [2], these are b =4.75 mm and h =0.25 mm for LBP1, b =6.5 mm and h =0.3 mm for LBP2 and b =9.4 mm and h =0.4 mm for LBP3 ( Fig. 4).…”