Diode Area Melting Use of High Power Diode Lasers in Additive Manufacturing of Metallic Components

Additive manufacturing processes have been developed to a stage where they can now be used to manufacture net-shape high-value components. Selective Laser Melting (SLM) comprises of either a single or multiple deflected high energy fibre laser source(s) (e.g. 200 – 400 W each) to raster scan, melt and fuse layers of metallic powdered feedstock. The beam(s) is(are) deflected by a Scanning Galvo Mirror System and an F-theta lens is used to provide a flat field at the image plane of the scanning system. However, this deflected laser raster scanning methodology is high cost (addition of multiple high-power deflected lasers in SLM for increase productivity can suffer penalties of ~£170K for each additional laser), energy inefficient (wall-plug efficiency of typical SLM fibre laser sources ~50 % [1]) and encounters significant limitations on output productivity due to the rate of feedstock melting (e.g. typical theoretical build rate of SLM of stainless steel <2.8 mm3/s (<10cm3/min) [2]). This work details the development of a new additive manufacturing process known as Diode Area Melting (DAM) featuring multiple high efficient laser sources (i.e. >60 % wall-plug efficiency [1]) with scalability potential (<£100 penalty per additional laser beam for increase productivity). This process utilises customised architectural arrays of low power laser diode emitters (i.e. ~5W laser power) for high speed parallel processing (theoretical build rate of scaled DAM of stainless steel >2.8 mm3/s (>10cm3/min)) of metallic feedstock. Individually addressable diode emitters are used to selectively melt feedstock from a pre-laid powder bed. 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 [3][4]. The melting capabilities of the DAM process were tested for low melting point eutectic BiZn2.7 elemental powders, AlSi12 and higher temperature pre-alloyed 17-4 and 316L stainless steel powders. The process was shown to be capable of fabricating controllable geometric features with evidence of complete melting and fusion between multiple powder layers.

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/mm3 were theorised as sufficient to enable processing of fully dense layers.

Finally, this investigation presents the first work modelling the DAM process, detailing the unique thermal profiles experienced with the laser processed powder bed. Process optimisation is improved through modelling thermal temperature distribution, targeting processing conditions inducing full melting for variable powder layer thickness. In this work the developed thermal model simulates the processing of 316L stainless steel and is validated with experimental trials.

Key findings that have been identified in the present research include the following:
• Edge emitting diode laser modules featuring multiple ~5 W emitters, can be used directly in AM of metallic components.
• Typical 808 nm diode lasers wavelength enables high laser absorption mechanisms in a metal powder-bed based AM process, which in turn allows the use of lower laser power (<5 W) than the conventionally used in SLM (100-400 W).
• Temperatures in excess of 1450 ºC can be reached in metallic powder beds (stainless steel) with <5 W diode-laser spots using appropriate optical mechanisms to collimate and focus the low-quality beam (27º and 7º divergence in the fast and slow axis respectively) down to <250 µm melting spots.
• It has been identified the ability to near-net shape and process material with melt temperatures in excess of 1450 ºC (i.e. stainless steel powder) using multiple individually addressable and non-deflected low power diode laser beams in order to scan in parallel, selectively melting material from a powder bed.
• DAM process parameters including laser beam profile (i.e. spot spacing and spot dimensions), particle size distribution (emissivity and conductivity of the powder), laser power and scan speed affect the porosity and melt-pool uniformity of DAM components.
• An energy density of 86 J/mm3 can be theorised as the minimum required for fully dense DAM (stainless steel) components.
• Effective melt area in DAM can be 6.67 % in excess of the actual spots size (i.e. 4.75 mm laser beam width has an effective melt width of 5.067 mm).
• Temperature gradients and cooling rates during DAM processing of metallic feedstock are similar to optimised pre-heated SLM mechanisms with low residual stress formation.