• Sara Hagmann

3D Printing and Electrochemical Machining

Metal 3D printing or additive manufacturing is a rapidly growing field, particularly in the aerospace and medical markets.


The most common metal additive manufacturing techniques include powder-bed, powder freeform, and wire freeform. Of the three, powder-bed additive is the most prevalent and uses a laser (e.g. DMLS, SLS, SLM) or electron beam (EBM) to melt or weld metal powder, layer by layer, to create a 3D geometry. The powder and wire freeform additive manufacturing methods can be used to form much larger parts with faster deposition rates but with worse feature resolution.


These metal AM techniques enable complex geometries, lightweight designs, and consolidated parts that would be impossible (or at least prohibitively expensive) with other manufacturing processes.


PROBLEMS WITH 3D PRINTING


However, metal 3D printing does have a few challenges including surface roughness, minimum wall thickness, thermal distortion, and cost.


Surface roughness in a 3D printed metal part can be a function of the powder size and distribution, layer heights, surface angle relative to the build platform, re-deposition of melted material (e.g. micro weld splatter), and layer recoating or powder quality. During the printing processes, surfaces that are overhung or “downskin” (i.e. <90° from the build plate) lead to surface roughness that can be multiple times worse than the sidewall or upskin surfaces.

turbine with spider web-like support structures - an excellent application for electrochemical machining
An additively manufactured part with support structures. Photo credit: 3dhubs.com

In addition, support structures of thin, removable metal lattices are often utilized to support downskin surfaces to prevent sagging, to help conduct heat, or to stabilize a critical section during the recoating operation or heat-treating steps. Even after removal, these support structures leave remnants or surface irregularities. This surface roughness can negatively affect the part either through incorrect form or fit, creating excess surface flow friction, initiating corrosion, creating crack formation sites via pits or crevices, or making part inspection (e.g. fluorescent penetrant inspection, FPI) more difficult.


Metal additive manufacturing is also an inherently thermal process based on welding principals. The heat generated during the build process can lead to residual stresses in the part which cause distortion and changes in geometry. In extreme cases, this distortion can lead to cracking and build failures. Although simulation and predictive algorithms are improving this issue, thermal stresses will always affect certain parts.


There are also limits to the wall thickness that are achievable with metal additive manufacturing. The free-form AM processes in particular are limited to relatively large thicknesses or order to achieve higher deposition rates but even powder bed AM processes have aspect ratio limitations that can be driven by thermal challenges, powder recoating forces, etc.

Turbine segments side by side, one with no post-processing, one with smooth surfaces created by electrochemical machining
Nickel-alloy turbine vane/nozzle segment finished by electrochemical machining

Thus far, most metal AM parts have found value in the market either for fast-turnaround replacement parts or by creating geometries / eliminating assemblies that improve performance or reduce downstream costs. As metal AM expands to cover broader markets, the cost pressures will increase. In some cases, those costs are driven the surface finish, wall thickness, or thermal distortion challenges. For example, the techniques used to improve the as-printed part surface finish (finer powders, smaller layers and beam sizes, etc.) also slow or add costs to the process.


Based on these challenges, post processing of 3D printed parts is a significant aspect of delivering a complete part. It is not unusual to employ multiple post processing methods including hand polishing, vibratory processing, CNC machining, and EDM to address the surface roughness, thermal distortion, or wall thickness limitations. However, one post-processing method that may be underutilized for this work is electrochemical machining, or ECM.


ELECTROCHEMICAL MACHINING: A POTENTIAL SOLUTION


Electrochemical machining (ECM) and pulsed electrochemical machining (PECM) are manufacturing processes that are neither heat- nor contact-based. With a gap between a custom tool and the metal workpiece, ECM uses a current and an electrolyte solution to machine the surface of the 3D printed part, atom by atom. Because of this precise material removal, ECM can create pristine surfaces in areas where finish is important while leaving other areas in their as-printed state.

Tensile bar samples processed by electrochemical machining laid side by side
Nickel-alloy tensile bar samples processed by ECM and achieving local surface finishing where required.

Particularly when applied to volume production of additive parts, ECM can be a cost-effective post-processing technique to address surface roughness. Although ECM has many similar attributes to electropolishing, the use of locally small gap between the electrode and workpiece is particularly efficient at removing the macro-level roughness characteristic of support structure remnants, downskin surfaces, EBM powder bed, or freeform AM methods.


Given some of the thin-wall and thermal distortion challenges, ECM can also be an effective secondary machining operation. Instead of trying to work within the wall thickness constraints of existing metal 3D printing methods, ECM could be used to readily create thin-walled features where they are critical to the design. In addition, if thermal distortion is a known potential challenge to achieving the correct profile tolerances, it may be more cost effective to add extra stock material to the surface while utilizing ECM to dissolve the extra material into the correct shape.

Small part with thin walls manufactured with electrochemical machining, next to a quarter for comparison
Thin walls possible with electrochemical machining

Finally, with effective post processing methods such as ECM, it may be more cost efficient to focus on increasing the 3D printing speed at the expense of surface roughness or feature size (e.g. thicker layers, larger powders, larger beam sizes, freeform AM methods) while utilizing a rapid post-processing technique like ECM to create the critical geometries where necessary. Utilize 3D printing to create internal lattice structures or organic shapes while ECM can help reduce surface roughness, minimize rib thickness, or perform other value-added operations.


IN SUMMARY


Metal 3D printing and ECM can be utilized in tandem to achieve lower costs or better performance than either could achieve on their own. Particularly for additive manufacturing applications which are producing medium to high part volumes, ECM should be considered as an effective post processing tool. In addition, Voxel is continuing to develop this technique such that ECM could be effective at improving surface roughness on single part applications, so stay tuned to this blog for future developments!