Surface roughness and finishing techniques in selective laser melted GRCop-84 copper for an additive manufactured lower hybrid current drive launcher

https://doi.org/10.1016/j.fusengdes.2020.111801Get rights and content

Highlights

  • Additive manufacturing enables rapid construction of lower hybrid launcher waveguides for fusion reactors.

  • Surface of selective laser melted (SLM) material is rough leading to arcing and RF losses.

  • Surface finishing of SLM printed components is a key enabling technology for use in RF structures.

  • Wet blasting and vibratory finishing methods reduce surface roughness.

  • Chemical etching results in anisotropic etch rates that follow laser hatch pattern in printed material.

Abstract

Recent advances in selective laser melting (SLM) 3D printing technology allows additive manufacture of lower hybrid current drive (LHCD) RF launchers from a new material, Glenn Research Copper 84 (GRCop-84), a Cr2Nb (8 at. % Cr, 4 at. % Nb) precipitation hardened alloy, in configurations unachievable with conventional machining. Rough surfaces in additive manufactured components are a limiting factor in RF structures. Surface roughness increases RF losses and impedes conditioning by trapping gas and contaminants that induce arcing by evolving from the surface at high power. SLM printed GRCop-84 is post-processed with mass finishing to reduce surface roughness. Wet blasting, vibratory finishing, and chemical etching remove adhered powder grains from SLM printed GRCop-84. Profilometry and scanning electron microscopy were used to quantify resulting surface quality. Chemical etching is examined to remove zinc contamination from wire electrical discharge machining slag prior to vacuum use. Etch rates in SLM printed GRCop-84 are anisotropic, and etch times should be limited to prevent pitting corresponding to the laser hatch pattern. Vibratory mass finishing and mechanical polishing produced surfaces suitable for low RF loss. Use of chemo-mechanical finishing is recommended for the interior of SLM printed structures.

Introduction

A novel lower hybrid current drive (LHCD) multijunction launcher on the high field side (HFS) of the DIII-D tokamak [1] is required to withstand multiple 16 h, 400 °C bake cycles and survive disruption loads. Selective laser melting (SLM) allow additive manufacture of RF launchers in configurations either difficult to achieve with conventional machining processes due to enclosed structures or uneconomical due to removal of large amounts of copper from a thin shell. Multijunction launchers meet the criteria of a large structure consisting of mostly free space, multiple adjacent waveguides partitioned by thin internal septa, variation of waveguide broad wall width in the phase shifter section, and internal RF structure detail required for power splitting and impedance matching. Copper alloys are ideal for RF launchers as the high conductivity reduces losses compared to steel or Inconel alloys. Copper alloys have traditionally been considered difficult to 3D print with SLM techniques due to high thermal conductivity and low coupling to Nd:YAG and fiber lasers in the 1030−1080 nm wavelength range [2] resulting in porosity and sub-unity density.

A promising alloy is Glen Research Copper (GRCop-84) [3,4], a Niobium Chromate (Cr2Nb) 8 atomic (at.) % Cr, 4 at. % Nb [5] precipitation hardened alloy with superior high temperature strength. GRCop-84 is selected for the launcher based on earlier additive manufacturing techniques that produce a fully dense material without hot isostatic pressing. This material can tolerate a 400 °C bake while retaining high strength to survive disruption loads. GRCop-84 has low creep and high strength up to 800 °C, with oxidation resistance improved by an order of magnitude over oxygen free copper (OFC) up to 650 °C by formation of a durable layer of Cr2Nb oxide in the form of Nb0.6Cr0.4O2 [6]. GRCop-84 has a thermal conductivity of 300 W/m∙K (75 %–84 % of pure Cu), a resistivity 2.5 μΩ∙cm [7] (140 % of pure Cu). Cr2Nb precipitates pin grain boundaries within a pure copper matrix; grain boundaries do not grow during long term exposure of 800 °C, with little reduction in tensile strength after exposure to 1000 °C [3], unlike precipitation hardened alloys which are substantially weakened permanently after high temperature exposure due to over-ageing. GRCop-84 may only be fabricated with rapid solidification methods such as SLM of gas atomized GRCop-84 powder, to prevent growth of the Cr2Nb precipitate size [3]. SLM printing techniques for GRCop-84 have been developed by NASA Marshall Space Flight Center, ASRC Federal Astronautics LLC [8], and Special Aerospace Services [9]. In tests performed by Hayes, et.al at Special Aerospace Services, GRCop-84 tensile samples achieved >99.9 % density with a 472 MPa 0.2 % yield strength and 714 MPa ultimate tensile strength (UTS) at 25 °C for samples printed perpendicular to the stress direction. The thermal, mechanical, and electrical properties of this material are ideal for use in a multijunction type LHCD launcher for low RF loss while allowing high temperature bakeout and preventing damage when exposed to mechanical disruption loads on the launcher structure.

Internal surface roughness on waveguide structures increases RF losses and contributes to power limitations at high power in a vacuum. Rough waveguide surfaces impede cleaning prior to installation and RF conditioning. Rough surfaces induce arcing by evolving trapped gas from the surface. Field enhancement from sharp points on a rough surface on the waveguide broad wall may act as initiation points for arcing. Rough surfaces in additive manufactured components are a limiting factor in the manufacture of RF components. Unlike parts machined from extrusion or billet where surface finish may be partially controlled by the milling process or subsequent grinding, the surface of additive manufactured RF components will require a mass finishing step for surface roughness control. Additively manufactured components will require surface finishing on interior surfaces that may not be easily accessible by conventional grinding or polishing methods. Although SLM printed GRCop-84 waveguides [10,11] indicated large losses in millimeter wave systems due to surface roughness, the longer wavelength at 4.6 GHz for LHCD applications reduces RF losses to acceptable levels.

In this paper we present studies of the surface roughness of SLM printed GRCop-84, and develop mass finishing techniques to improve surface quality. Scanning electron microscope (SEM) images of as-printed and mass finished GRCop-84 surfaces are compared. Mechanical grinding and polishing of GRCop-84 is presented as a comparison of surface roughness. Roughness and brass slag deposition on Wire Electrical Discharge Machined (EDM) waveguide parts are analyzed along with mechanical and chemical slag removal techniques. Profilometry is used to quantify surface roughness on printed and finished surfaces. Wet-blasting and subsequent vibratory mass finishing is shown to remove adhered powder granules from the waveguide resulting in a smooth surface that will not inhibit RF conditioning in a vacuum environment and reduce RF losses.

Section snippets

Additive manufacturing process for test samples and launcher components

Additively manufactured GRCop-84 samples evaluated in this paper are produced by ASRC Federal Astronautics LLC [8] (Now Quadrus Corp. as of May 2020) using selective laser melting in a Concept Laser M2 Cusing machine. GRCop-84 powders sourced from ATI Powder Metals and Carpenter Powder Products range between 10 μm and 45 μm in diameter with an average size of 25 μm. The SLM process is conducted under an inert argon atmosphere. A single 400 W (max) fiber laser set at 180 W scans vertically from

LHCD launcher fabrication process

The chosen assembly process and part geometry determines which finishing methods may be utilized to reduce surface roughness. Components with deep internal geometry that prohibits direct access to mechanical polishing, such as phase shifters, will require chemical or vibratory finishing techniques, while waveguide components that are enclosed after fabrication by welding, such as poloidal power splitters, may be finished by mechanical polishing techniques.

Each launcher module [1] in the LHCD

Surface profilometry

The surface roughness of SLM printed GRCop-84 samples in as-printed and finished conditions was measured with a Mitutoyo SJ-210 profilometer configured with the following parameters:

  • ISO1997, Profile R, Parameter 3

  • Filter: Gauss, λc = 0.8 mm, λs = 2.5 μm, N = 5

  • Speed: 0.5 mm/s

  • Probe tip radius: 5 μm

Ra, Rq, and Rz parameters are listed in Table 1 for as-printed and finished GRCop-84 surfaces along with number of measurements per surface, n, and standard deviation of the Ra value. Initial surface

Electron microscopy of GRCop-84

Surface characteristics of GRCop-84 samples are images scanning electron microscopy (SEM) using a Zeiss Merlin SEM.

Optical microscopy is used when beneficial, however the superior depth of field with a SEM is preferable when imaging with varying depth at high magnification. During SEM imaging, a beam energy of 5 keV and current near 100 pA are selected. Imaging on the Zeiss Merlin SEM is typically at a 0° or 45° angle to vertical, as shown in Fig. 7 (a) and (b). A 45° tilt angle improves the

Mechanical and abrasive mass finished surfaces

The vertical sidewalls of SLM printed GRCop-84 in as-printed condition exhibit a small ripple and adhered powder granules partially melted into the surface of the bulk material as shown in Fig. 8 (a,b) and a surface roughness of Ra = 3.3 μm, whereas an extruded WR-187 OFC waveguide (c,d) had a surface roughness of Ra = 0.39 μm.

The top surface of SLM GRCop-84, shown in Fig. 9 (a,b) shown a repeated hatch pattern from the scanning of the laser.

The hatch is arraigned in 3 mm x 3 mm squares with

Chemical etched surfaces

Chemical etching was tested for surface finishing of as-printed and mass finished GRCop-84 surfaces. Samples were etched in a 3:1 mixture of vinegar to 3% hydrogen peroxide at 40 °C in an ultrasonic bath. Etch rates measured by thinning of test coupons indicated an etch rate of 275 nm/min. A test coupon polished with 6 μm diamond in the plane of the printed layers and etched for 1−4 hours, as shown in Fig. 15, demonstrates an anisotropic etching rate in a pattern corresponding to the scanned

Wire EDM cut surfaces

Fabrication of thin sheets of GRCop-84 from SLM printed material requires a wire EDM method to cut thin sheets from an SLM printed block as shown in Figs. 2(b) and 4 (d,e), as printing a single 0.5 mm thick sheet vertically would lack stability, and printing flat on the build plate would be uneconomical as each sheet would fill the majority of the plate surface area. Wire EDM cutting used spark erosion of the metal work piece by a traveling wire within a water bath. The combination of the

Conclusions

Additive manufacture of GRCop-84 for use in fusion reactor RF components provides superior strength compared to CuCrZr alloys. Surface roughness on additive manufactured parts increases RF losses by increasing surface resistance, impairs the ability to clean a surface, and provides potential locations to initiate arcing a high-power vacuum waveguide. Finishing techniques were explored to determine optimum methods of producing a smooth GRop-84 waveguide surface. To reduce RF losses to within 10

CRediT authorship contribution statement

A.H. Seltzman: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Validation, Visualization, Writing - original draft. S.J. Wukitch: Funding acquisition, Project administration, Resources, Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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