How will you finish your metal part design? Some metal parts do their jobs inside equipment away from prying eyes. Others are out in the open where at least utilitarian surface treatments are expected. And t hen there are parts that might have a functional use but because of their location require the finest surface finish possible - it’s the showroom appearance that sells the goods.
A design engineer will first ensure that the component laid out in his CAM program meets the physical constraints of the mating parts. The alloy is chosen for its physical and environmental compatibility properties. And finally the surface finish is selected for properties of lubricity, corrosion resistance, wear resistance, electrical conductivity, and naturally - appearance.
There are two families of surface treatment involving either addition or modification. In the former case addition includes diffusion, cladding, overlay, plating, galvanizing, powder coating, ceramic or vitreous glazing, plasma spray, and vacuum sputtering. These processes range from centuries old to state-of-the- art and from inexpensive to suitable only for the space program.
Surface modification involves chemical or mechanical processes being brought to bear on the base metal itself. In these cases the metal is unchanged and nothing of consequence is added to the structure. Shot peening for example is commonly used to work harden the surface while at the same time to smooth the surface and change its luster. Other techniques include buffing, etching, pickling, anodizing and electro polishing.
Mechanical buffing employs a carrier (soft buffing p ads of fabric or leather or hard plates of soft metal like copper). Between the work piece and the carrier a buffing compound is applied, commonly as a slurry. If the buffing compound is harder than the work piece then the compound actually cuts into the piece leaving scratches. Finer and finer grits are used until an acceptable appearance is achieved. This is how the harder gemstones are polished. If however the compound is softer than the work piece then the peaks of the scratches are burnished (deformed) until the peaks fill in the valleys and the surface is smooth.
In the case of anodizing, the surface of the aluminum is oxidized to the depth of 10 nanometers – about the thickness of 100 atoms. Pickling steel with either hydrochloric or sulfuric acid removes surface scale and oxidation but the resulting bare metal could quickly oxidize unless further treated. On the other hand phosphoric acid could be used to clean and passivate stainless steels by selectively removing iron and leaving behind a thin superficial layer high in protective chromium and nickel.
Rather than covering a metal surface using additive techniques or protecting it in the case of the passivation techniques, it is possible to leave the native metal unaltered chemically and merely alter its surface roughness to achieve a desired appearance. (Surface roughness details available in the ASME document B46.1-2009 and subsequent issues).
One method to reduce surface roughness is a combine d electrical and chemical process called electro-polishing or anodic dissolution which involves immersing the metal parts into an acidic bath and applying an electric potential.
Just as lightning strikes elevated and pointed structures, the dissolving of a rough surface occurs mainly on the elevated points of a surface scratch, more so than in the depths or valleys of the surface imperfection.
The voltage is typically 20 to 45 VDC and the bath is blend of concentrated sulfuric and phosphoric acids, although additions of perchloric acid, sodium phosphate, sodium perchlorate, ammonium fluoride, or hydrofluoric acid are sometimes Plasotec Examples made depending on the alloy of the workpiece. This aggressive approach allows a fast throughput and therefore lower application costs but the high rate yields edge rounding and reduced fidelity to the machined design. In other words it develops an overall rounded part appearance which in some applications is highly desired. Surface smoothing is possible to 0.2 um.
A variation of electro polishing, plasma polishing, uses a weak acid to neutral solution with a higher voltage, commonly 200 VDC but in some cases as high as 350 VDC. The resultant micro-roughness is commonly a mirror-like 0.01 um. This process is used extensively for stainless steels but could also prove effective for red metal alloys, as well as nickel, chrome, titanium, and some magnesium alloys. Because the surfaces are potentially truer and smoother and because the solutions used are nearly neutral and the wash is de ionized water this technique is environmentally friendlier and is increasingly used throughout the industry.
In the plasma polishing process the work piece is connected as the anode (positive pole) which is placed into the bath along with the negatively charged stainless, graphite or platinum cathode. The bath solution varies according to the metal being processed with ammonium sulfate and aluminum chloride being most common, in concentrations ranging from 3% to 10%. Glycols and other agents may be added to increase the bath viscosity. Rather than a still pool the baths are agitated using mechanical stirring, jet pumps, or bubbling compressed air.
The DC voltage applied is tailored to the metal being etched, the bath composition and temperature, and the rate of etching desired. Current and voltage are controlled as the process moves from dissolution to passivation, preventing the passive layer from breaking down which would cause pitting.
The current density, size of the part, and initial surface roughness and desired final smoothness all contribute to the duration in the tank.
The chemical process involves metal being ionized on the surface of the part, resulting in the positively charged ions being attracted to the negatively charged cathode. In essence the process electroplates the cathode with the metal sacrificed selectively from the workpiece high points – the reverse of electroplating. As byproducts hydrogen is given off at the cathode and oxygen at the anode where it protects the workpiece from hydrogen embrittlement. Common metal removal rate from the work piece range s from 2 um to 8 um per minute.
Plasma and other electro-polishing techniques perform best when the surface defects to be corrected exhibit imperfections such as scratches that project above the surface norm. Imperfections that are depressed areas or gouges are not as readily corrected. In the latter case much more material would have to be removed to achieve a planar surface.
What could electro and plasma polishing do for your products? Give them protection from the environment and eye-appeal
Call USTEK Inc. for all your metal part requirements:
4663 Executive Dr
Columbus OH 43220-3627
Robert Sergio Simon