Magnesium takes on aluminum and steel

Aug. 10, 2006
U.K.-based Keronite PLC and Thixomat Inc., Ann Arbor, Mich., have joined forces to produce electrolytic-treated Thixomolded magnesium alloy parts.

Senior Editor

The Keronite process is a chrome-free surface treatment based on plasma-electrolytic oxidation. It is widely used to transform aluminum and magnesium-alloy surfaces into hard, dense ceramic with outstanding corrosion and wear resistance.

Keronite surface treatment quickly converts the surface of light alloys into a dense, hard ceramic oxide. The 5 to 50- m-thick layers have porous surfaces that accept topcoats and a hardness of between 36 and 55 Rc.

U.K.-based Keronite PLC and Thixomat Inc., Ann Arbor, Mich., have joined forces to produce electrolytic-treated Thixomolded magnesium alloy parts. The Keronite surface treatment reportedly will let magnesium gain a foothold in automotive applications where conventional magnesium alloys are too soft.

Thixomolding is well known for producing near-net shapes that need little finishing — molded parts have smoother surfaces than conventional castings, without flow marks. Surfaces also have lower porosity and fewer defects and, thus, better mechanical properties than die-cast magnesium. That's because Thixomolding eliminates high-velocity turbulence as molten metal enters a die. This prevents oxide and flux inclusions common with conventional casting operations. The process may also promote aluminum enrichment on surfaces, giving greater corrosion resistance than conventional die-cast alloys.

Keronite is a chrome-free immersion process that creates an extremely hard layer on inner and outer surfaces of complex shapes. The process faithfully follows the contours of the substrate surface as it creates a ceramic layer of uniform thickness with high dimensional accuracy. The Keronite thickness can be adjusted to optimize performance. At 400 to 700 HV, Keronite on magnesium has similar hardness to hardanodized aluminum. The process quickly produces a layer typically between 5 and 50 μm at a controllable rate of 1 to 5 μm/min. Magnesium with Keronite coating is an ideal candidate for environmentally friendly, highprecision parts.

A recent EU-funded project called Nanomag tested Keronite on conventionally formed magnesium pistons for small engines used in lawnmowers, motorcycles, and chainsaws. The tests showed Keronitetreated magnesium is as wear resistant as high-silicon cast aluminum. Coupled with Thixomolding this lets magnesium replace aluminum but also steel and plastics in small or lightweight structures that need high precision and corrosion resistance.

Keronite PLC has also formed a partnership with Gramm Oberflaechentechnik of Germany. The pair is developing a costeffective automated process for selective masking of high-volume components at high speed.

Thixomolding at a glance

In a single step, the Thixomolding process transforms room-temperature magnesium chips (heated to a semisolid slurry inside a barrel and screw) into precision-molded components. No sintering or debinding steps are required as in the MIM (metal-injection-molding) process.

High-dimensional stability and tight tolerances are a direct result of laminar flow of the semisolid magnesium into the mold cavity, as well as high mold-filling pressures and rapid solidification.

Typical tolerances are classified as follows:

Linear tolerances are ±0.001 in. (0.003 cm) for each inch of dimension.

Across parting line and moving die components need the appropriate linear tolerance plus an allowance of an additional 0.002 to 0.01 in. (0.005 to 0.025 cm) depending on projected area of the component from 10 to 100 in. 2 (19 cm2), respectively.

Flatness tolerances range from 0.003 to 3 in. (0.008 to 8 cm) with an additional 0.001-in. (0.003-cm) flatness allowance for each additional inch. The largest dimension of the flat surface should be used.

Machine stock allowance of 0.010 in. (0.03 cm) maximum is recommended if machining requires extremely tight dimensions.

Thixomolding is not as aggressive on tooling as die-cast aluminum and, thus, increases tool life considerably.

Keronite basics

Keronite technology is based in the principles of plasma-electrolytic oxidation. Parts suspend from a bus bar and submerge into a proprietary electrolyte solution inside a stainless-steel electrode cage. A modulated electrical current passes through the bath and converts the surface of light alloys into a dense, hard ceramic oxide without subjecting the substrate itself to damaging thermal exposure.

The Keronite layer attaches to the substrate by a strong molecular bond. The fused ceramic layers closest to the substrate surface protect against corrosion and wear. The outer surfaces of the Keronite layer are, however, porous and lend themselves to the application of scratch-resistant, decorative topcoats including paints and lacquers as well as PTFE, adhesives, and metal coatings. Additionally, the proprietary electrolyte solution contains no chrome, ammonia, or other toxic chemicals. The nonhazardous liquid requires no special treatment prior to disposal and presents no danger to those handling it.

Typical properties of Keronite on magnesium:

Corrosion resistance: Surfaces resist atmospheric and galvanic corrosion and withstand over 1,000 hr in salt fog (ASTM B117).

Scratch resistance: The porous outer layer and atomic bond with substrate provides excellent adhesion for top coats or composite layers. Treated magnesium parts are reportedly three times more scratch resistant than their anodized counterparts.

Hardness: Typically, hardness ranges from 400 to 600 HV (36 to 55 Rc), Independent tests have shown, however, that hardness of around 700 HV is possible on a AZ91 magnesium alloy, making it harder than most hard-anodized aluminum.

Wear resistance: Eliminates high friction and galling. Wear abrasion tests (Taber) suggest that Keronite performs 20 times better than bare magnesium and is twice as durable as anodizing in a two-body abrasive wear scenario. Additionally, once polished, Keronite surfaces on magnesium have friction coefficients of less than 0.15 against steel.

Heat resistance: The dense nanoscale microstructure of Keronite ceramic surfaces also makes them ideally suited for applications where components are exposed to extreme temperatures or repeated thermal cycling. Treated magnesium parts withstand short exposures to temperatures up to 1,832°F (1,000°C).

Keronite PLC,
+44 (0)1223 89322,
Thixomat Inc.,
(734) 995-5550,

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