SURFACE TREATMENT

Deposition of aluminium takes place in electrolytes free of water, consisting of alcalihalogenid combinations and aluminium trialkyles being solved in hydrocarbon.
One of the advantages of using anhydrous electrolytes is the absolute absence of hydrogen during the deposition of aluminium because no decomposition of water occurs and consequently no hydrogen arises at the cathode.
After the deposition has been finished the Aluminal-layer can be processed with any post-treatment known from aluminium: it can be passivated or sealed which enhances the effect of corrosion protection. A lubricant can be applied for adapting properties of friction.

No hydrogen embrittlement with high strength parts

When coating high strength parts by means of aqueous galvanic systems it is possible that atomic hydrogen diffuses into the base material. This can lead to brittle fracture, being dreaded as it may happen spontaneously without any advance warning. Rising material strengths means also rising danger of hydrogen embrittlement. Therefore aqueously coated steels beyond 1000MPa have to be baked out in order to loose containing hydrogen. Due to the entirely non-aqueous composition of the Aluminal electrolytes there is no danger of hydrogen embrittlement. The examination of the high strength steel 1.7033 under constant load of 75% of the notch bar impact strength of approximately 2000MPa according ASTM F 519-93 resulted in no hints of hydrogen embrittlement. All samples passed the test of 200 hours without brittle fracture, verified by a scan of an electron microscopy. This is a unique property for galvanic coatings. It enables to combine the advantages of high strength steel with an excellent galvanic corrosion protection, e.g. homogeneity of the layer, excellent adhesion, clear outlines after coating and low friction threads. The Aluminal-layer is already pore free and corrosion resistant at 6µm thickness. This enables Aluminal to keep narrow dimensional tolerances depending on the geometry of the part. For plain parts such as retaining rings it is possible to keep fluctuation of thickness below half of 1/100. This is achieved by optimising the plating racks permanently.
Since 2007 the Seeger Orbis company delivers Aluminal electroplated retaining rings to an established German OEM.

New potential for development of spring technology

Electroplated sheet springs and retaining clamps are already applied to serial production assemblies of vehicle manufacturers.
The serial use of electroplated springs made of high strength steel increases the requirements for the coating as well as the customer’s benefit by three additional aspects:
– no negative influence of the Aluminal-layer on the dynamic performance of the base material
– excellent resistance to stone chipping
– potential for weight reduction of >10% by using ultra high strength springs >2.100Mpa
Dynamic testing of several 100,000 load cycles have shown, that Aluminal is resistant till the collapse of the base material.
At the critical contact area between spring and suspension support, where dirt leads to abrasive friction, the advantage of the cathodic protection by pure aluminium leads to another positive effect. There is no corrosion of the spring steel in this area, too. This is another advantage of galvanic aluminium over serial paint coatings, as paint does not provide any cathodic protection. Aluminal – coatings avoid spring breaking by draft corrosion due to oscillation or tension.
Allevard Rejna Autosuspensions (ARA) is the European leader in design, manufacture and marketing of elastic suspension elements for the automobile industry and a top European supplier of precision springs in diversified applications. ARA produces suspensions in 25 plants around the world and employs almost 2000 people with the headquarter being located in France, turnover is 430 Mio. Euro.
ARA manufactures a comprehensive range of springs and stabilizers specifically designed for individual suspension requirements of cars and utility vehicles. The treatment of high-strength micro-alloyed steels calls for total mastery in the fields of heat treatment, shot-peening and anticorrosion treatments.
In order to further reduce the weight of steel springs the strength of the applied micro-alloyed steels needs to be increased. This allows for the wire diameter to be reduced. Significant weight reductions of more than half a kilogram per spring could be realised. However, lower spring diameters also call for increased corrosion protection in order to ensure spring service life.
During several years of joint development between ARA and Aluminal it became clear that Aluminal delivers significant advantages for spring surfaces such as extreme corrosion protection in salty water environments coupled with self-healing and cathodic protection properties that have shown to withstand the most severe stone-chipping tests both, in laboratory as well as on numerous test vehicle runs. The high ductility of pure Aluminium combined with plating temperatures much lower than for the serial powder coating especially ultra high strength steels will perform without any losses in strength and show high durability in vehicles. The joint team effort between ARA and Aluminal has led a major German car maker to start series introduction coil springs with Aluminal surface on a volume platform.

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