Metal treatment – Process of modifying or maintaining internal physical... – Producing or treating layered – bonded – welded – or...
Reexamination Certificate
1999-10-12
2001-10-16
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Producing or treating layered, bonded, welded, or...
C148S698000, C148S701000
Reexamination Certificate
active
06302976
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of coating with polymer and surface treating an object of high-strength aluminium.
BACKGROUND ART
Objects of high-strength aluminium are often used as structural materials for machine parts on which demands are placed for light weight and high strength, for example in aircraft structures.
High-strength aluminium is obtained by precipitation hardening (or so-called age hardening) of a so-called heat treatable aluminium alloy by a two-stage heat treatment process. In the first stage, during the so-called precipitation treatment, the material is heated to an elevated temperature at which all alloy components are dissolved in the crystal lattice structure of the aluminium and are transformed into so-called solid solution. The greater the proportion of alloy components which the alloy contains, the higher will be the temperature required for solution. The solution treatment is terminated in that the object is rapidly cooled with water, water mist or air. In the second stage, during the so-called ageing process, hardening precipitations are formed in the material. Ageing of high-strength aluminium takes place at elevated temperature for a relatively short time, so-called artificial ageing, as opposed to so-called cold ageing, i.e. ageing at room temperature over a relatively lengthy period of time.
Aluminium material is generally highly resistant to corrosion in a neutral environment because of the fact that the aluminium surface is oxidised and the thus formed oxide layer is relatively corrosion-resistant. In acidic (pH<4) and alkaline (pH>9) environments, this oxide layer becomes, however, unstable and so the material corrodes.
In order to achieve improved corrosion resistance, machine parts and structures for use in acidic or alkaline environments can be surface treated by means of coating with a suitable chemical-resistant polymer possessing superior internal strength and adhesion to the surface of the aluminium object, such as, for example, polymers containing fluorine. Fluorine-containing polymers normally also possess superior thermal resistance, which is an advantage in many fields of practical application.
One particular field of application for such polymer-coated corrosion-resistant aluminium objects is machine pats in filling machines intended for the packing of liquid foods of the type which fills, forms and seals packages in the same machine. In the handling of foods, extremely high demands are placed on hygiene and cleanliness, these demands being satisfied in that those parts of the machines which are in direct content with the food are regularly cleaned (i.e. at least once a day) by means of efficient detergents or cleaning agents. Such cleaning agents often contain alkaline chemicals. In cleaning, it is inevitable that detergents and cleaning liquids splash and drop onto other parts of the machine. In particular, machine parts which are included in the sealing unit such as, for example, sealing jaws, are often located beneath the filler pipe and the conduits which lead to and from the filler unit, which, on cleaning of the filler unit, inevitably results in cleaning agent dripping down onto these machine parts.
Surface treatment of high-strength aluminium by means of polymer coating today is put into effect in that the finished, already precipitation hardened and ready-to-use aluminium object is coated with a layer of polymer and then heated to elevated temperature in order to sinter or melt the polymer coating fast to the aluminium surface. How high the temperature which is to be selected is a matter of discretion taking into account the properties of the polymer and the temperature resistance of the aluminium. The term sintering (also known as agglomeration) is taken to signify the physical process which takes place when more or less solid material particles bond or frit to one another by molecular diffusion in the surface layer and thus “migrate together” to form a continuous microporous network.
Commercially available polymer compositions with a high melting point and which are sintered at high temperatures such as, for example, approx. 400° C. display generally better adhesion, mechanical properties and resistance to chemicals. Heating to such elevated temperatures entails, however, that the aluminium material loses both hardness and mechanical strength by more than 50% up to approx. 65-75%. In practice, polymers are therefore employed which melt and sinter at lower temperatures, such as, for example, at approx. 200° C. Coatings of such polymers unfortunately display poorer adhesion to the aluminium surface and, as a result, afford a poorer corrosion protection, while, on the other hand, the hardness and mechanical strength of the aluminium object are retained on heating up to at most approx. 200° C.
Even though such plastic-coated machine parts of high-strength aluminium today constitute the most corrosion-resistant alternatives on the market which also satisfy other design and construction requirements, they must be replaced after a relatively short service life because the polymer coating has been attacked and weakened by the alkaline substances and no longer affords blanket protection for the aluminium object which, as a result, will be destroyed by corrosion. It is, thus, an as yet unsolved problem within the prior art technology to surface-coat objects of high-strength aluminium in order to achieve improved corrosion resistance to a sufficiently high degree without negatively influencing the mechanical strength and durability properties of the aluminium object.
OBJECTS OF THE INVENTION
One object of the present invention is, therefore, to realise a novel method of surface-treating objects of high-strength aluminium as described by way of introduction, without consequential problems of the type inherent in the prior art technology.
A further object of the present invention is to realise a method of producing surface-treated objects of high-strength aluminium with improved corrosion resistance.
A particular object of the present invention is to realise a method of producing objects of high-strength aluminium possessing improved corrosion resistance and retained pristine high strength and superior mechanical properties.
Still a further object of the present invention is to realise a corrosion-resistant object of high-strength aluminium which is surface-treated with polymer and is produced using the method according to the present invention.
SOLUTION
These and other objects will be attained according to the present invention by the method which carries the characterizing features as set forth in the characterizing clause of appended claim
1
. Variations and modifications of the method according to the present invention are apparent from appended subclaims
2
to
12
.
Further, the present invention realises surface-treated objects of high-strength aluminium according to appended claim
13
, with improved corrosion resistance and retained pristine mechanical properties.
OUTLINE OF THE INVENTION
In so-called heat-treatable aluminium alloys, one or more of the alloy components are selected such that a strength increase is achieved by precipitation hardening, also called age hardening. The precondition for precipitation hardening to be able to take place is that the solubility of the added alloy components in aluminium reduces with falling temperature. Thus, precipitation hardening is achieved by solution treating, in a first stage, i.e. for a relatively short time heating the heat-treatable aluminium alloy to such an elevated temperature that the added alloy components merge into solid solution within the aluminium structure, and subsequently rapidly cooling the alloy so that a saturated solution of alloy atoms in the aluminium material remains and, thereafter, in a second stage ageing the aluminium alloy for a relatively long period of time, when the actual precipitation takes place, for the formation of small finely dispersed precipitations distributed in the basic material.
Thus, the present i
Burns Doane Swecker & Mathis L.L.P.
Tetra Laval Holdings & Finance S.A.
Wyszomierski George
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