The problem of microporosity of porous metals
What is the reason of leaks?
As we know, cracks, fissures and porosity are the cause of them. These deficiencies are originated when a metal is fused, especially with alloys and/or parts with large section changes.
All of this is due, among other reasons, to the material, that is not suitable for a particular type of parts, to cooling defects, to the gas formation or contents of extraneous matter during melting, which produces cracks, cavities and porosity.
It may not even exist deficiency of the material structure, but because of the thickness of the pieces they may present escapes through intermolecular spaces.
All of these deficiencies, which are not easily discovered at first sight, produce scraps in castings.
In case the microporosity produce leaks during common use of pieces, these leaks can be treated with our impregnation process and be 100% recovered. Even tough the pieces will be treated with impregnation process as an additional impregnation process during the prodution, they will have a guarantee for a long term durability. In fact the impregnation treatment has to be considered as integral part of the production process.
Types of microporosity during smelting
There are three categories of microporosity during smelting:
a) Porosity without permeability towards outside. In this case, leaks will result only in presence of mechanical operations. This can affect the structural strength of the piece, according to its dimensions and morphology.
b) Blind porosity. This is a porosity that starts from the surface and penetrates to the body of the piece. The morphology of this porosity can be extremely varied; it may have a small entrance and deep cavities.
The presence of blind porosity negatively affects the results of superficial treatments.
In fact, the pores become receptacles of air, of treatment fluids, of cutting oil, etc, which may flow out of the metal, corroding treated surfaces and producing the following problems:
– in case of oven-treated painting, there may be a “blistering”effect (bubbles on the surface of the painting);
– in galvanised pieces, discoloration;
– in anodization, “white spotting”.
c) Permeable Porosity. This category of porosity is the most relavant because it may cause discarding of components that have already been mechanically processed and which, therefore are very expensive.
Advantages of anti-porosity treatment
Considering the impregnation costs of porous metals, the largest firms, especially those in the mechanical sector, have calculated that with leaks in more than 5% of the pieces, applying the impregnation treatment to 100% of the production gets interesting, in order to limit the final control to a single lot sampling. This percentage is variable, because it depends on the mechanical processings the pieces are previously subjected to.
It is also possible to monitor impregnated pieces. It must be evaluated whether it is more economical to impregnate and recover pieces or monitor them impregnating only those with leaks. In this case the impregnation treatment must be considered as part of the production process, therefore control and impregnation costs have to be added.
What is a leak and measuring leaks
An important point to consider with this type of problem is the definition of “leak” and how it can be discovered. A “leak” can be defined as the process of filling or loss of a fluid through the wall of a piece, which jeopardize the piece itself. Therefore without the jeopardising effect, the word “leak” is not proper.
What has to be decided, is if the leak is acceptable or not.
To get to this conclusion, a unit of measure is necessary. Since the flow of gas is used in most of the processes to identify leaks, we must specify a volume and pressure in order to determine a quantity of fluid.
The most common unit is cm3, at atmospheric pressure per second (cm3atm/sec).
Since leaks are very tiny in most of the cases, it is used a system of negative exponents of 10.
The measurement of a possible leak, in its industrial application, with its visual equivalents, is shown in the following scheme:
|10-1 1cm3/10 sec.
|10-2 1cm3/100 sec.
||10 bubbles per second
|10-3 3cm3/10 hrs
||1 bubble per second
|10-4 1cm3/3 hrs
||1 bubble every 10 seconds
|10-5 1cm3/24 hrs
|10-6 1cm3/2 weeks
|10-8 1cm3/3 yrs
|10-9 1cm3/30 yrs
A bubble is considered as having a volume of 1 mm3.
It must be kept in mind that the maximum leak acceptable for a product depends upon its nature and upon the fact that static systems require more restrictive specifications than dynamic ones.
For instance, in a dynamic chemical process, a leak of 10-1 at 1 cm3 in N.C. may be tolerable.
Every producer must determine the tolerable level of leak and perform tests in conditions similar to the real situation, since different conditions of temperature or fluid would make the test results not valid.
Leaks greater than 10-1 can be discovered with visual or acoustic methods.
It must be kept in mind, additionally, that looking for leaks 100 or 1000 times smaller than acceptable limits only adds expenses, without improving the dependability of the product.
Nevertheless, it is recommended to work during tests with values twice those of normal use, in order to obtain a reasonable degree of leak discovery.
The mission of Munari System impregnation procedure is to solve, first of all, the problems of porosity created during casting.
The technological process and the market requirements have in fact lead the designers to realize increasingly complex types and use increasingly lightweight alloys.
Because all of this it is impossible to avoid porosity of castings and it is necessary looking for a procedure that make the restoration of defective elements possible.
With the anti-porosity treatment, restoration is possible. Castings that are excessively porous or damaged can therefore be “saved”, as well as elements whose homogeneity, endurance and quality can be improved in advance (such as electrical windings).
The success of Munari plants is due to the combination of two great advantages: on one hand, the incomparable efficacy and the effective convenience of Munari impregnating products and, on the other hand, the choice of realizing exclusively custom-made plants.
The relation that the Munari firm establishes with each company interested in the supply is, in fact, a type of consultancy: in the initial phase, the client’s requirements are analysed by a technical-scientific team; after that are following the research and testing of the specific impregnating product; at last, a calibrated plant is designed, according to the productive volumes needed and the space that the client has available.
The proposed solution always contains two possibilities: the first one is “contingent”, in order to meet practical current requirements. The other one is “a prevision”, based on potential future requirements.
In Munari’s plants the use of Munari impregnating products obviously is expected. They are competitive in price, practical, extremely resistant, even at high temperatures (700-750 °C), non toxic and above all, ecological.
In case Munari products were not suitable for the specific treatment required by the client, other impregnating products can also be used.