Gas Porosity Chart

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Take it from everyone who deals in metal casting and foundry operations, Casting Porosity will ruin a good day if not constantly keeping in check. Sometimes it will do so quietly and naggingly, while other times its the center of attention. Almost always, it will have both supplier and end-user reeling at the lost time and headaches it causes.

The good news is that with the right mix of foundry know-how and modern foundry technology, porosity can be contained to a great length from what it used too.

What is Porosity?

Salesman Definition: Porosity can be referenced to holes and imperfections in the metal derived by several variables during the casting process. These holes will effect surface finish and/or cause leakage – ultimately having a negative effect on the castings structural integrity. Variables leading to porosity can be caused by gas formation, solidification shrinkage, or non-metallic compound formation, all while the metal is in its liquid state.

At the start of the casting and melting process, an important measure is to keep excess elements (like hydrogen) from building in the melt to help take away porosity from the start. After gas formation is controlled, then shrinkage and other porosity factors can be better managed. If not controlled, porosity will start to take form as soon as metal is poured and compounded along with the other porosity causing elements during solidification.

Metallurgist Definition: (See at bottom for thorough explanation.)

CONTROL IS KING:

The best means of tackling porosity is to be ahead of it. Proactive and stringent melt control is the most effective means of maintaining efficiency and limiting any lost time.

Berntsen has invested heavily into equipment and quality procedures that keep our processes in check and to the expectations that we have promised. Berntsen has built a system of control throughout the casting process and these measures  combined with foundry know-how and proper equipment have allowed us to supply at high levels of casting integrity.

 

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Clean vs Gaseous Metal

Leak-Free Casting

We have many customers who require castings to be leak-free due to either water related reasons (valves) as well as applications that require oil to be contained (gear boxes, oil pans, etc). Controlled metal allows for an efficient and controlled supply chain from the start and contributes much to our successful history as a quality oriented foundry.

Better Machined Castings in both cosmetic appeal and leak-proofing

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Breaking down Porosity into Metallurgy:

POROSITY BY GAS FORMATION

Hydrogen gas naturally diffuses into liquid aluminum.  As the aluminum solidifies, excess hydrogen will come out of solution as bubbles, which are seen as gas pits in the casting.  The quantity of bubbles is mainly determined by the amount of hydrogen in the melt at the time of casting.  The standard shop floor method for determining hydrogen level in the melt is the reduced pressure test (RPT).  The range of results when an RPT is conducted is shown in the attached Gas Porosity Chart.  Reminder!  The appearance of the gas bubbles in the samples is exaggerated by the vacuum condition of the test.  A casting cooled under normal atmospheric pressure will look much different(better) than its RPT counterpart.  But we include this to underscore the wide range of hydrogen possible in any given melt.

Hydrogen gas pits have a negative effect on the mechanical properties of the casting.  They can also affect the leakproof integrity, the quality of sealing surfaces and the ability to achieve required machined surface finishes.

Because hydrogen diffusion in the melt is an inherent problem, the hydrogen level needs to be reduced to an acceptable level prior to pouring molds.  There are various tools and methods for removing gas from the melt.  How effective they are is major factor in the quality of the melt, which is in turn a major factor in the quality of the casting.  Since the control of hydrogen level in the melt is so critical, Berntsen recently purchased a ‘fluxing’ instrument.  The tool has many advantages over other methods.  It utilizes both rotary degassing and chemical flux injection.  In operation, nitrogen gas is injected through the impeller shaft.  The impeller shears the nitrogen gas into very fine bubbles and disperses them throughout the melt.  The nitrogen has the effect of displacing the hydrogen but that method alone is ineffective if too many aluminum oxides are present in the melt.  Oxides also occur naturally, and have negative quality effects of their own.  But with regard to degassing, hydrogen has a tendency to cling to the oxides and nitrogen gas alone is not very effective in removing oxides from the melt.  “Flux” as it is used in this respect represents a salt based melt additive that de-wets the oxides and allows for their easy removal.  With the aluminum oxides reduced, the hydrogen level can be driven down to acceptable levels.  The Flux instrument combines nitrogen rotary degassing while concurrently injecting flux through the impeller.  This combination provides a consistent, highly efficient , automated method of removing hydrogen gas and oxides from the melt.