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Developing coatings for Mg: a permanent mold coating traditionally used with aluminum has been optimized for use with magnesium.

After remaining in the shadows for a long time, magnesium castings now occupy a small part of the automobile manufacturing market. Most of these castings are diecast and normally used for non-critical applications (e.g. valve colors and instrumental panels). However, if other processes were developed for the metal, particularly permanent mold, it could become a significant part of the structural automotive market.

So, why is the permanent mold process ignored when it comes to magnesium? The combination of the metal and permanent mold should reveal similar property advantages as those seen with aluminum's marriage to the process.

The only rationale is that magnesium alloys are rarely chosen for structural parts, very much like in the early use of aluminum. But the fact that the mechanical properties of magnesium and aluminum are similar should lead to an increasing demand for magnesium structural castings. And according to the Structural Cast Magnesium Development Program of the U.S. Automotive Material Partnership, permanent mold could be a viable processing route to produce quality components of varying size and thickness.

Several roadblocks have been preventing successful permanent mold casting of magnesium, the most important of which is a lack of suitable mold coatings. However, recent developments have put one on the market. It also has been found that boron nitride, a coating normally used with aluminum, can be optimized for magnesium use.

Turning a Coating

Previous work has tested the majority of the coatings commonly used with aluminum permanent molds, but none of them have been found to be satisfactory for magnesium. Coatings used with aluminum create a reaction with the sodium silicate binder, producing a black oxide which results in the coating peeling off from the mold surface.

In the absence of a satisfactory, ready-made mold coating, older research was consulted to determine that boron nitride might be a suitable material. That determination was based on the fact that boron nitride is inert to the contact of liquid magnesium and provides superior wear resistance and finish while reducing the mold/casting interface heat transfer. While the high cost and lack of insulation provided by boron nitride make it impractical for long term casting production, proper use of the coating can create a niche solution for producing magnesium components through the permanent mold process.

Earlier research also suggested the factors that would need to be optimized in the use of the coating--slurry dilution, mold temperature on application and final thickness of the coating. Also based on previous research, the method of application of the coating already had been optimized for use with a spray gun, dictating a pressure of 30 psi and a distance of about 1 ft. (Fig. 1).


Optimal Primer

To determine the optimal conditions of application of the coating for best wear resistance for each of the conditions tested, 12 to 16 castings were poured in two identical molds (Fig. 2), and the coating thickness at eight selected locations (Fig. 3) was measured before and after the pouring operation.


Because of the number of pours necessary and a limited magnesium melting capacity, aluminum A356 was used in the first trials rather than magnesium. Given the inertness of magnesium towards boron nitride, it was expected that the optimal application condition determined with aluminum would hold true for magnesium alloys. That assumption could be tested once the optimal conditions were discovered.

The mold halves were sprayed at temperatures of about 300F (150C), 390F (200C), 480F (250C) and 570F (300C). When the mold was too cold at 300F, the water carrier was slow to evaporate, producing a gray halo on the impacted surface. On the other hand, the coating applied on too hot a mold at 570F blistered and came off readily in several spots even before the first pour.

Applying a coating of uniform thickness over the molds was difficult, the average thickness varying generally from 40 to 90 micrometers on the four faces of the two molds. The wear rate measured after 12 to 16 pours was 1-2 micrometers per pour. No differences in wear rate were noticeable between the sprue find the locations at the bottom and top of the mold cavity. The coating applied on a mold at 390F was found to be the most durable, with the wear rate being less than 1 micrometer per pour vs. 2 micrometers per pour for application temperatures of 300 and 480F.

Using this data in conjunction with past findings, the optimal conditions for the application of boron nitride were found to be:

* 30 psi spray air pressure;

* 1 ft. spraying distance;

* 9 degrees Baume slurry density;

* 400F mold temperature on spraying.

Optimal Pouring

The two molds were sand blasted, and the coating was applied according to the optimal conditions. The coating was first tested by pouring aluminium A356 at 1,350F. The results obtained indicated that the initial thickness varied from 58-119 micrometers on one face of the mold to 25-77 micrometers on the other face. Only in the regions of abrupt change in flow direction did the coating wear to the bare mold, i.e. at the bottom of the sprue and, to a lesser extent, at the far end of the runner.

Similar tests were carried out at a pouring temperature of 1,560F. The average coating thickness on one set of mold halves initially was 71-76 micrometers and 29-31 micrometers after 15 pours, while the figures for the other set of mold halves initially were 43-48 micrometers and 30-33 micrometers after 15 pours.

The mold coating thickness was measured on the four mold halves at the same eight locations as in previous trials (Fig. 2). The changes in thickness confirmed that the thin coating (45 micrometers) wore off at a rate of 1 micrometer/ pour, while the thicker coating (75 micrometers) wore off at about twice that rate. Similar to what was observed in the tests carried out at 1,350F, two locations of extreme wear were noticeable at the bottom of the sprue and the far end of the runner.

The same two molds used for the aluminum pouring studies were then sand blasted and coated with a new application of boron nitride at optimal conditions for magnesium. The molds were preheated to 660F in a resistance furnace and poured six times in succession with magnesium AZ91E at 1,350F.

Coating Conclusions

For the optimal application of the boron nitride mold coating with magnesium (9 degrees Baume density, mold at 400F), the wear rate is about 1 micrometer/pour. The wear is more severe with thicker applications of the coating, as well as in zones of impingement of the melt or abrupt changes in flow direction.

Metalcasters wishing to apply the permanent mold process to magnesium also should recognize that a commercial coating specific to that operation now has been developed. This coating presents the advantage of being more insulating than the boron nitride coating investigated, which is imperative when pouring thin, rangy castings.

For More Information

"Magnesium Driving to Permanent Mold," D. Weiss and S.T. Robison, MODERN CASTING, September 2005, p. 26-29.

"Shrinkage Behavior of AM5O, AM6O and AZ91D," J.F. Wallace, D. Schwam, D. Druyor and Y. Zhu, 2005 AFS Transactions (05-028).

Optimal Conditions for Boron Nitride Use with Magnesium

* 30 psi spray air pressure.

* 1 ft. spraying distance.

* 9 degrees Baume slurry density.

* 400F mold temperature on sprayng.

* 45 micrometer coating thickness for 1 micrometer of wear per pour.

* 1,350F pouring temperature.

Commercial Magnesium Coating Developed

Particularly if a thin, rangy casting is needed, metalcasters may want to turn to a newly developed coating specifically tailored to use with magnesium.

Because most coatings react with magnesium and are pulled out of the mold when the casting is removed, researchers developed a commercially suitable product that would minimize that reaction. By adding fluoride to an existing coating developed for squeeze casting, the chemical reaction can be stopped, and the new coating is not pulled away from the mold.

The newly developed coating does not come with the prohibitive price tag of boron nitride, and it offers the insulating properties that specifically are necessary for thin-walled magnesium castings.
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Article Details
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Author:Morin, G
Publication:Modern Casting
Date:Feb 1, 2007
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