Solvability of hydrogen in aluminum.

Budic, Ivan ; Novoselovic, Daniel ; Gros, Josip 等

1. INTRODUCTION

Pure metals and alloys produced by any procedure whatsoever contain besides other mixtures also a certain amount of gases. Gases in castings of different metals occur in the shape of gas bubbles, solutions (interstitiously diluted), chemical compounds. Solubility of gases in metals depends upon:

--temperature

--composition of metal

--partial gas pressure

Under the same conditions (temperature and pressure) solubility of gases depends on kind and structure of the metal. Solubility of gases in metals from kinetic point of view is considered as complex of elementary particles.

Solvability of hydrogen in aluminum, magnesium and some other harder solvable metals is significant.

2. CASTING DEFECTS DUE TO DISSOLUTION OF GASES

Porosity belongs to one of the main factors regarding quality of the products in casting industry. Porosity in some degree is always present, has bad influence on quality of the surface of the casting, its mechanical properties and resistance to corrosion. Hydrogen as the only gas capable to solve in significant quantities in aluminum melt is the main factor influencing gas porosity. Another kind of porosity occurs due to reduction of metal volume due to process of solidification.

[FIGURE 1 OMITTED]

3. SOLVABILITY OF HYDROGEN IN ALUMINUM

Based on results of numerous investigations it has been determined that solvability of hydrogen in aluminum increase at high temperatures in the following order: [N.sub.2], S[O.sub.2], CO, C[O.sub.2], [O.sub.2], and [H.sub.2]. This proves that solvability of hydrogen is the greatest.

During melting of aluminum and its alloys there is a violent reaction between aluminum and the water vapor in the oven atmosphere, according to the following reaction:

2Al + 3[H.sub.2]O [right arrow] [Al.sub.2][O.sub.3] + 3[H.sub.2] (1)

Parallel with the aforementioned reaction, another reaction between carbides and aluminum nitrides and water vapor is possible, whereby a certain quantity of gases (methane, H[N.sub.3], CO) arise.

Penetration of hydrogen into aluminum develops in three process stages, as follows:

--absorption

--diffusion

--dissolution

Absorption represents accumulation of gases on the metal surface layer (alloy), from where they enter into interior of the metal by process of diffusion. Gas retained in the interior of the metal in atomic or molecular condition is present in the solution or is incorporated into the metal.

Absorption on contact surface alloy--gas is the first stage of the process in which gas is dissolved into metal. Gas diffuses into metal as consequence of gas solvability at specific temperature. The effect of oxide layer created on the surface of aluminum alloy, which protects it from further oxidation, should be added to the process of absorption.

The quantity of hydrogen which aluminum can take over from water vapor during melting surpasses balance values for dry hydrogen.

Due to reaction of aluminum with water vapor, several times greater gas quantities can pass over to the melt. How much gas will pass over depends upon partial pressure of the water vapor and the speed of chemical reaction. Table 1. shows solvability of hydrogen in aluminum at different temperatures (Casting manual, 1985).

4. EXECUTION OF RESEARCH

Establishes the present day state of knowledge, we choose the most influential parameters to get solvability of hydrogen in aluminum alloy. Tests were conducted on AlSi12 casting in single sand mold, where we measure casting temperature and pressure fig.2. Figure 3. shows the measurement equipment and figure 4 shows aluminum casting (Budic, 1994).

Measurement results for solvability of hydrogen in aluminum alloy are given in the diagram fig. 5.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

5. REMOVAL OF HYDROGEN

There are many methods of degassing of the melt, to prevent bubbling of the casting. This paper gives description of only two methods which are frequently applied.

5.1 Degassing by gases

Degassing is mainly conducted by blowing of inert gases into the liquid alloy. Gases such as nitrogen, argon, chlorine or freon are blown through submerged graphite pipe lance with holes to the bottom of the melt. As the bubbles of the blown in gas travel towards surface, they are exposed to bubbles from melt, and bring it up to the surface. The most favorable way would be to introduce larger number of tiny gas bubbles into the melt during the process, because this increases possibility of their contact with those gas bubbles which should be brought to the surface (Rapp, 2008). The application of rotation machines leads to mixing up of the melt, causing better efficiency of the process, because blades brake gas bubbles into numerous smaller ones, and distribute them evenly into the melt.

5.2 Degassing by ultrasonic vibrations

This procedure uses high intensity ultra sonic vibrations to create oscillating pressures in the aluminum melt. The oscillating pressure creates cavitations and "tears" bubbles into tinier and more dispersed bubbles, which can absorb hydrogen easily. The result is smaller consumption of blown gas and decrease of porosity of castings.

6. METHODS OF CONTROL OF POROSITY

Test at decreased pressure--A simple and successful method of testing is test at decreased pressure, i.e. vacuum reduced pressure test (RPT), also known as Straube-Pfeiffer test.

Control by radiography--Radiography is method of internal testing where metals are exposed to the beam of X-radiation or gamma radiation. Differences in thickness, density, absorption caused by internal defects can be seen on the shady picture which appears on the fluorescent screen or photographic film.

Control by ultra sound--Control by ultra sound demands certain knowledge and experience, so as to apply corresponding test techniques and to interpret the results. Surface roughness and size deviations disperse the sound impulse, and herewith aggravate discovery of discontinuities.

7. CONCLUSION

Test results confirm some previous research that greater pressure and higher pouring temperatures lead to higher solvability of hydrogen.

Solvability can be avoided by regular choice of manufacturing technology of the castings. Hydrogen in aluminum castings might appear by mechanical grips during pouring, due to chemical reactions and changes in solvability during melting and process of formation of the casting.

8. REFERENCES

Budic, I. (1994). Contribution to the mechanism of formation of bubbling in castings, Doctoral thesis FSB, pp. 18, Zagreb

Casting manual (1985). Croatian Foundry Association, Zagreb

Rapp, R.A. (2008). The Closed-circuit degassing of liquid aluminum by Argon, Available from: http://www.springerlink.com Accessed on: 2009-07-31

*** (2009) http://mix.msfc.nasa.gov--Porosity inside a metal casting Accessed on: 2009-07-31

*** (2008) http://www.osti.gov--Degassing of Aluminum Alloys Using Ultrasonic Vibration Accessed on: 2009-07-31

Tab. 1. Solvability of hydrogen in aluminum at different
temperatures

Temperature, Hydrogen solvability,
[degrees]C [cm.sup.3]/100 g Al

300 0,001
400 0,005
500 0,012
600 0,026
658 solid 0,036
658 liquid 0,69
700 0,92
800 1,67
850 2,1