Abstract
Because of its high strength to weight ratio aluminium alloys are in great demand and extensively being used in aerospace industires. The addition of alloying elements such as silicon along with other minor elements increases its mechanical strength. In this study we evaluate the effect on casting quality of an ultrasonic method for treating the melt. An experimental set up was built for the degassing of hydrogen and other inclusions from aluminium melt. First an impeller is rotated in an inert gas environment to draw out hydrogen bubbles by increasing the surface area to volume ratio. Then using ultrasonic vibrations at a frequency of 20 kHz and power of 1.5 kW, cavitation bubbles induces dispersion and degassing action. Analysis of microstructure and mechanical properties was done using SEM (scanning electrom microscopy) and brinnels test. Various relationships such as the melt temperature, volume melt, grain size and their effects on degassing efficiency have also been established.

Origin Of The Problem
Porosity is as serious issue in casting process. It’s an unavoidable defect, that no matter, how many precautions are taken, the slightest of the traces will still be left. The defects not only leave the surface uneven but also have adverese effect on the strength of the material.
Gas porosity which mainly takes place due to entrapment of small air bubbles (the main being Hydrogen gas). Scientists and the expermentalists have been looking for the best of the methods for removing these defects, but still facing difficulties.

Introduction
Aluminium alloys are light metals with low weight and excellent corossion resistance properties and therby are extensively used in aerospace and automotive industries. Most Aluminium alloys are produced using die casting methods, but certain defects are also present which reduce mechanical strength and degrade the casting quality. The scope of this report is using certain techniques to reduce the porosity levels therby making the alloy of superior quality.
A413 LM6 which is a not only a good corrosion resistant casting alloy but also with good mechanical strength and brittle nature has been considered for the experiment.
In this report Ultrasonic degassing method has been our main technique in use, to reduce the gases causing porosity. Although other methods such as rotary impeller degassing and spray degassing have also been discussed, but our main stress has been on Ultrasonic technique.

No doubt Spray Degassing, a former technique still used in industiries, is one of the effective method to eliminate hydrogen gas by the spraying inert gases (better known as purge gas) which combine with aluminium drops in atomized form thereby removing the hydrogen content.

Rotary impeller Degassing which is being substituted by the above mention degassing technique is also in use but the problem with it is the environmetal hazards caused by the use of hexacholoroethane tablets (which on reaction with aluminium in its melt forms aluminium chloride gas bubbles, thereby collecting hydrogen gas along with it and bringing to the surace being absorbed by the atmosphere).Therefore the maximum hydrogen content that can be reduced by this technique is less than about 0.1cm3/100 g Al.[3] Efficiency of Spray degassing is far better than that of Rotary Impeller Degassing both in terms of time consumed and considering environmental issues.

In order to remove Hydrogen gas, a new technique - ultrasonic method is in use. Ultrasonic Degassing uses ultrasound vibrations of high intensity, which introduces alternating/oscillating pressure waves in the molten material. These oscillating pressure waves then creates small cavities in huge amount, which takes in the dissolved Hydrogen from melt. The Hydrogen bubbles coalesce and float to the surface.
According to one of the sources, aluminium melts have been found to contain impurities in tiny form. Cracks or cervixes are generally found on the surface of these impurities. In alumium melt these defects becomes sites fro adsorbing hydrogen, which finally diffuses with air bubbles and then mixes in the melt. But however when treated with ultrasonic vibrations, the pressure induced is sufficienr for developing cavitation bubbles on the impurity surface thus refining the micro-structure.[7,8]
Though research on Ultrasonic degassing method was initiated by Soviet Union [6], scientists are still woking North America to furher develop this technique. In Oak Ridge National Laboratory, an experimental device was built for the hydrogen removal from aluminium using ultrasound vibration at a frequency of 20 KHz.[2]

Research Objective
The objective of this report is to design efficient method to reduce the porosity levels present in the aluminium melt by the combination of more than one methods.
In order to achieve this objective, first a rotary degassing method is used followed by ultrasonic inspection.

METHODOLOGY

1.Stirring

Here an impeller in form of a stirrer is being used to draw out hydrogen bubbles.
The impeller rotating creates swarm of bubbles over wider area, thus increasing surface-area-to-volume ratio. The bubbles though smaller in size, stay long enough in the melt causing the hydrogen bubbles to collect over time.

Mixing chlorohexaethane (C2Cl6) tablets, also helps in degassing from the melt. The tablets react with melt aluminium forming aluminium chloride gaseous compunds. Since the partial pressure Hydrogen gas is almost negligble it fuses with the bubbles formed by the aluminium melt. The bubbles along with hydrogen float to the surface and escape vi exhaust system.

When both techniques are combined, the mechanial stregnth of the metal increases (due to decrease in porosity level) which is better than using individual methods.
Using rotating impeller is a two-way benefit. First The turbulence created by rotating impeller forces the solid particles to collide and form clusters. The clusters are formed due to weak vander wall forces of attraction between solid paticles. Now either the particles settle down to the bottom or they rise to the surface with the bubbles and are skimmed off.[5]
Second it helps in degassing, reducing the porosity levels as mentioned above.
Several Factors like initial hydrogen level, volume of vessel, impeller speed (rpm), surface effects (vortexing and splash), gas flow have to be considered when developing a suitable degassing rotor technique.[4]
Best rotor speed selected for one of the experiments was found to be 500 rpm.
After the experiment was conducted, there was a significant improvement in the tensile strength as well as the elongation of the metal. With increase of degassing rate, the hydrogen concetration levels decreases in the melt, increasing the density of the material and therby decreasing number of pores in aluminium.
The combined method (rotary impeller and hexachloride tablets) was far better and effective than the individual methods used. The degassing rate was found out to be 76.9% with final hydrogen concentration 0.51cm/Kg, provided with the gas flow rate kept at 0.2m3/hr, rotational speed of 500 rpm and a refining time of 15 mins.

2.Ultrasonic Inspection

The ultrasonic system consists of Ultrasonic generator (1.5kW), transducer, radiator (for transmitting vibrations to melt). Aluminium and its alloy ingots were melted in a furnace surrounded by bricks. The chemical composition of the alloys are lised in the table.

% | Fe | Si | Cu | Mn | Mg | Zn | Ni | Ti | Sn | Al | A413 LM6 | 0.6 | 11.5-13.5 | 0.1 | 0.4 | 0.1 | 0.1 | 0.1 | 0.15 | 0.05 | Balance | A356 LM25 | 0.09 | 6.5 | 0.05 | 0.05 | 0.3-0.45 | 0.05 | - | - | - | Balance |

3 Kg of aluminium alloy was heated for 10 s at a temperature of 700 C, before the ultrasonic treatment. Then the treatment was applied for a time duration of 1 – 3 minutes at a resonant frequency of 20 kHz. Samples for the analysis of microstrusture and mechanical properties were taken by pouring molten metal into a water colling system and obtaining chilled samples 40 mm in diameter.

3.Grain Size
After the experimentations it was found that the grain size actually reduced after the treatment compared to the the microstructure of the alloy without treatment exhibiting larger grain size. The ultrasonic method enhances the degassing action due to the formation of cavitation bubbles which also enhances nucleation therby promoting the microstructure refinement [9]. To study the microstructure of the constituents, Scanning Electron Microscopy (SEM) analysis was used.

4.Results
Maximum alloy density of 2.67 Kg/dm3 was obtained after 2 minutes of ultrasonic experimenting. Although after a minute the density had reached the value of 2.629 kg/dm3 (about 98.5% of the peak value). The kinetics of ultrasonic degassing shows that the rate of degassing slows down and finally reaches a steady state is reached. No more gas can be surfaced out after the steady sate.[2]
The number of large pores decreased as the experiment was carried out as a consequence of ultrasonic vibrations (by the formation of cavitation bubbles). SEM showed hydrogen bubbles, of diameter 22um and below still remained (even after 1 minute of processing).[10]
The nucleation of cavitation bubble depends mainly on surface tension and vapor pressure of the melts.[14] With smaller surface tension and larger vapour pressure, cavitation bubble are readily formed. The surface tension and vapour pressure at the melting point for aluminium is 0.871N/m and 0.000012Pa[11]. Because of which vapor bubbles are not likely to be induced by ultrasonic treatment of aluminium melt and hence decreasing the gas pore size and to a good extent increasing the mechanical properties of aluminium alloy.[6-8,12,13]

5.Limitation
Surface roughness (for order > 50um) and dicontinuities found at the cast surface makes ultrasonic testing more difficult as it scatters the ultrasonic waves.

6.Conclusion 1. Ultrasonic Degassing method is reliable and fast. It took only few minutes (infact within 2 minutes) to degass a small volume until the steady state gas concentration was reached. [2] 2. The melt temperature effects the efficiency of degassing. For temp range of 700 – 740 C, the degassing rate was much higher as compared in the range of 620 – 660 C. [2] 3. For a large volume melt, the ultrasonic degassing rate was lower as compared to that in small volume melt. However the steady state concentration didn’t change with the changing volume of melt. [2] 4. The effect of grain size refinement increases with increasing frequency, power and processing time but upto a limit.[9]

7.Study Of Gases Used in Degassing

An environment of inert gases is kept while using the stirring method to make sure that hydrgoen which was absorbed to the atmosphere doesn’t get reabsorbed back to the melt (since hydrogen reacts easily with aluminium melt).

Nitrogen
Because of its less cost, Nitrogen is commercially used as Degassing agent in rotary degassing.
It creates a metal dross (wet) which is rich in aluminium.

Argon
Though more costlier than nitrogen, it is more efficient in removing Hygrogen entrapped bubbles.
Being inert in nature and more denser (compared to nitrogen and helium), argon makes a proective sheath over the melt therfore blocking further oxidation and hydrogen reabsorption.

Since Argon is an inert gas and more available compared to other inert gases, which makes it less expensive, it is commonly used as purging gas be it in Rotary Impeller or Spray Degassing.
Moreover Nitrogen reacts to some extent with aluminium forming Aluminium Nitride (AlN), we prefer using Argon.

8.References

1. Influences of different degassing processes on refining effect and properties of 4004 Al alloy. March 2013

2. Degassing of molten aluminum A356 alloy using ultrasonic vibration. – Habing Xu. August 2004

3. Spray Degassing as a Method for Hydrogen Removal in Aluminum Melts. April 2007

4. Understanding Aluminium Degassing – Dr. David V. Neff – May 2002

5. Removal of Hydrogen and Solid Particles from Molten Aluminum Alloys in the Rotating Impeller Degasser: Mathematical Models and Computer Simulations – Virendra S. Warke. June 2003

6. G.I. Eskin, News of the Russian academy of natural sciences, Metallurgist 42 (No. 8) (1998) 284.

7. G. I. Eskin: Ultrason. Sonochem. 1 (1994) 59–63. 12)

8. G. I. Eskin: Ultrason. Sonochem. 2 (1995) 137–141

9. Materials Transactions, Vol. 50, No. 2 (2009) pp. 401 to 408 #2009 The Japan Institute of Metals

10. The influence of melt treatment process on the quality and performance of Al based components – H. Puga, J. Barbosa, F. Silva

11. J. J. Jasper: J. Phys. Chem. 1 (1972) 841–1010

12. J. Campbell: Int. Met. Rev. 2 (1981) 71–108

13. G. I. Eskin: Ultrason. Sonochem. 8 (2001) 319–325

14. E. Apfel: J. Acoust. Soc. Am. 48 (1970) 1179–1186