Vacuum Gas & Inclusion Testing Apparatus

Application:

Z-Met analyzes the presence of dissolved hydrogen gas and inclusions inside molten aluminum on a qualitative real-time basis. Z-Met is used by process metallurgists when immediate knowledge of cleanliness of molten aluminum is required for quality control and process correction. The analyzer has application in plants which manufacture highly sophisticated aluminum alloys and MMCs for aerospace, defense, nuclear, automotive, and electronic industries.

Method of Testing:

150 gms of molten aluminum (2” dia x 1” height) is collected in a specially designed crucible and vacuum chamber. The sample is solidified under ultra high vacuum generated very quickly in the solidifying chamber. The solidification process is viewed over a CTV screen. A video camera helps the operator to analyze the extent of hydrogen gas and the eruption of inclusions present in the sample. The Z-Met in-line test is extremely rapid and takes about 5 minutes from sample collection to completion of the test.

Z-Met Unit Specifications:

  • Size: 42” (1060 mm) x 42” (1060 mm) x 72” (1828 mm)
  • Weight: 200 lbs (91 kg) on mobile base
  • Voltage: Available in 120/220 volts 50/60 Hz
  • Gas Detection: From 0.08 cc/100 gms to 0.42 cc/100 gms
  • Inclusions: From 20 microns to 3.0 mm qualitative
  • Process Consumables: None

Warranty:

1 year parts & labor except for vacuum pump.

Application Background:

Molten aluminum alloys contain four types of impurities:

  • Dissolved hydrogen gas
  • Inclusions (metallic & non-metallic)
  • Dissolved alkali & alkaline earth metals
  • Salts of alkali & alkaline earth metals

These impurities have to be detected and removed prior to casting. Z-Met provides an immediate real time qualitative answer as to how much gas and inclusions are present inside molten aluminum. The Z-Met technique is based on the fact that, during solidification of the molten alloy under vacuum, the impurities behave in a definite manner. The solubility of hydrogen in the solidified metal is 20 times lower than its solubility in the molten metal. As a result, when vacuum is applied on top of solidifying melt, the hydrogen gas that is rejected ahead of the solidification front is lifted to the top of sample and the size of the gas bubbles and their number are directly proportional to the amount of gas present in the dissolved state. Rejected gas & inclusions are detected under the video camera. The hydrogen gas always preferentially nucleates at a site where an inclusion is present. Also, whether the inclusions are metallic or non-metallic (i.e. simple or complex ceramic particles) they are always floated to the top of the solidifying metal if vacuum is applied on the surface of the molten metal. Thus, the observation of the sample of molten metal “during” the solidification process reveals maximum information on the internal cleanliness of the molten metal.

For example, when gas content of molten alloy is 0.10 cc/100 gm, under 0.5 Torr vacuum, an alloy will show 3 small bubbles (0.1” dia) if inclusions are present or only one large bubble (0.25” dia) if the metal has low inclusion count. So the total number of bubbles released in the vacuum test gives a composite reading on the presence of gas and inclusions in the metal.

The presence or absence of molten salts is also detected by virtue of the fact that the salt changes the surface tension of the alloy at the boundary of the gas bubble. A typical bubble under vacuum is allowed to grow to good extent inside the solidification chamber if salts are present in the molten state of the alloy. In absence of the salt the bubble size remains relatively small before a break results in the surface of the bubble by the combined effect of vacuum and the buoyancy force of the hydrogen gas inside the bubble.

Therefore, by observing as to how long a bubble has remained as a bubble of same size (i.e. without growing or rupturing) in equilibrium with continuously applied vacuum an experienced operator can make qualitative decision regarding presence or absence of salts hand-in-hand with gas and inclusion detection. Z-Met thus provides a simultaneous qualitative determination of presence of impurities in the molten aluminum and allows process metallurgists to implement a process correction in the right direction.

A video recording device (available separately) can be attached to the camera or CRT screen output and batch to batch data can be maintained on the quality of the melt as a function of heat number.