Measurement methods of inclusions in aluminum melt

Measuring inclusions in aluminum melt and therefore monitoring its quality are considered important factors in production. Besides classical metallography, which involves cutting, polishing and observing inclusions, other techniques have been developed to measure inclusion amounts in aluminum melt. Each method contains advantages and disadvantages . These methods are discussed below:

a) LiMCA (liquid metal cleanliness analyzer)

In this method, a DC current is made in an orifice with a small diameter (300µm) and then aluminum melt is drawn to flow through the orifice. The non-conductive inclusions in the melt increase the electrical resistance when they pass through the orifice, changing the voltage. This is an in-line technique, which can be continuously carried out, making it preferable to use in industry. But the high cost of this method can restrict its use. This technique is suitable to detect small inclusions in the range of 20µm to 300µm , but for larger inclusions are not suitable as they get stuck in the orifice. This technique cannot distinguish between bubbles and particles, showing the same result.

b) PoDFA (porous disk filtration analysis)

This is an off-line technique with the most reliable results in comparison to other methods. It is possible to identify the types of inclusions with this technique, but it is a time-consuming method, which limits its usage. In this method, a certain mass of aluminum is forced through a fine filter, and then the area of concentrated inclusion on the filter surface is calculated by using an optical microscope. The unit is expressed in inclusion area over mass of filtered metal mm2/kg.

c) Prefil-Footprinter

This method is based on the PoDFA technique. When the amount of melt is forced through the filter, the weight of filtered melt is recorded by a balance and the diagram of filtered melt versus time is sketched. The inclusions concentrated on the surface of filter act as a barrier to decrease the flow rate of the melt. The diagram of filtered melt vs. time is used to analyze the level of inclusion in the melt; a curve with a sharper slope means a faster flow rate and a cleaner melt.

d) Ultrasonic detection method

This technique works by inserting a probe in molten metal and uses a pulse-echo technique to measure the amount of inclusions. This method is desirable because of its non-destructive feature, capability to probe a great amount of liquid and also, it can be used for the continuous, on-line observation of liquid cleanliness

e) K-Mold test

In this method, the fracture points in a cast sample are provided by the notches contained in moulds. Then, these fracture points are visually investigated and the metal cleanliness evaluated by the value K, which is calculated from the following formula: K= s/n , S: total number of inclusion in (n) number examined pieces from one sample ; n, number of pieces. This technique is mostly used in cases of large inclusions and specifically more sensitive to oxide films.