Aluminium identification
There are many aluminium alloys. Some are best for casting, some for extrusion, some for diecasting and some for machining. Most amateur casters use scrap aluminium. For critical castings then it is best to use scrap castings to make new castings. However for non critical applications then I tend to use what ever scrap is available. Most usually this is extruded material such as old tubing from scrapped garden furniture, old window frames, carpet joining strips etc. These things are normally easily identifiable as being made of aluminium. However, identification is not always easy and an aluminium diecasting can look very similar to a zinc diecasting..
Some metals look very similar to aluminium but will cause problems if added to molten aluminium. Magnesium in particular will catch fire and burn very fiercely and maybe destroy the crucible and the furnace. Zinc added to aluminium may burn feebly producing toxic fumes. Zinc may also cause the aluminium to become brittle. Copper and iron alloys melt at much higher temperature and present little danger but both may cause the aluminium to become brittle if they dissolve significantly in the melt. Aluminium, magnesium and zinc alloys are all commonly cast or diecast so there is some potential to confuse the three metals.
The following table summarises the properties of common engineering metals. The numbers will depend somewhat an the particular alloy.
Some metals look very similar to aluminium but will cause problems if added to molten aluminium. Magnesium in particular will catch fire and burn very fiercely and maybe destroy the crucible and the furnace. Zinc added to aluminium may burn feebly producing toxic fumes. Zinc may also cause the aluminium to become brittle. Copper and iron alloys melt at much higher temperature and present little danger but both may cause the aluminium to become brittle if they dissolve significantly in the melt. Aluminium, magnesium and zinc alloys are all commonly cast or diecast so there is some potential to confuse the three metals.
The following table summarises the properties of common engineering metals. The numbers will depend somewhat an the particular alloy.
Metal
Aluminium alloys Magnesium alloys Zinc alloys Copper and alloys Cast iron and steel |
Colour
Silvery Silvery silvery/grey red/yellow silvery/grey |
Density, g/ml
2.7 1.7 7.1 9.0 7.9 |
Melting point, degrees C
circa 660 circa 650 circa 420 850-1100 1200-1600 |
Looking at the table it can be seen that there is a big difference in density between aluminium, magnesium and zinc so this provides an easy way to differentiate these materials. If the metals are nice geometric shapes like square bar, round bar, or tube it is easy to measure the density just by weighing the piece and then calculating the volume of material from the dimensions. The weight divided by the volume then gives then density. However, scrap rarely comes in convenient shapes for accurate volume estimation so some other method id required to estimate the volume. This problem was solved by a Greek called Archimedes 2000 year ago and his principle provide a simple and accurate way to measure density. Put simply Archimedes principle says that when an object is immersed in water then it experiences an upthrust equal to the weight of water displaced. Thus the object is weighed in air and then weighed in water. The difference in the weights gives the weight of water displaced and since the density of water is 1 g/ml or 1kg/l then the difference in grams equals the volume in ml or the difference in kg is the volume in litre.
The item shown in the header photo is a digital luggage scale and this greatly facilitates making the measurements to determine density. It was purchased on ebay and it cost less than £2 (<3$US). It can weigh up to 25 kg and it has a resolution of 5 g. The scale is turned on using the centre button and the units flash. Whilst flashing the units can be selected by further presses of the button. Once selected the scale displays 0 after a few seconds. An object can be hung from the hook and the weight will show on the display. A further press on the button will tare the scale and the reading will return to zero.
In order to measure density the following procedure is followed:
The item shown in the header photo is a digital luggage scale and this greatly facilitates making the measurements to determine density. It was purchased on ebay and it cost less than £2 (<3$US). It can weigh up to 25 kg and it has a resolution of 5 g. The scale is turned on using the centre button and the units flash. Whilst flashing the units can be selected by further presses of the button. Once selected the scale displays 0 after a few seconds. An object can be hung from the hook and the weight will show on the display. A further press on the button will tare the scale and the reading will return to zero.
In order to measure density the following procedure is followed:
The scale is turned on and the units are set to kg. The object to be measured, in this cas a con rod from an old lawn mower engine, is then hung from the hook using thin wire. The scale is read and the weight noted down. Call this weight W.
After weighing the button is pressed again and this tares the scale and resets it to zero
After weighing the button is pressed again and this tares the scale and resets it to zero
The object is then lowered into water so that it is completely submerged and not touching the sides or the bottom of the bucket. The scale is read and the reading is noted. Call this weight w.
Note that the reading will be negative because the upthrust makes the apparent weight of the object less.
Note that the reading will be negative because the upthrust makes the apparent weight of the object less.
The conrod weighed 0.175 kg and the upthrust was 0.065 kg. The density is then given by W/w = 0.175/0.065 = 2.69 kg/l =2.69 g/ml. This is very close to the density for aluminium and it is safe to assume that the object is aluminium.
A carburetter casting weighed 0.130 kg and the upthrust was 0.050 kg giving a density of 2.6 kg/l. This is also very close to the density for aluminium.
Another die cast lamp holder part was also tested. This weighed 0.115 kg and had an upthrust of 0.025 kg giving a density of 4.6 kg/l. This is obviously not aluminium but another alloy, probably ZAMAC.
This method is reliable for determining density provided there are no pressed in component made from steel, brass, bronze etc. When weighing in water be careful to ensure that there are no trapped air pockets that could lead to false values for the upthrust.
It is really a very simple and quick procedure to carry out. Although in the photos the balance is suspended from a cantilevered arm this was only done to permit photography. In practice the scale is held in the hand, the object attached and the weight noted. The scale is then tared and the object then lowered into water and the upthrust noted. This whole procedure takes only circa 20 seconds.
A carburetter casting weighed 0.130 kg and the upthrust was 0.050 kg giving a density of 2.6 kg/l. This is also very close to the density for aluminium.
Another die cast lamp holder part was also tested. This weighed 0.115 kg and had an upthrust of 0.025 kg giving a density of 4.6 kg/l. This is obviously not aluminium but another alloy, probably ZAMAC.
This method is reliable for determining density provided there are no pressed in component made from steel, brass, bronze etc. When weighing in water be careful to ensure that there are no trapped air pockets that could lead to false values for the upthrust.
It is really a very simple and quick procedure to carry out. Although in the photos the balance is suspended from a cantilevered arm this was only done to permit photography. In practice the scale is held in the hand, the object attached and the weight noted. The scale is then tared and the object then lowered into water and the upthrust noted. This whole procedure takes only circa 20 seconds.