Possible reasons are:

1) Aluminium was lost in the form of dross.

2) There may be macro-segregation of elements in the samples studied.

Possible reasons are:

1) Aluminium was lost in the form of dross.

2) There may be macro-segregation of elements in the samples studied.

3) The chemical composition of the master alloys used and other calculations may be re-checked.

Fe: did you use tools for melt cleaning which showed wear?

Si: did you add Master-alloy which wasn't fully dissolved?

All: did you just remelt ingots or did you prepare it synthetically - then check the questions above. 

Please let me know, what the solution of the problem is. Thx, Bernhard 

Dear Bernhard Stauder,

Thanks for your response.

the alloy was sourced from the brake master caliper scrap and the remelted in a steel container. which is believed to be more refractory than the alloy.. No additives (flux) was used.

Dear Olawale,

Then. ... the Fe-pick up is from the Steel Container.

Normally we use steel crucibles fro Mg-melts but not for Al-melts. Because of higher solubility of Fe in molten Aluminium .

Dear Prof Prasada

I am very grateful for all  the three suggested answers you have have give me. I believe I can rely on them. Thanks

Dear Olawale

Please allow this question:

Why didnt you apply an coating (e.g. Boron Nitride or some paint based on ceramic fibers)? That is common practise.

You should generally be very careful with metal- metal interfaces, no matter which alloy you are actually melting.

Cheers

Dear Dr Thomas

I note your advice

thanks

Dear

You ask about explanation, which might be required in manuscript writing as you already done casting and I hope microstructural analysis also has been done so far. Through XRD analysis and microstructural analysis you got exact information about the intermetallics formation than you able to automatically  correlate the outcome.

the most obvious explanation is that you lost aluminium through oxidation

unnecessary liquid metal motion increases dross formation and can also cause oxide film entrainment. quiecent flow is paramount to control these phenomena. read papers and book Castings by John Campbell. 

I agree with Laurens, which I mentioned in my answers above.

Dear Olawale Ajibola,

1. Check compositions just before and after casting

2. Use ceramic crucibles for melting

3. Check your Al-Si master alloy

4. Check mass before and after casting

5. Check wether or not the sands used for your sand casting forms are free of plant residues like straw

How clean was your sand?

I believe the quality of sand has nothing much to do with changes in composition during melting and casting. It will however affect the soundness of your final casting. It is best to you use ceramic crucibles as suggested here and avoid any Fe based tools for melt handling as much as possible. Melt under cover-flux & minimise causing surface turbulence to molten metal - this minimises oxidation which not only lead to loss of elements such as Al & Mg but also leads to oxide defects in the casting. As Lauren says, John Campbell has written quite a lot about this. There is also an excellent review by John A. Taylor that can be helpful. Finally, be careful when preparing samples for composition analysis - ensure they are rapidly solidified to prevent the macro-segregation mentioned by Prasad Rao. Otherwise the result may be misleading.

I have picked up 0.5% iron melting pure Al.

Not uncommon to use the same furnace for different metals and it can be hard to prevent cross contamination.

Use of Fe tools probably does not help.

In this case the dross theory sounds reasonable, due to the increase of Fe and Si.

I had the same problem, Fe comes from your crucible or any other steel tools you've used during casting, as Fe is easily dissolved in Al

1. Check compositions just before and after casting

2. avoid any Fe based tools for melt handling 

3 Aluminium was lost in the form of dross..

I add two remarks:

1. The observed Si-increase tends to be overlooked in the present discussion.

2. Assuming that Si- and Fe amounts are very small as compared to the Al-amount present, can anyone calculate the amount of Al-loss due to dross formation corresponding to a marginal increase of Si- and Fe-contents?

At about 660 degree centigrade aluminium alloys start to melt and any additional heat above that (660 oC ) is super heat,. when this super heat is above 50 oC some of the aluminium start to oxidise and form dross, so also there is tendency for the aluminium to dissolve some gases in its environment then, whereas at this temperature Fe and Si that may be present will be unaltered. so by volume fraction you must have lost some aluminium due to excessive superheat at the favour of some other stable alloying element in the system, thereby making their % increased in totality compared to that of the aluminium.

Alternatively, as some other contributors have said avoiding any iron tools such as iron crucible, tong and stirrer during melting may be helpful. Meanwhile, in the course of solidification macro segregation can also occur, and  hot working of the material to minimize the segregation may not be of utmost significant like observing macro segregation in ferrous material based which can be minimized by hot working.

I agree with Mr. van Mourik: the losses of Al to dross would only lead to a "substantial" increase in Fe and Si if the losses in Al would be substantial as well. If that is the case you may want to change your casting procedure.