Optical microscopy requires a high degree of lustrous finish/ appropriate etchant for suitable duration (Have used Keller's Reagent for etching of AA6061 MMC's the matrix being an Al-Si-Mg alloy).

If optical microscopy doesnt reveal the grains, then better go for Scanning electron microscopy.

Dear Anad,

Thank you for your response, however, the alloy you've mentioned is an wrought aluminium alloy. As far as I've seen in the literature, revealing grain boundaries in these alloys is not problematic. Unfortunatelly I am dealing with casting alloys, with high Si content. Procedure for wrought alloys most probably will not be suitable.

Thanks once again.

Dear Mr. Dybowski,

Spontaneous idea: Could you try Barker etching? This gives you a crystallographic orientation-dependent colouring of grains under polarised light. We have tried it in the past on AlSi7 metallic foams - it is kind of an metallographic equivalent of EBSD, you could say.

Hope this helps.

Best regards,

Dirk Lehmhus

Hello Dirk,

Thank you for the idea. Yes, I have tried Barker, without effects unfortunatelly. However, I guess I have to experiment some more with the etching parameters. Do you remember which parameters did you use? 

Hi

I used Keller's Reagent for etching of A356 alloy,    Al-7%Si-0.3%Mg alloy

Dear Mohamed,

Thank you very much. Can you give any further information about the imaging? Polarized light? Sensitive tint? Was the etching long or only few seconds long?

Deep echting with Keller's reagent and looking with the SEM:

http://martin-moeser.de/Aluminiumguss/Umschmelzen_Kolbenlegierung.html

Hello everybody.

I am doing now trials with Barker and Tuckers. If I will have any results I'll share.

Thank you for the participtaion.

Use Barker's reagent (4-5 ml HBF4 48% with 100 ml H2O) 20 V DC for 90 second. Use aluminium cathode (-), sample is anode (+). Try to observe it using polarized light (crossed polarizers).

I have tried 0.5% HF in water. I works well if you are patient as the reaction rate is very slow. However, the etchant can be more easily prepared compared to others.

Dear Malur,

How long do you etch the alloys in HF?

It is difficult to give a precise etching time with this etchant. The major problem is that an oxide layer forms on aluminum a little after  you complete polishing which tends to counteract reaction of HF with aluminum . I have tried repeated immersion in HF and repolishing to surmount this problem. With an average of 4 such repetitions I have been able to see the microstructure under a microscope after about 8 minutes of final etching  for 6061 samples, but it should be close to this for cast alloys also. Good contact between the sample surface and HF is essential and so dipping the sample in HF is far better than mere swabbing the surface with HF.

I'll agree with Mr Gierth. HF does not seem to be good.

Recently I've tried chemical etching with reagent containing H2SO4 as well as HF. It gave quite good results, but still far from optimal ones.

Mr Gierth, I've tried to etch with Barkers at 30V and it completely failed, did not give any result. 

Dear Sebastian,

Olympus GX71. We have also Nikon Eclipse E200. Also, as we are investigating both HPDC and sand cast alloys, I someteimes try Olympus SZX9 stereoscope.

Next week I'll try lower parameters.

Dear Sebastian,

You have worried me. The HPDC alloys are I think a bit more important for me, however, they are mainly heat treated. 

What do you think about older microscopes? Neophot 32, Reichert MeF2?

I found this publication by George Van der Woort, an authority on metallography. Please read through it. Weck's reagent seems to work well for aluminum alloy castings.

Regards,

Malur

Dear Malur,

I've seen many Van der Woort's publications - it was one of my first ideas. Unfortunatelly, this does not work. Maybe if I'll try another microscope, it will be better. Thank you for the participation.

I now agree with the views expressed by others that wrought aluminum alloys can be etched with traditional etchants more easily than cast alloys. My experience is mainly with cast aluminum alloys and as I have said before repeated polishing and etching with 0.5% HF has worked for me and obviously not for you. I now feel that the relative ease with which a wrought aluminum alloy (say 6061) is etched has to do with the high dislocation density present in wrought aluminum alloys; this would result in higher surface energy in wrought alloys.Castings have low dislocation density and this could be the factor for the difficulty in etching. I went through the literature and found a patent for etching cast aluminum alloys after plating it. The document is attached. The argument in the patent is that the plated layer has high microporosity and the etchant is able to penetrate through the layer on to the substrate (cast aluminum). After etching away the plated layer, etched surface of the casting is revealed.  The patent was issued in 1997 and I do not know the legal problems involved in using this technique.   

Electrolytic Etching with Barker's reagent (2% HBF4) 20 V DC for 1-3 minutes can reveal grains of  Al-Si alloy. Use SS cathode and keep sample as anode (+). Observe under polarized microscope. For better results pls use a quarter filter. normal light optical microscopy will not be effective.

You may get your favorable result by using electro-etch. The conditions are as below:

Solution: 2.5 mL HBF4 + 100 mL distilled water

Voltage: 20 V and time: 90 sec

You should prepare the surface carefully so that no damages present on the surface.

After eletro-etch you must observe the specimen surface through Polarized light in the optical microscopy.

I have tried the above method for 1000, 4000, and 5000 aluminum alloys and I have got the favorable result. The attached file indicates a sample of microstructure of 1050 aluminum alloy obtained with the above procedure.

For better result try by varying the concentration slightly or time of etching

Hi,

As noted by Malur Srinivasan, Weck’s reagent works very well. But I advise you to do before an attack with a solution of NaOH (2%).

Note that a better result is obtained after electrolythic polishing

Go to phase diagram Al-Si

Dear Debowski tabke of etchants in reference engineering metallurgy by Higgins is useful

Hello

For microscope examination of A356 alloy, use Kellers agent.

Distilled water 95.0 Hydrochloric acid (37% concentration) 1.5 Hydrofluoric acid (40% concentration) 1.2 Nitric acid (66% concentration) 2.5

This is the as-cast A356 alloy microstructure

Dear Dybowski what is the composition of your alloy?