Andrew, Its difficult to be sure from this distance but I suspect that you may not have etched enough as there is still evidence of flow in the post etch sample - the "flap" that seems to be still in the cavity at about 1.39 for example, proper etching should have got rid of that. We are talking about metal removal process so some artefacts will vanish with the metal removed and others will be uncovered. The idea of the etch is to removed metal flowed over defects by the machining/polishing process. Metallographic preparation is a bit of an art form that may include several stages of polish etch polish etch finishing with 1 micron diamond abrasive (some times 1/4 micron) and a final light etch although the latter would not commonly be caustic soda. Many different etchants are used but 0.5% HF is common Also although you don't seem concerned with the smaller defects its worth noting that each and every defect including the barely big enough to see is the sign of a bifilm. You are unlikely to see an actual film at the magnification you are using indeed even with much higher magnifications they are difficult to see and generally only become visible with optical microscopy when they are what John Campbell would call older thicker films. I have occasionally seen these but different etching techniques are usually necessary too - caustic etching is a bit brutal on some constituents. By the look of al the defects that show up both pre and post etch I would think that there are rather a lot of bifilms present. A reflection perhaps of the high surface area metal melted - did I hear Cans?.. Martin
Thanks for taking the time to review and comment, Martin. I etched with a lot of bubble activity but not long enough to get black deposits on the surface. I need to repeat this a few times with different levels of etching. If you could repeat what you have done with a known grit sandpaper that would be most helpful. I will try to repeat your exercise with 800 grit. No doubt my castings are full of oxides. And having been remelted, should have copious amounts of hydrogen. Ha! I've worked around enough HF I don't want any around me. 1/2% seems pretty mild, however. In a refinery HF Alkylation unit there are a few thousand gallons of nearly pure HF acid. Nice thing about strong acids they are not corrosive to mild steel. I do have diamond polishing paste, down to 1/2 micron. It is very time consuming for me without proper equipment and at my level of magnification I don't see much more. Interesting to note that bifilm are hard to see with proper equipment. I'm not likely to acquire a proper microscope and don't expect to see any grain structure at 300X. I arrive at the point of recognizing if you can't see it and it can hurt you I need to look at other methods of inspection to verify whether remelted aluminum of various sources is suitable for use. In part that's why I embarked on the pipe pressure testing challenge. My initial o-ring grooves were inadequate for moderate pressure so I plan to make some new ones and test thin sections at a couple of thousand psi. It also seems appropriate to do some tensile testing and have embarked on making a simple tensile test machine. I don't think I can measure percent enlongation accurately enough to establish yield strength but expect to show ultimate load before failure. Yes, you heard cans although I've lost interest in melting cans after several hundred pounds melted since I now have a seemingly unlimited source of window frames. My interest in non-commercial ingot casting stems from my belief that the average home caster wants to use found materials, not go out and buy ingots to melt, and be restricted to melting them only one time. Since people are making things from cans, ladders and window frames I have an interest in demonstrating how the best results can be obtained from those raw materials. A similar argument can be made for wheels and previously cast materials. At another level are previously forged materials like pistons.
Andrew, Its difficult to be sure from this distance but I suspect that you may not have etched enough as there is still evidence of flow in the post etch sample - the "flap" that seems to be still in the cavity at about 1.39 for example, proper etching should have got rid of that. We are talking about metal removal process so some artefacts will vanish with the metal removed and others will be uncovered. The idea of the etch is to removed metal flowed over defects by the machining/polishing process. Metallographic preparation is a bit of an art form that may include several stages of polish etch polish etch finishing with 1 micron diamond abrasive (some times 1/4 micron) and a final light etch although the latter would not commonly be caustic soda. Many different etchants are used but 0.5% HF is common
Also although you don't seem concerned with the smaller defects its worth noting that each and every defect including the barely big enough to see is the sign of a bifilm. You are unlikely to see an actual film at the magnification you are using indeed even with much higher magnifications they are difficult to see and generally only become visible with optical microscopy when they are what John Campbell would call older thicker films. I have occasionally seen these but different etching techniques are usually necessary too - caustic etching is a bit brutal on some constituents.
By the look of al the defects that show up both pre and post etch I would think that there are rather a lot of bifilms present. A reflection perhaps of the high surface area metal melted - did I hear Cans?.. Martin
Thanks for taking the time to review and comment, Martin.
I etched with a lot of bubble activity but not long enough to get black deposits on the surface. I need to repeat this a few times with different levels of etching.
If you could repeat what you have done with a known grit sandpaper that would be most helpful. I will try to repeat your exercise with 800 grit.
No doubt my castings are full of oxides. And having been remelted, should have copious amounts of hydrogen.
Ha! I've worked around enough HF I don't want any around me. 1/2% seems pretty mild, however. In a refinery HF Alkylation unit there are a few thousand gallons of nearly pure HF acid. Nice thing about strong acids they are not corrosive to mild steel.
I do have diamond polishing paste, down to 1/2 micron. It is very time consuming for me without proper equipment and at my level of magnification I don't see much more.
Interesting to note that bifilm are hard to see with proper equipment.
I'm not likely to acquire a proper microscope and don't expect to see any grain structure at 300X.
I arrive at the point of recognizing if you can't see it and it can hurt you I need to look at other methods of inspection to verify whether remelted aluminum of various sources is suitable for use. In part that's why I embarked on the pipe pressure testing challenge. My initial o-ring grooves were inadequate for moderate pressure so I plan to make some new ones and test thin sections at a couple of thousand psi.
It also seems appropriate to do some tensile testing and have embarked on making a simple tensile test machine. I don't think I can measure percent enlongation accurately enough to establish yield strength but expect to show ultimate load before failure.
Yes, you heard cans although I've lost interest in melting cans after several hundred pounds melted since I now have a seemingly unlimited source of window frames.
My interest in non-commercial ingot casting stems from my belief that the average home caster wants to use found materials, not go out and buy ingots to melt, and be restricted to melting them only one time. Since people are making things from cans, ladders and window frames I have an interest in demonstrating how the best results can be obtained from those raw materials. A similar argument can be made for wheels and previously cast materials. At another level are previously forged materials like pistons.