Grain Characterization of Aluminum DC Cast Ingot and Downstream Products Andris Innus In collaboration with Yves Raymond Arvida R & D Centre Analytical Technologies Group ASTM Subcommittee E04.08 Workshop Nov. 17, 2010 San Antonio, Texas
Content Introduction ASTM methods referenced Key messages Micro-evaluation Specimen etching Not all microscopes give the same results Grain size measurement Grain size reference materials Macro-evaluation Specimen etching The comparison methods Structures Wrap up ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 2
Introduction Aluminum a metal other than Fe base Relevance of grain structure for aluminum ingot and downstream products? Ingot: Meet customer & internal specifications. An indicator of control of process parameters. Downstream Products: Relationship with tensile & bending properties. An indication of control of process parameters. ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 3
Introduction Grain structure evaluation techniques used within Rio Tinto Alcan labs. 1. Micro-evaluation via electrolytic etching (anodizing) and polarized light microscopy. 2.. Macro-evaluation by comparison against reference materials. 3.. Semi-macro evaluation by comparison against reference materials. ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 4
ASTM methods referenced E 883 E 1951 E112 Reflected Light Microscopy Calibrating Reticles and Light Microscope Magnifications Determining Average Grain Size E1382 Determining Average Grain Size Using Semi-Automatic and Automatic Image Analysis ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 5
Key Messages 1. Tailor the grain size assessment method to the need. Case for the application of reference materials (as compared to images). 2. For micro-grain size, the polarized light capability of the microscope is just as important as the etch. 3. An in-house made electrolytic etch set-up is simple and can offer greater flexibility. 4. EHS first when handling macro-etchants containing HF. ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 6
Micro-evaluation Specimen etching - introduction The electrolytic etch process (Barker s) of a polished specimen yields anodic (aluminum oxide) interference films of different thicknesses as a function of grain crystal lattice orientation. Upon applying polarized light, the grains yield different colors as a function of the interference film thickness and the interface (metal oxide) grains crystal lattice topography. Light grains Majority of light returned Dark grains Majority of light scattered ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 7
Micro-evaluation Specimen etching - components As with any anodizing process, we need the following: 1. A clean, well polished surface to be etched. 2. An electric circuit, i.e. good contact between the specimen to be etched and the circuit. 3. An electrolyte of suitable volume. In our case it is a dilute solution fluoroboric acid, aka Barker s. 4. A controllable DC power source: 40 V max.* Specimen (Anode) V A - + DC Source Cathode Stirrer ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 8
Micro-evaluation Specimen etching RTA ARDC set-up Power supply Anode Cathode (graphite / stainless steel) Thermometer (if needed) Specimen Container (high density polyethylene) Magnetic stirrer Set-up installed in fume hood ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 9
Micro-evaluation Specimen etching specimen surface area We can etch the entire surface of anything from foil cross-sections to blocks up to several inches. Etch Polish Gas Current Density (Amps / dcm²) Resistance (Ohms) Voltage (V) ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 10
Micro-evaluation Specimen etching etch time Under-etched Over-etched ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 11
Micro-evaluation Not all microscopes give the same results - polarizer ¼ λ plate Analyzer λ Plate No λ plate Polarizer λ plate ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 12
Micro-evaluation Not all microscopes give the same results Microscope 1 Microscope 2 with λ plate Images from other microscopes ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 13
Micro-evaluation Grain size measurement image analyser ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 14
Micro-evaluation Structures (Misc.) Feathery grain. Longitudinal section grain structure variation et orientation ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 15
Grain size reference materials Not all products need a precise measurement obtained via quite a time consuming process. We may not appreciate the forest from the trees with micro-sections, e.g. as for sheet (rolling) ingot. ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 16
Grain size reference materials Specimens with different uniform grain size were polished, electrolytically etched, and the grain size assessed by image analysis. Sets of specimens with observable difference in grain size increment were lightly ground, and macroetched. The respective image analysis values were assigned, and the sets are used to estimate grain size of similarly macro-etched ingot surfaces. ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 17
Macro-evaluation Specimen etching etch effect Why physical references and not just pictures? The etchants have a similar effect as for electrolytic etching. It is the subtle differences in the light reflected from the grain bodies that helps to discern grain size against the reference materials. Sample macro-etched specimen area, but change light angles. ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 18
Macro-evaluation Specimen etching safety first The etchants contain hydrofluoric acid, as well as nitric and hydrochloric acids, e.g. Tuckers & Poultons. While all concentrated acids must be handled with respect and using recognized safe practices, HF can be particularly bad. HF seeks out Ca in the body and will attack bones and reduce the Ca level in the blood, which may in the extreme case provoke cardiac arrest. Calcium gluconate required on site. Environmentally safe spent acid disposal (neutralization, and solids disposal) is required because of the fluorides. ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 19
Macro-evaluation Specimen etching practice A well machined (lathe, milling machine) or a ground surface free of cold work and surface contaminants. A heated surface will yield a crisper and more defined grain structure. Set-up to pour the acid mixture onto a horizontal surface. Do not dunk large specimens into a bath an ooops may result in a significant acid splash. Incremental applications to make sure that surfaces do not over-etch. Rinse with hot water in-between applications. Dry the surface and compare against the set of grain size references. ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 20
Macro-evaluation Comparison methods 1. With the unaided eye ( Macro ) Compare against the set of grain size references. 2. With a stereo-microscope ( Semi-macro ) Two color fiber optic illumination. An additive effect of the colors as a function of grain crystal lattice orientation. Two fibre optic lights with red and green filters ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 21
Macro-evaluation Structures Columnar grain Feathery grain. Equiaxed grain ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 22
Wrap-up Tailor the grain size assessment method to the need. Case for the application of reference materials. There is no ASTM method for macro-evaluation of grain size. For micro-grain size, the polarized light capability of the microscope is just as important as the etch. If grain contrast is poor, don t necessarily blame it on the etch. Comment in E 883? An in-house made electrolytic etch set-up is simple and can offer greater flexibility, e.g. grain size and structure variations. EHS first when handling macro-etchants containing HF. ASTM E 2014 Standard Guide on Metallography Lab Safety is insufficient and inadequate be more informed. ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 23
Wrap-up Questions? Thank You ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 24