Grain Characterization of Aluminum DC Cast Ingot and ...

Andris InnusIn collaboration with Yves RaymondArvida

R & D CentreAnalytical Technologies Group

ASTM Subcommittee E04.08 WorkshopNov. 17, 2010San Antonio, Texas

Grain Characterization of Aluminum DC Cast Ingot and Downstream Products

ASTM Subcommittee E04.08 Workshop, San Antonio, Nov. 17, 2010 2

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


Introduction

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.

Aluminum –

a metal other than Fe base


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 methods referenced

E 883

Reflected Light Microscopy

E 1951

Calibrating Reticles and Light Microscope Magnifications

E112

Determining Average Grain Size

E1382

Determining Average Grain Size Using Semi-Automatic and Automatic Image Analysis


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.


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 grainsMajority of light returned

Dark grainsMajority of light scattered


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.*

V

- +DC Source

A

Stirrer

Cathode

Specimen (Anode)

Presenter

Presentation Notes

Power source: Above 40 V additional safety precautions would be need to ensure against electric shock from exposed surfaces. Reference National Fire Protection Association NFPA 70E Standard for Electrical Safety In the Workplace, 2004, section 110.7 (E) at 50 V or more.


Micro-evaluation Specimen etching – RTA ARDC set-up

Magnetic stirrer

Specimen

Thermometer (if needed)

Cathode(graphite / stainless steel)

Container(high density polyethylene)

Anode

Power supply

Set-up installed in fume hood


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.

Voltage (V)

Cur

rent

Den

sity

(Am

ps /

dcm

²)

Etch Polish Gas

Resistance (Ohms)


Micro-evaluation Specimen etching – etch time

Under-etched

Over-etched

Presenter

Presentation Notes


Micro-evaluation Not all microscopes give the same results - polarizer

Polarizer

Analyzerλ

Plate No λ

plate¼

λ

plate

λ

plate

Presenter

Presentation Notes


Micro-evaluation Not all microscopes give the same results

Microscope 1 Microscope 2 with λ

plate

Images from other microscopes

Presenter

Presentation Notes


Micro-evaluation Grain size measurement – image analyser

Presenter

Presentation Notes


Micro-evaluation Structures (Misc.)

Longitudinal section –

grain structure variation et orientation

“Feathery”

grain.

Presenter

Presentation Notes

Some structures are not measurable Careful of orientation effects


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.

Presenter

Presentation Notes


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 macro-

etched. The respective image analysis values were assigned, and the sets are used to estimate grain size of similarly macro-etched ingot surfaces.

Presenter

Presentation Notes


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.


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.

Presenter

Presentation Notes


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.

Presenter

Presentation Notes


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


Macro-evaluation Structures

“Feathery”

grain.

Columnar grain

Equiaxed

grain


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.


Wrap-up

Questions?

Thank You

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