hydrogen in alumimium
TRANSCRIPT
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Hydrogen in Aluminium
Solubility, Characterisation and Removal
R. N. Chouhan
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Principles of solubility and removal of hydrogen
Sources of hydrogen
Defects caused by presence of hydrogenDetection of Hydrogen
Removal
Contents
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Principles
Solubility
Hydrogen Precipitation
Principles of hydrogen removal
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Solubility
3H2O + 2Al 6H + Al2O3
the equilibrium reaction for dissolution ofhydrogen in aluminium can be expressed as
H2 (gas) [H] (metal)
For above equation equilibrium constantK is given by
KH = [activity of hydrogen]
(partial pressure of H2)1/2
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If the dissolved gas is sufficiently dilute that it obeys
Henrys law
KH = [Wt%H] / PH21/2
This expression is called Sieverts Law
above expression can be simplified as
Log(wt%H) = - A/T + Log(PH2
) +
Constant
Sieverts Law
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Empirical relationship for solubility of hydrogen
log CH
= -2761/T + 2.768 (L)
log CH= -2580/T + 1.399 (S)
Where, CH represents cc of hydrogen gas/100 g of
aluminium at 1 atm and T is the absolute temperature
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0
0.5
1
1.5
2
2.5
400 500 600 700 800 900
Temperature (C)
Hydroge
n(ml/100g)
Solubility of hydrogen in aluminium at 1 atm
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Ce, Li, Th, & Ti increases
Si, Cu, & Sn decreases, and
Fe, & Cr have a marginal effect on solubility
Effects of other alloying elements
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Hydrogen Precipitation
Concentration gradient ahead of the solidification front
Co = Cl
Cs
= 0.05
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Gas nucleation
For a gas bubble to nucleate remain stable or grow
the pressure inside the bubble must be equal to or
exceed the sum of the hydrostatic pressure, the
pressure of the atmosphere above molten metal
and surface tension forces
PH2 Ph + Pa + Surface Tension forces
Where PH2 =equilibrium pressure of hydrogen
Ph = hydrostatic pressure of metal
Pa = atmospheric pressure
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Gaseous cap formed on a solid substrate is shownalong with the various interfacial energies which act.
cos = ( SL - SG) / LG
Gas nucleation
LGcos + SG = SL
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Gas pores nucleated due inclusion which is pushedahead of the solid-liquid interface
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Bubbles trapped between dendrite arms
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Forces acting on existing bubble
Pintr
Pext
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For mechanical equilibrium it can be simplified as
Pint Pext =2T/r
where T = surface tension 1 N/m2 for aluminium
So if Pext = 0.1 atm =0.1 x105 N/m2 and neglecting Pext
r2T/ Pint 0.5 m
Equilibrium pore size
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Spherical pores
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Typical porosities
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The principle of hydrogen removal
KH = [Wt%H] / PH21/2
Log(wt%H) = - A/T + Log(PH2) +Constant
Application of vacuum
Gas purging
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H2
H2
P3>P2
P2>P1
P1>0
P1 = 0
Movement ofgas purge
bubble
Atmosphere
Melt containingdissolvedHydrogenH2
H2
H2
H2
H2
H2
The principle of gas purging
S f
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Furnace atmosphere, which contains water vapour
in addition to some H2
Moisture from refractories, dirty skimmers, and
other furnace tools
Hydrated corrosion products, which form part of the
charge, such as
Weathered ingot and scrap
Sources of hydrogen
S f h d
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Oil contaminated turnings, chips, or scrap
Damp fluxes, and
Oil and hydroxide coating on metallic
sodium used for modifying AlSi alloys
Metal mould reaction
Metal turbulence, improper gating
Sources of hydrogen
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Dissolved hydrogen in molten aluminium results in
porosity, the size and shape of which is dependent on
Composition of the alloy,
Its solidification characteristics
Microstructural featuresPresence of porosity nucleation sites.
Interdendritic porosity, which is encountered when
hydrogen content is sufficiently high ( > 0.15 cc/100 g)
D f t d b f h d
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Porosity
Low toughness
Inferior surface finish
Lower weldability
Leakage of pressurised castings in serviceBright flakes in forgings
SCC in Al-Zn-Mg-Cu alloys
H2 embrittlement
Defects caused by presence of hydrogen
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Detection of Hydrogen
Various quantitative and semiquantitative methods for the
measurement of the hydrogen content in aluminium and its
alloys featuring various degrees of sophistication have been
evolved. The choice of the appropriate technique is often
difficult since it can be affected by several factors, viz.
Stage of production,
Speed of analysis,
Detection range,
Capital investmentDesired accuracy,
Ease of operation etc.
T h i f S lid S l
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Techniques for Solid Samples
Vacuum Tin Fusion
Vacuum Fusion
Nitrogen carrier Fusion
Hot Vacuum Subfusion Extraction
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Vacuum Tin Fusion
The technique involves fusing of the aluminium alloy
sample in a molten tin bath at about 500C and
extraction of the hydrogen evolved during the fusion
process is delivered into an evacuated vacuum
system. Depending on the alloy and its solubility in
tin, complete solution of the sample requires 1-1.5
hrs. The apparatus consists of a furnace sectionwhere the hydrogen is extracted, a section for
collection and measurement of gas evolved from the
sample and a section for the analysis of the collected
gas . The collected gas is analysed for hydrogen witha mass spectrometer or by diffusion through a
palladium tube heated to 600c. Its accuracy is 0.07-
0.08 cc/100 g for pure aluminium
V F i
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Vacuum Fusion
This technique involves melting theprepared solid sample in a thoroughly
degassed boron nitride, graphite, or
alumina crucible in a high vacuum (10-3). A
rapid evaluation of gas occurs as themetal is melted. This reaches completion
after 3-5 mins. The hydrogen evolved is
extracted and separated from background
gas by a palladium tube or by analysingwith a mass spectrometer
Nit i F i
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Nitrogen carrier Fusion
The sample is placed in an outgassed graphite crucible in a
silica (quartz) tube. The graphite crucible is baked out anddegassed at a high temperature (~2000C) by induction
heating for 3-5 mins. The inside of this tube is isolated from
the ambient atmosphere by a flow of nitrogen. The sample is
introduced into the crucible. During surface contamination
removal the surface temperature of the sample is reported to be
at 400C-480C for 60 seconds. The sample is allowed to cool
down for 5 mins before commencement of the extraction. The
system is sealed and the sample is melted by induction heating
by raising the plate current and gas is extracted for 3-5 mins.
Hydrogen is extracted by diffusion from the liquid sample into
the nitrogen stream and is then detected by katharometer
Hot Vacuum Subfusion Extraction
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Hot Vacuum Subfusion Extraction
A dry-machined cylindrical sample, is heated in an
evacuated clear silica tube to a temperature beloweutectic or solidus temperature, until a definite
endpoint is obtained on the gas evolution or
extraction curve recorded with a strip chart recorder
via a Pirani, Baratron or ionization gauge. Theextraction time varies from 1-2 hrs. When extraction
is complete, the gas is subjected to a simple
analytical test at constant volume with either the
mass spectrometer or by heating the palladium tube
to diffuse the hydrogen out.
T h i f M lt M t l S l
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Techniques for Molten Metal Samples
Straube-Pfeiffer (Vacuum Gas) Test
The Initial Bubble Test
Recirculating Gas Methods
S b Pf iff (V G ) T
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Straube-Pfeiffer (Vacuum Gas) Test
The Initial Bubble Test
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The Initial Bubble Test
Sample of molten aluminium contained in anelectrically heated crucible is placed in a closed
chamber, and vacuum is gradually applied until
the first bubble is observed at the molten metal
surface. The pressure and temperature at whichthe first bubble appears are recorded. A
nomograph relating pressure, temperature and
hydrogen solubility of the alloy being tested is
used to obtain the hydrogen content.
R i l ti G M th d
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Recirculating Gas Methods
It operates on the principle of monitoring hydrogenactivity developed in a small quantity of inert gas
continuously recirculated through the molten metal
under test until the gaseous hydrogen diffused into the
purged gas bubble is in equilibrium with the solute
hydrogen in the molten metal in accordance withSieverts law.
If the solubility of hydrogen in the alloy at a given
temperature and pressure is known then hydrogen C
content can be obtained from:
C = STP + (Pi/P)1/2
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ALCOA Telegas IITM Instrument
Diff t R i l ti G M th d
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ALCOA Telegas IITM Instrument
QRG Test Unit
SLM Hydrogen Determinator
Different Recirculating Gas Methods
Sample preparation and handling
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Sample preparation and handling
Preparation and handling of samples are very critical in
hydrogen content determination because contamination ofthe sample is one of the principle sources of spurious
hydrogen in the determination of the hydrogen content in the
solid alloy samples. In majority of the cases, surface
preparation determines the reproducibility of the test resultsand thus should be standardised Samples by dry turning for
direct method to be prepared with caution and immediately
tested. After 24 hrs remachining is required. For liquid
samples (indirect method), preheated mould permitting rapid
solidification with smooth feeding is required to prevent lossof hydrogen and formation of porosity
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Removal
Various methods of removing H2 from melt existsuch as gas purging, vacuum and flux
degassing. The most common methods (fig. 9)
rely on bubbling gases through the melt to carry
H2 to the surface. The efficiency of this processdepends mainly on the size of the bubble
produced- small bubbles give better efficiency.
The types of gases used can be split into two
categories, reactive and inert.
R ti
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The gases react with the melt in various ways.They usually contain Cl2, Cl2 compounds or Cl2
mixtures e.g.
Cl2Ar with 50% Cl2 Mixture
N2 with 10% Cl2 Mixture
Freon CompoundHexachloroethane Compound
Reactive gases
I t
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Nitrogen or argon is used to remove hydrogenfrom the melt. There is much less removal of
solid particles by floatation. Some removal of
Na and Mg may occur but this process can be
carried out after SrAl modification.
Inert gases
Diff t d i t h i
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Different degassing techniques
Natural degassing
Vacuum degassing
Ultrasonic treatment
Gas purging
Tablet/Flux degassing
Gas p rging
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Gas purging
Rotary degassing
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Rotary degassing
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Comparison of degassing techniques
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Lance Degassing
Bubble Size: 2-3 cmDispersion: Poor
Efficiency: Poor-Med
Consistency: Poor-Med.
Rotary Degassing
Bubble Size: 2-5 mmDispersion: Very Good
Efficiency: High
Consistency: Good
Porous Plug
Bubble Size: 2-10 mmDispersion: Fairly Poor
Efficiency: Med.-Good
Consistency: Medium
Tablet Degassing
Bubble Size: VariableDispersion: Poor
Efficiency: Variable
Consistency: Poor
Comparison of degassing techniques
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