chapter 10 ‐aluminum alloys and main manufacturing processes · 2018-12-10 · aluminum product...
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Chapter 10 ‐ Aluminum alloys and main manufacturing
processesFabrizio D’Errico
Politecnico di Milano Dept. Mech. Engineering
Main reference: 2004 ASM International ‐ Aluminum Alloy Castings: Properties, Processes, and Applications ; J.R. Davis, Aluminum and Aluminum Alloys, 2001 ASM International
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General features
• Cristal lattice: Face Centered Cubic (FCC)• Melting point: 660 °C • Density: 2.7 g/cm3• Young Modulus: 70 GPa• High reactivity to oxygen, forming Al2O3 protective nonsoluble passivated film
• High electrical conductivity• High thermal conductivity, 237 W/(m∙K)• High surface reflectance of metal
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Corrosion resistance of aluminumand aluminum alloys
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Negative factors to control:
High thermal conductivity heat is absorbed from base metal, thus requires high energy density sources for weldingStrong perturbation of
microstructure in zones close to welding jointNecessary removing of Al2O3
oxide layer
Welding principles for aluminum alloys
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Aluminum product manufacturing
Aluminum may be fabricated into any form is one of its most important assets:
- Hot deformation processes (e.g. stamping, forging, extruding,..) applied to wrought aluminum alloys, specifically tailored for this process;- Casting processes, employing specific chemical composition developed for casting aluminum alloys.
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Solubility in Aluminium: strengtheningmechanims on solute addition
Solute atoms take positions ofAl in the lattice
Anne
aled
proof
strength
(MPa)
1.0Weight % of solute
2.0 3.0
Mg
ZnCu
Si
Mn
50
40
30
20
10
00.0
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Aluminium Association (AA) Wrought Alloys
Table 10.1 - Classification of wrought aluminum alloys according to their strengthening mechanism.
Table 10.2 - Strength ranges of various wrought aluminum alloys.
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Heat treating of Al alloys (1)
• Typical of bi-phasic diagrams with incomplete solubility of solute element with stable second phases (not growing)
• Precipitates capable of disturbing crystal lattice are formed during artificial aging or ambient temperature
No HeatTreat!
HeatTreat.
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Heat treatment of aluminum alloys (2)Summaring, the heat treatment for mechanical properties improvement of Al alloys (also named hardening of aluminum alloys) basis on multiple stage treatment:
1. Solution Heat Treatment Triple objectives: a) dissolution of soluble phases formed during and after previous solidification; b) reduction/mitigation of microsegregation; c) Residual stresses caused by solidification or by prior quenching are reduced;
2. Rapid quenching water submerging for retention of the highest possible degree of solution, with the lowest level of induced residual stresses and the least warpage or distortion. A metastable, supersaturated solution heat treated condition is created.
3. Precipitation Heat Treating or Aging either natural (i.e. ambient temperature) or artificial (i.e. by furnace) precipitation hardening process. Formation of coherent clusters of solute atoms causes crystal lattice strain because of mismatch in size between the solvent and solute atoms. Perturbing lattice reduces dislocation motion capability.
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Example of Time‐Temperature characteristics of alloy 6061 sheet
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Strain Hardening
Increase UTS, YS,HB e A%
Increase UTS, YS and HB (reducessligthly A%)
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Aluminium Association (AA) Casting Alloy Systems
Series Major alloying element(s)
1xx.x 99.00wt% min. Al2xx.x Cu3xx.x Si + Cu/Mg4xx.x Si5xx.x Mg6xx.x unused7xx.x Zn8xx.x Sn9xx.x Other
Table 10.3 - Strength ranges of various cast aluminum alloys.
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Aluminum silicon phase diagram
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Aluminum Casting ProcessesSome process selection criteria are:• Casting process metallurgy issues: requirements for fluidity,
resistance to hot tearing, minimization of shrinkage tendencies.• Casting design considerations: draft, wall thickness, internal
passages.• Mechanical property requirements: strength and ductility, hardness,
fatigue strength, toughness, impact strength, specification limits.
Industrially, however, most attention is put to major external factor for the selection of casting process is the number of castings estimated:
• sand cast prototypes or when only one or a few parts are required, expendable die is preferred (when production is limited, tooling cost dominates);
• For increasing number of casting, automated molding processes are preferred with permanent molds.
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Casting process with expendable mold
• Process includes green sand anddry sand casting.
• The mold must have sufficientstrength to maintain its shapethrough the casting process andsufficient permeability to permitthe air, and gases formed duringpouring, to evacuate the moldcavity as the metal enters.
• Among disadvantages are: relatively low dimensional accuracy and poorsurface finish.
• Strength is typically lower as a result of slow solidification rates.
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Mold Gravity Feed Casting Process• Permanent mold tooling is typically more
expensive than that required for sandcasting and other expendable moldprocesses and is justified by the volumeof production.
• The volume of production also dictatesthe extent of process automation.
• Molds can be manually operated orextensively automated.
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Low‐pressure die casting with permanent mold• Among advantages, metallurgical
quality achieved in the low‐pressure process is high, and no risers (extra material) are necessary.
• Relative low investment costs (a high‐pressure die casting machine costs around four times as much as a low‐pressure system) is also external key‐factor for choosing low pressure die casting compared to high pressure die casting
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High‐pressure die casting with permanent mold
• Locking pressure defines the capacity ofthe machine.
• The larger the plan area of the castingand the greater the hydraulic pressureapplied.
• Die casting machines are designed withlocking pressures from as little as 25tonnes to more than 4500 tonnescorresponding to injection pressures ofup to 500 MPa.
• Rapid filling of the mold (20 to 100 μs)and rapid solidification under pressurecombine to produce a consistentlydense, fine grained and highly refinedsurface structure with excellentproperties
• Solidification conditions require alloys ofsuperior castability and that displaygood resistance to cracking at elevatedtemperatures.