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

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

Corrosion resistance of aluminumand aluminum alloys

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

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.

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

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.

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.

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.

Example of Time‐Temperature characteristics of alloy 6061 sheet

Strain Hardening

Increase UTS, YS,HB e A%

Increase UTS, YS and HB (reducessligthly A%)

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.

Aluminum silicon phase diagram

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.

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.

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.

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 

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.