Aluminium alloy 6061

Aluminium Alloy 6061 - Composition, Properties,
Temper and Applications of 6061 Aluminium
Topics Covered:
 Background
 Composition
 Key Properties
 Physical Properties
 Mechanical Properties
 Thermal Properties
 Typical Heat Treatment/Temper States
 Typically Available Forms
 Applications

Background:
Aluminium alloy 6061 is one of the most extensively used of the 6000 series aluminium
alloys. It is a versatile heat treatable extruded alloy with medium to high strength
capabilities.

Composition:
Table 1. Typical composition of aluminium alloy 6061

Component Amount (wt.%)
Aluminium Balance
Magnesium 0.8-1.2
Silicon 0.4 – 0.8
Iron Max. 0.7
Copper 0.15-0.40
Zinc Max. 0.25
Titanium Max. 0.15
Manganese Max. 0.15
Chromium 0.04-0.35
Others 0.05

Key Properties:
Typical properties of aluminium alloy 6061 include:

 Medium to high strength
 Good toughness
 Good surface finish

 Excellent corrosion resistance to atmospheric conditions
 Good corrosion resistance to sea water
 Can be anodized
 Good weldability and brazability
 Good workability
 Widely available

Physical Properties:
 Density: 2.7 g/cm3
 Melting Point: Approx 580°C
 Modulus of Elasticity: 70-80 GPa
 Poissons Ratio: 0.33

Mechanical Properties:
Temper Ultimate Tensile 0.2% Proof Stress Brinell Hardness Elongation
Strength (MPa) (MPa) (500kg load, 10mm 50mm dia (%)
ball)
0 110-152 65-110 30-33 14-16
T1 180 95-96 16
T4 179 min 110 min
T6 260-310 240-276 95-97 9-13

Thermal Properties:
 Co-Efficient of Thermal Expansion (20-100°C): 23.5x10-6 m/m.°C
 Thermal Conductivity: 173 W/m.K

Electrical Properties:
 Electrical Resistivity: 3.7 – 4.0 x10-6 Ω.cm

Typical Heat Treatment/Temper States:
Treatment Definition
F As fabricated
0 Annealed to obtain lower strength temper
T1 Cooled from an elevated shaping process and naturally aged1
T4, T4511 Solution heat treated and naturally aged2,3
T51 Cooled from an elevated shaping process and artificially aged
T6, T6511 Solution heat treated and artificially aged2,3

Note:
 This designation applies to products which are not cold worked after cooling from an elevated temperature
shaping process, or in which the effect of cold work in flattening or straightening has no effect on
mechanical properties
 This designation applies to products which are not cold worked after solution heat-treated, or in which the
effect of cold work in flattening or straightening has no effect on mechanical properties
 3. This designation applies to products which are not cold worked after solution heat-treatment, or in
which the effect of cold work in flattening or straightening does not effect mechanical properties.

Typically Available Forms:
Being and extruded grade of aluminium, alloy 6061 is typically available as:

 Tube
 Bar
 Pipe
 Rod

Applications:
Typical applications for aluminium alloy 6061 include:

 Aircraft and aerospace components
 Marine fittings
 Transport
 Bicycle frames
 Camera lenses
 Driveshafts
 Electrical fittings and connectors
 Brake components
 Valves
 Couplings

What is the definition of white layer?
A thin layer of hardened material caused by a dull insert that gives the false impression of
a successful part until the surface fails. Hard turning often requires predicting the
wearing of inserts so they can be changed before they cause the white layer.

(b)
(a)
Figure 6: A sample without white layer (a), and a sample with white layer (b).
Whitelayers formed during machining have negative effects on surface finish and fatigue
strength of products. The whitelayer is generally a hard phase and leads to the surface
becoming brittle causing crack permeation and product failure. This is a major concern
with respect to service performance especially in the aerospace and automotive
industries. Numerous authors have investigated the formation of whitelayer under
different manufacturing processes. In turning, it was suggested that the whitelayer
structure is a martensitic phase whose formation is correlated to tool wear. Past studies
have tended to concentrate on the formation of whitelayers at conventional cutting
speeds, but never examined the formation at high cutting speeds.
Properties:
White layer occurs on the surface of steel and may be up to 10 μm thick. The dark layer
underneath it may be two or three times thicker. (Ramesh, 2002) The transition
between
white and dark layer is usually abrupt, and occurs within a transition zone less than 1 μm
in depth. (Akcan, Shah et al., 2002)
Observation of white layers using a scanning electron microscope and an optical
microscope suggest that it has a nanocrystalline structure due to large strain deformation
and dynamic recrystallization. It has been proposed that the white layer does not have
visible grain boundaries because the grains are small enough that they scatter light—not
necessarily because it is resistant to chemical attack. (Akcan, Shah et al., 2002)
White layer hardness has been measured to be significantly greater than the martensite in
the bulk of the material. (Akcan, Shah et al., 2002)
Using nano-indentation hardness measurements, the hardness was found to be
approximately 12.85 GPa, compared o about
10.70 GPa for the bulk material. The grain size has been measured between 30-500nm.
(Akcan, Shah et al., 2002 )
Formation:
The surface of a sample that has been electrodischarge

machined also has a white layer. White layer has also been observed on
surfaces that experience wear, such as on the surface of railroad tracks or a pin-on-disk
test. (Griffiths, 1984)
White layer is sometimes referred to as untempered martensite, and dark layer is referred
to as over tempered martensite, because of their similar properties to heat treatment
by models that use heat effects as the primary cause of white layer formation.
(Chou andEvans, 1999)
In turning, when aggressive cutting parameters are used, even using a new tool, white
layer accompanied by tensile stress is expected. This is an unacceptable condition that is
typically undesirable. If less aggressive parameters are used with a new tool, one expects
no white layer and a compressive residual stress. As the tool wears however, these
desirable characteristics diminish and a white layer develops. White layers may form at
either low or high cutting speeds. At low speeds it forms due to grain refinement, at high
speeds it forms due to rapid heating and quenching. (Akcan, Shah et al., 2002;
Ramesh,2002) defects.

.

Counters EDM "White" Layer effect:
The use of Electrical Discharge Machining (EDM)in the production of forming tools to
produce plasticsmouldings, die castings, forging dies etc., hasbeen firmly established in
recent years. Developmentof the process has produced significant refinementsin
operating technique, productivity andaccuracy, while widening the versatility of the
process.
Electrical Discharge Machining (EDM), while providing a rapid and relatively less
expensive means for producing die casting die inserts, at the same time sets up some very
high and detrimental surface stresses. These stresses if not completely and properly
removed, can accelerate thermal stress cracking.

Metal is removed by a series of electrical spark discharges. The steel in the contact area
melts or vaporizes then solidifies on the surface of the cavity. Each spark erodes a tiny bit
of metal, leaving a small crater in the surface of the tool. This leaves the immediate
surface in a high residual tensile stress condition. The topmost or recast "white" layer is a
brittle, hard layer prone to cracking. This is the material that has melted and rapidly
solidified. The white layer is densely infiltrated with carbon and has a distinct separate
structure to that of the parent metal.

Below this layer is the heat affected zone that has been structurally altered by the heat
produced during EDM. This layer reaches the austenizing temperature of the steel. The
zone may contain re-hardened or hard, brittle "untempered" martensite formed during
the rapid cooling from the austenizing temperature. This can increase crack susceptibility
since this microstructure stores considerable strain energy that decomposes with heat.

(Reference: www.metallife.com)

After mandatory removal, by polishing, of the top "white cast" layer" it is important to
protect the next exposed layer. MetaLLife compressive stress removes the scratch stress
risers created during polishing and closes the cracks that have propagated below the
recast layer into the heat affected "untempered martensite" zone. This restores the
desired residual compressive stress benefits to the tool.

EDM layer zones*

White layer - 5-15 microns - crack prone
un tempered - 25-40 microns - crack prone
tempered - 40-85 microns
1 micron = 0.00003937"

*Depth of zones is dependent on the spark density, volts, and amps of EDM equipment.

EDM’s Effect on Surface Integrity:
Article From: MoldMaking Technology, Jerry Mercer

Posted on: 2/1/2008

Understanding the various layers of a cavity that are thermally altered by the EDM
process will help you understand how EDM affects the integrity of the mold surface.

Protecting the surface integrity of the cavity is one of the most critical facets of EDM. The
integrity of the surface finish in the cavity is determined by the formation of thermally
altered layers created by the EDM process, which involves the transference of a controlled
electrical discharge between an electrode and the workpiece. The current applied to the
workpiece during this discharge melts and vaporizes the metal, creating the thermally
altered layers of the cavity. To understand how EDM affects the integrity of the mold

surface, you must first understand the various layers of the cavity that are thermally
altered. EDM changes not only the surface of the work metal, but also the subsurface
layers.

Thermally Altered Layers:
The various layers affected by the EDM process will be referred to as the altered metal
zone. Figure 1 shows that the altered metal zone is comprised of two thermally affected
sub-layers of material: the recast layer or white layer and the heat affected zone.

The white layer is the layer that has been heated to the point of a molten state, but not
quite hot enough to be ejected into the gap and be flushed away. The EDM process has
actually altered the metallurgical structure and characteristics in this layer as it is formed
by the unexpelled molten metal being rapidly cooled by the dielectric fluid during the
flushing process and resolidifying in the cavity.

This layer does include some expelled particles that have solidified and been re-deposited
on the surface prior to being flushed out of the gap. The white layer is densely infiltrated
with carbon to the point that its structure is distinctly different than that of the base
material. This carbon enrichment occurs when the hydrocarbons of the electrode and
dielectric fluid break down during the EDM process and impenetrate into the white layer
while the material is essentially in its molten state.

The first layer of the heat-affected zone is
the re-cast or “white layer”. This layer has been heated above the melting point of the tool

steel and quickly cooled, subsequently producing an extremely brittle surface subject to
micro-cracking. If this condition is left untreated, then propagation of the cracks can
ultimately lead to failure of the tool. Immediately below the re-cast layer is the re-
hardened layer. This layer has been heated to the austenizing, or hardening temperature

and rapidly cooled, leaving an un-tempered brittle surface condition. Below the re-
hardened layer is the re-tempered layer. The re-tempered layer has been heated above the
normal tempering temperature of the tool steel, leaving the area with a lower hardness
than typically useful for the tool. The unaffected base layer is the last layer of the HAZ
and this layer is in the same condition as it was prior to the EDM process.

Multiple passes using a lower
current, on the final passes, can minimize the amount of heat-affected zone, and
therefore reduce the chance of the tool cracking while in service. The use of newer EDM
equipment, with better control of the amperage and frequency, produce less “white
layer”. Removal of the white layer is vital to the longevity of the tool. Grinding or stoning
and polishing of the HAZ should be performed on the tool to remove the white layer.
Stress relieving of the tool may also reduce the chance of cracking by tempering the re-
hardened layer. Stress relieving should be performed about 25-50 degrees F below the last
tempering temperature. See the tool steel data sheets for specific tempering
temperatures.

Influence of machining parameters on the surface
integrity in electrical discharge machining:
Purpose: The aim of this research is to make a study of the influence of machining
parameters on the surface integrity in electrical discharge machining. The material used

for this study is the X200Cr15 and 50CrV4 steel for dies and moulds, dies castings, forging
dies etc.

Design/methodology/approach: The methodology consists of the analysis and
determination of the white layer thickness WLT, the material removal rate MRR, the
electrode wear ratio EWR and the micro hardness of each pulse discharge energy and
parameters of electrical discharge machining.

Findings: The Results of the tests undertaken in this study show that increasing energy
discharge increase instability and therefore, the quality of the workpiece surface becomes
rougher and the white layer thickness increases. This is due to more melting and
recasting of material.With the increase of the discharge energy, the amount of particles in
the gap becomes too large and can form electrically conducting paths between the tool
electrode and the workpiece, causing unwanted discharges, which become electric arcs
(arcing). these electric arcs damage the electrodes surfaces (tool and workpiece surfaces)
and can occur microcracks.

Keywords: EDM; Energy discharge; White layer thickness WLT; Metallographic
aspect; Cracks; HAZ

Reference to this paper should be given in the following way: M. Boujelbene, E.
Bayraktar, W. Tebni, S. Ben Salem, Influence of machining parameters on the surface
integrity in electrical discharge machining, Archives of Materials Science and Engineering
37/2 (2009) 110-116.

Fig. 1.
(a) The composition of the heat affected zone HAZ, (b) Influence of the white layer
thickness WLT on the discharge energy W in EDM

Fig. 2. Analyse of the White Layer Thickness WLT
as a function of the machined energy W observed under an optical microscope; (a) W =
38.4 J, WLT = 13.7μm; (b) W = 99.84 J, WLT = 18.7μm; (c) W = 384 J, WLT = 31.95μm

Fig. 3. The influence of the
material removal rate MRR and the electrode wear ratio EWR on the thickness of the
white layer.

Fig. 4. Influence of the tool material on the white layer thickness WLT in roughing EDM;
(a) Copper electrode WLT = 53.65 μm, (b) graphite electrode WLT = 51.88 μm

Detecting White Layer in Hard Turned Components
Using Non-Destructive Methods:

Title: Detecting White Layer in Hard Turned Components Using Non-Destructive Methods

Author: Harrison, Ian Spencer

Abstract: Hard turning is a machining process where a single point cutting tool removes material
harder than 45 HRC from a rotating workpiece. Due to the advent of polycrystalline cubic
boron nitride (PCBN) cutting tools and improved machine tool designs, hard turning is an
attractive alternative to grinding for steel parts within the range of 58-68 HRC, such as
bearings. There is reluctance in industry to adopt hard turning because of a defect called
white layer. White layer is a hard, 1-5 쭠deep layer on the surface of the specimen that resists
etching and therefore appears white on a micrograph. When aggressive cutting parameters
are used, even using a new tool, white layer is expected. If more conservative parameters are
selected, one does not expect white layer. There is some debate if white layer actually
decreases the strength or fatigue life of a part, but nevertheless it is not well understood and
therefore is avoided. This research examines the use of two different non-destructive
evaluation (NDE) sensors to detect white layer in hard turned components. The first, called a
Barkhausen sensor, is an NDE instrument that works by applying a magnetic field to a
ferromagnetic metal and observing the induced electrical field. The amplitude of the signal
produced by the induced electrical field is affected by the hardness of the material and
surface residual stresses. This work also examines the electrochemical properties of white

layer defects using electrochemical impedance spectroscopy. This idea is verified by
measuring the electrochemical potential of surfaces with white layer and comparing to
surfaces without any. Further corrosion tests using the electrochemical impedance
spectroscopy method indicate that parts with white layer have a higher corrosion rate. The
goal of this study is to determine if it is possible to infer white layer thickness reliably using
either the Barkhausen sensor or electrochemical impedance spectroscopy (EIS).
Measurements from both sensors are compared with direct observation of the microstructure
in order to determine if either sensor can reliably detect the presence of white layer.

Type: Thesis

URI: http://hdl.handle.net/1853/6982

Date: 2005-01-20

Publisher Georgia Institute of Technology

Subject: Residual stress
White layer
Hard turning
Barkhausen effect
Electrochemical impedance spectroscopy

Departm Mechanical Engineering
ent:

Advisor: Committee Chair: Kurfess, Thomas; Committee Member: Liang, Steven; Committee Member:
Melkote, Shreyes

Degree: M.S.

Title: White layer formation and tool wear in high speed
milling of 57HRc tool steel using coated and
uncoated tools:

Author: Paul T. Mativenga, Aamir Mubashar
Address: Manufacturing and Laser Processing Research Group, School of
Mechanical, Aerospace and Civil Engineering, The University of
Manchester, M60 1QD Manchester, UK. ' Manufacturing and Laser
Processing Research Group, School of Mechanical, Aerospace and Civil
Engineering, The University of Manchester, M60 1QD Manchester, UK
Journal: International Journal of Agile Systems and Management 2007 - Vol.
2, No.2 pp. 172 - 185
Abstract: Advances in process technology have opened new possibilities for rapid
manufacturing. High Speed Machining (HSM) is one of these innovative
areas. One demanding application is the HSM of hardened steels for die
and mould tooling applications. A significant impediment in the wide-
spread use of HSM in hard machining is a lack of understanding and
subsequent control of possible micro-structural changes to the surface of
machined components. These changes can occur in the form of surface
and sub-surface layers induced by grain refinement, rapid heating and
quenching and or reactions with the environment. Some surface layers are
known for decreasing the material fatigue life due to their brittleness.
Generally, these affected surface layers are referred to as the white layer.
This paper focuses on formation of white layers during high speed milling
of hardened tool steels. The machining was carried out using uncoated
and TiAlCrN coated micro-grain carbide end mills. The cutting tools were
also analysed for tool wear. The paper explores the correlation of white
layer formation to tool wear progression and how this is affected by the
PVD coating. Surface hardening, sub-surface tempering, surface finish
and compositional changes are also presented. The results show that in
milling, tool wear is a significant driver for white layer thickness
progression. Moreover, increased oxygen content suggests that oxidation
could also play a role in white layer formation.
Keywords: tool coatings; high speed machining; HSM; microhardness; white layer
formation; tool wear; high speed milling; tool steels; rapid
manufacturing; agile systems; hardened steels; microstructure; carbide
end mills; PVD coating; oxidation.
DOI: 10.1504/IJASM.2007.015787

Aluminium alloy 6061

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