Mechanical Properties of Aluminum Welds - Course Notes

MECHANICAL PROPERTIES OF
ALUMINUM WELDS FOR
AUTOMOTIVE STRUCTURAL
APPLICATIONS
Jennifer Hyde
Supervisor: Dr. McDermid
MATLS 701 Seminar
Feb 17, 2012

Outline
2
Motivation
Background/Literature Review
Project Outline
Experimental Procedure
Preliminary Results
Summary
Future Work
Acknowledgements

Motivation
3
Increased use of aluminum for light weighting of
vehicles due to its relatively high strength to weight
ratio
The Aluminum Association. Aluminum in Transportation
http://aluminumintransportation.org/main/commercial-vehicle/commercial-vehicle (Accessed Feb 2012)

Background: GMAW
4
Gas Metal Arc Welding
(GMAW)→ two metals are joined
by heating from an arc between
the metal and continuously fed
filler wire electrode. This process
uses a shielding gas (argon or
helium) to protect the molten
weld pool from oxidation.
Popular welding technique for Al
alloys
Results in 3 distinct regions:
weld; HAZ; base material
S. Kou, Welding Metallurgy, 2 nd ed., Hoboken,
New Jersey: John Wiley and Sons Inc., 2003.
Microhardness Profile

Mechanical Properties of Welds
5
Problem
One of the easiest ways to get the mechanical
properties of a material is from uniaxial tensile testing
This is a problem for welds since the mechanical
properties of welded samples from tensile testing is
limited to the instability of the weakest region of the
welded joint
How can we measure mechanical properties
of each region of the weld

Mechanical Properties of Welds
6
Solution:
1) Gleeble/thermal
treatment apparatus
costly
time consuming
Sensitive to variation of
welding conditions
2) novel shear test; tensile
testing combined with DIC
modified ASTM B831 shear
samples
Mechanical behaviour up to
large strains from shear test
Gleeble 3180 Dynamic Systems Inc.
http://www.gleeble.com/index.php/products/productsoverview.html
(Accessed Feb 2012)

Modified Shear Samples
7
Machined using wire and sink EDM (Electric Discharge Machining)
Pulled in a tensile testing machine

8
Digital Image Correlation (DIC):
ARAMIS System
Spray paint test sample to get
random speckle pattern and
using a CCD camera to take
snapshots during testing.
ARAMIS calculates
displacement between each
snapshot and a reference
picture to get strain
able to calculate strain within
the shear zone and also
localized strain during tensile
test (i.e. strain in weld; HAZ
and BM regions)
Relate stress to strain by time.
Facets in the undeformed and deformed state
(Aramis v5.3 manual, GOM mbH, Braunschweig, Germany (2004))

Shear Test: Kang et al [1]
9
These modified samples first
used on DC and CC 5754 sheet
materials
This geometry prevents end
rotation of the shear zone as
compared to the ASTM B831
samples
Compared shear test to uniaxial
tension tests
They concluded that effective
stress and strain for shear test
only matched tensile results when
using the Barlat-Lian yield
function which incorporates
planar anisotropy.
[1] J. Kang et al, Journal of Engineering Materials and Technology, 031004-1-5, 2008.

10
Yield Functions:
For simple shear:
Von Mises:
Barlat-Lian:
M=8 for FCC materials

Barlat-Lian Yield Function:
r= plastic strain ratio
Equal to 1 for an isotropic
material
11

Effective stress, MPa
[2] Kang J, McDermid , J.R., Bruhis, M., SAE Paper, 2012-01-0181 (2012)
Resistance Spot Welds
12
More recently this modified
shear sample geometry has
been extended to the
investigation of AA5754-O
RSW welds and AA6022 T4
RSW welds [2]
350
300
250
200
Used Barlat-Lian equation
on base materials to match
tensile and shear effective
stress-strain curves
150
100
50
AA5754 Uniaxial tension
AA6022-T4 Uniaxial tension
AA5754 von Mises
AA6022-T4 von Mises
AA5754 Barlat-Lian
AA6022-T4 Barlat-Lian
0
0 0.2 0.4 0.6 0.8
Effective strain
Uniaxial Tension and Shear effective stressstrain
curves for the base materials: 5754 and
6022 T4

True stress, MPa
RSW welds graph
13
350
300
s = 487.24e 0.2194
R² = 0.99
s= 479.83e 0.2941
R 2 = 0.9898
s= 321.97e 0.1409
R 2 = 0.9813
250
s= 226.33e 0.1497
200
R 2 = 0.9542
150
AA5754-O
100
50
0
6022-T4
Case 8 Tensile
Case 6 Tensile
Case 8 Shear
Case 6 Shear
Case 6 Shear
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
True strain
Case 6: RSW 6022 T4 Case 8: RSW 5754-O

Research Objective
14
GM currently cannot accurately predict the failure
location of gas metal arc welded aluminum
structural components
This research meant to provide the necessary
mechanical property information for both the weld
region and the HAZ so that their FE models are
accurate

Project Outline
15
Weld Cases
Case Material 1 Thickness Material 2 Thickness Weld wire
1 Aural 2 T7 3mm 6063 T4 4mm 4043
2 Aural 2 T7 3mm 6063 T6 4mm 4043
3 5754-O 3mm 6063 T4 4mm 4043
4 5754-O 3mm 6063 T6 4mm 4043
9 5754-O 3mm 5754-O 3mm 4043
These are in a butt joint configuration for testing purposes
Also testing base materials mechanical properties

Base Materials
16
AA 5754-O
Main alloying element is Mg
Non-heat treatable
O temper → fully annealed
AA 6063 T4 and T6
Mg and Si are the main alloying elements
heat treatable→ strengthening from metastable β'' precipitates (Mg 2 Si)
T4 → naturally aged; T6 → optimally aged
Aural-2 ® T7
Mg, Si and Mn are the main alloying elements
die cast
T7 → stabilized

Weld Configuration
17
(A)
(B)
RD
RD
RD
RD
Cases 1 and 2
(C)
Case 1
RD
RD
5754
Cases 3 and 4

Paint Bake Cycle
18
Al alloys used for automotive applications undergo
a paint bake cycle which may have an effect on
the material’s properties given that many Al alloys
are artificially aged around this temperature
Baking Conditions:
180°C for 20 mins then air cool, reheat to 180°C for 30 mins
All base material samples with the exception of
5754-O tested in this condition
All welds were tested in this condition which is
meant to simulate the paint bake cycle.

Experimental Procedure
19
Mechanical Testing
Tensile Tests
Base Materials
Welds
Shear Tests
Base Materials
Welds
HAZ
Locating the HAZ to place
shear zones
Microhardness Profiles
Microstructure
OM (Electrolytic etching using
Barker’s and viewing under cross
polarized light)
Fractography
SEM on fracture surfaces

Tensile Tests
20
Base Material:
Tensile samples cut parallel (0°), 45° and 90° to RD
Extensometer as well as ARAMIS (DIC) for strain measurements
Strain rate: 6.7 x 10 -4 /s
Welds:
Weld in middle of tensile sample
ARAMIS used for strain measurements
Strain rate: 2.8 x 10 -4 /s
10 KN Instron Testing Machine

6063 T4 Stress-Strain Curves
21
6063 T4 AR 6063 T4 B

Tensile Test of Welds: 1A:6063 T4 and Aural-2 T7
22
1A AR
1A B

Shear Test samples: welds
23
Welded Plate
Shear Sample

24
Locating the HAZ for Shear Test

Shear Tests- Base Materials
25
Same machine as tensile test
Testing base material samples parallel and transverse to
RD (0° and 90°)
Strain rate: 2.6 x 10 -3 /s
Using Aramis/DIC to directly measure shear angle
at each stage
Shear stress and strain converted to effective stress and
strain using von Mises criterion

Aural-2 T7 Shear Curves
26

Summary
27
Tensile tests on base materials were completed with general
agreement between samples and some differences in
sample orientation (i.e. 0°, 45°, 90° wrt RD)
Tensile test on welds using ARAMIS (DIC) were able to show
strain locally; in both the weld region and region where
fracture occurs
It has been found that only 6063 T6 has a noticeable HAZ
Shear tests on base materials were completed. The effective
stress-strain curves show differences for shear and tension.

Future Work
28
Locate the fusion zone of 6063 T4, 5754-O,
Aural 2 T7 and then place the shear zone
beside this region
Shear tests on welds and HAZ
SEM of fracture surfaces
Possibly use Barlat-Lian yield function to
incorporate material anisotropy

Acknowledgements
29
General Motors of Canada Ltd for financial
support and experimental materials
Initiative for Automotive Manufacturing Innovation
(IAMI) for financial support
Supervisor: Dr. McDermid
Technical assistance: Jidong Kang; Mike Bruhis;
and Doug Culley

QUESTIONS