Friction stir welding (FSW) is an exciting new solid-state welding process with the
potential to advantageously impact many fabrication industries. Current take-up of
the process by industry is hindered by lack of knowledge of suitable welding
parameters for any particular alloy and sheet thickness. The FSW process
parameters are usually chosen empirically and their success is evaluated via
simple mechanical property testing. There are severe drawbacks with such
methods of determining manufacturing conditions. These include indirect
relationships between tensile and fatigue properties, particularly for welds, and a
high probability of totally missing real optimized conditions.
This research is therefore undertaken as a first step in providing information that
will assist manufacturing industry to make sound decisions with respect to
selecting FSW parameters for weldable structural alloys. Some of the key issues
driving material selection for manufacturing are weld quality in terms of defects,
fatigue strength and crack growth, and fracture toughness. Currently a very limited
amount of data exists regarding these mechanical properties of FSW welds, and
even less information exists regarding process parameter optimization. This is due
to the mechanical microstructural complexity of the process and the relatively large
number of process parameters (feed, speed, force and temperature) that could
influence weld properties. In order to advance predictive understanding and
modeling for FS welds, it is necessary to develop force and energy based models
that reflect the underlying nature of the thermo-mechanical processes that the
material experiences during welding.
This project aims at determining the influence and effect of Friction Stir Welding
process control parameters on the microstructure of the thermo-mechanically
affected zone, the defect population in the weld nugget, hardness, residual
stresses, tensile and fatigue performance of 6 mm plate of 5083-H321 aluminium
alloy, which is known to be susceptible to planar defect formation. Welds were
made with a variety of process parameters (that is feed rate and rotational speed)
to create different rates of heat input. Forces on the FSW tool (horizontal and
vertical), torque and tool temperature were measured continuously during welding
from an instrumented FSW tool.
Detailed information on fatigue performance, residual stress states, microstructure,
defect occurrence, energy input and weld process conditions, were investigated
using regression models and contour maps which offer a unique opportunity to
gain fundamental insight into the process-structure-property relationships for FS
welds.
Weld residual strains have been extensively measured using synchrotron X-ray
diffraction strain scanning to relate peak residual stresses and the widths of the
peak profiles, taken from a single line scan from the mid depth of the FS welds,
with the weld process conditions and energy input into the welds. Several residual
stress maps were also investigated.
The optical and scanning electron microscope were used to determine the type of
intrinsic defects present in the FSW fatigue and tensile specimens. Vickers
hardness measurements were taken from the mid depth of the welds and were
compared with the weld input parameters.
The main contribution of this thesis is as follow:
(i) the relationship between input parameters and process parameters;
(ii) the relationship between input weld parameters (that is feed rate and
rotational speed) and process parameters (that is vertical downwards
force Fz, tool temperature, tool torque and the force footprint data),
energy input and tensile strength, fatigue life and residual stresses to
obtain regions of optimum weld conditions;
(iii) identification of the defects present in FSW, their relationship with
process parameters and their effect on tensile strength and fatigue life;
and
(iv) the usefulness of the real time process parameter monitoring automated
instrumented FSW tool to predict the mechanical properties of the welds.
Date of Award | 2007 |
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Original language | English |
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Awarding Institution | |
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OPTIMIZED FATIGUE AND FRACTURE PERFORMANCE OF FRICTION STIR WELDED ALUMINIUM PLATE: A STUDY OF THE INTER-RELATIONSHIP BETWEEN PROCESS PARAMETERS, TMAZ, MICROSTRUCTURE, DEFECT POPULATION AND PERFORMANCE
LOMBARD, H. (Author). 2007
Student thesis: PhD