الفهرس 
CHAPTER ONE: AN INTRODUCTIONOnly 14 pages are availabe for public view
 |  | from | 283 |  |  |
| | | from | 283 | | |
Abstract
The technique of vertical compensation friction
stir welding (VCFSW) has advantages in welding huge sheets
of aluminum alloys, especially in structural industries like
aerospace, automotive body, shipbuilding, and rail
transportation. The friction stir welding (FSW) process has
advantages over traditional fusion welding processes such as
the reduction of energy usage, no need to consumables, high
quality of weldments with no porosity and fumes, reduced bad
environmental effects, reduced waste, and minimized impact on
the safety and health. The VCFSW technique has benefits over
traditional FSW: reinforcement the joints, elimination grooves,
tunnels and gap defects, enhanced microstructures, improved
mechanical properties and hence obtaining high-quality joints. The
industrial requirements for this work grow from the wish to
apply the technology described in this present work in airplane
manufacturing in Arab Organization for Industrialization, AOI
located in Cairo, Egypt. The adoption of a new manufacturing
process e.g. airplane structures creates many challenges, as in the
case dealt with in the present thesis, repercussions that require a
nearly full product developments affecting the primary structure
design, with many or at least, reengineering of the different parts.
The main obstacles of conventional welding processes in huge
structures were related to the lack of mechanical properties due to
the increase heat input, lack of defect control and the difficulty to
iii
weld precipitated hardened alloys (as AA2024 and AA7075). The
main obstacle in VCFSW technique is the use of an additional
material in the gap between two workpieces of the joint; this thin
plate (compensation material) of the substrate which results greatly
increases the difficulties of the joining process. It’s known to be
particularly difficult. That creates a number of challenges. It also
creates several opportunities.
PURPOSE: the present work is motivated by two main issues. First
is the industry need to adapt the technology to the welding of
aluminum alloys in applying the industries of motor car, the
transportation of rail, shipbuilding, and aircraft. The second is a
need to better understand the metallurgical characterization and
mechanical properties of VCFSW process and how they affect the
joint efficiency.
APPROACH: In this study, an extensive experimental program
was undertaken in order to identify the parameters of variables and
their influences. In this respect, AA2024 aluminum alloy was
chosen as base metal (BM). On the other hand, AA7075 aluminum
alloy was chosen as compensation material at two heat treatment
conditions solution heat treatment with artificial aging (T6) and
annealing heat treatment (O). Three variables of process parameters
in VCFSW technique were investigated like tool traverse and
rotational speed and tool inclination angle in order to obtained best
parameters. Nevertheless, three main parameters have been studied
in this study; the first case is FSW without addition interlayer strip
iv
width and VCFSW with different addition interlayer strip width
values at 1, 1.5, 2, 2.5 and 3 mm in case of solution heat treatment
with artificial aged (T6), while the second case is FSW without
addition material and VCFSW with different addition interlayer
strip width values at 1, 1.5, 2, 2.5 and 3 mm in case of annealed heat
treatment (O) and finally third case is FSW of T-joints with
different combinations geometries e.g. T-butt-lap joint, T-doublebutt
joint and T-lap joint.
FINDINGS: The obtained results from this study in VCFSW joints
revealed that an increasing rotational speed at 2000 rpm led to
increase tensile strength and elongation. On the other hand,
decreasing traverse speed at 20 mm/min resulted higher tensile
strength and elongation of VCFSW joints. While best tool tilt angle
was at 2.5°. With the technique of VCFSW in producing welded
joints revealed high homogeneity without any defects if it compared
to conventional FSW process in case of T6. The material flow in
VCFSW joints was balanced material flow while in FSW joints
were insufficient material flow in case of T6. The optimum
mechanical characteristics (hardness, tensile and bending) of the
fabricated-welded joints were gotten during the using of the width
of compensation strip at 1.5 mm in case of T6. The fracture surface
of the welded joint of the width of compensation strip at 1.5 mm
revealed the typical ductile fracture in case of T6. While the
obtained results from this VCFSW in case of O revealed that the
quality of the welded joints in VCFSW technique are based on the
capacity of compensation material in filling in and mixing with the
v
BM. The material transfer in the pin-driven region takes place layer
by layer. On the other side, the shoulder driven material flow can be
described as the effectiveness of the shoulder to keep the
compensation material in the weld cavity. The optimum mechanical
hardness, tensile and bending characteristics of the fabricated
welded joints were gained during the using of the compensation
strip at 3 mm was used in case of O. The fracture mode in VCFSW
with interlayer strip width at 3 mm showed a ductile fracture mode
which happens at 45° in BM without clear decreasing in area.
Finally, the results in case of FSW of T-joints showed that the joint
efficiencies of all produced T-joints at different geometries along
the stringer are lower than those of all joints along the skin.
IMPLICATIONS: the compensation or additional material has
crucial role in excluding the cavities, enhancing joints efficiency
and producing extra sound joints by inserting strip of compensation
material between both edges of the base metal (BM). The effect is
acute when found a huge gap between two aluminum plates before
welding process of the joint. To adopt this industry successfully, the
process needs strong control before welding process of type of
materials and thickness (base metal and compensation material),
width of compensation material, heat treatment of materials, type of
joints and feature fabrication. Additionally process parameters in
VCFSW technique such as spindle speed, travel speed and
inclination angle need to be adjusted during welding process
according to type of materials, thickness and width of compensation