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Keyhole repair in precipitation hardening aluminum alloys using refill friction stir spot welding
Citation Link: https://doi.org/10.15480/882.1878
Publikationstyp
Doctoral Thesis
Publikationsdatum
2018
Sprache
English
Author
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2018-10-16
Aluminum alloys are widely used in transportation industries because of the increasing need to reduce
the environmental impact. With advances in technology, the demand for complex parts and
components that must be produced using several processing methods has increased. During
fabrication and service, a wide range of defects can appear in aluminum components and structures,
which could be repaired using a suitable through hole closure method. The search for a friction-based
solid-state keyhole repair technique that fulfills the requirements for high-quality repair welds has
become an important research topic because conventional fusion welding is difficult to apply in many
aluminum alloys. However, many commonly available friction-based welding methods are complex
and multistage processes that require specially designed equipment and are not suitable for sealing
through holes. The development of an adequate keyhole repair process is thus actual necessity.
The present study addresses the development of a suitable keyhole repair procedure of structural
aluminum parts using the refill friction stir spot welding process (RFSSW). For this newly developed
repair method, a plug made of a similar material is applied as a filler element into the keyhole and
RFSSW is used to weld the plug to the surrounding workpiece. To cover a wide range of alloys and
potential applications, the repair method was investigated in different precipitation hardening
aluminum alloys as well as different keyhole diameters and workpiece thicknesses. A fundamental
analysis of the process and resulting material properties considering the alloy-dependent
metallurgical transformations was conducted. Moreover, a knowledge-based process analysis
approach was chosen to study the behavior of the base material during high-shear-rate plastic
deformation and exposure to typical thermal cycles, which are both associated with the conditions
found during friction welding. The influence of the base material composition and properties on the
energy input during friction welding was investigated and a comprehensive analysis of the friction
condition and flow stress development was conducted.
The developed keyhole repair process using RFSSW is a universal through-hole closure method with
advantages such as defect-free welds, high weld efficiencies and superior surface appearance on both
sides of the weld. Within the scope of the present work, processing conditions were defined that lead
to defect free repair welds for all investigated materials and workpiece dimensions. The area of lowest
strength was in all cases found outside of the weld spot, mainly in the heat affected zone. For all
welded precipitation hardening aluminum alloys, metallurgical analysis revealed that the evolution
of the strengthening precipitates during and after the weld primarily determines the final mechanical
properties. Significant differences observed in the response of the base materials to the process were
found to be caused by the alloy composition, specifically by the characteristics of the present
precipitates. The fundamental process analysis revealed that in precipitation hardening aluminum
alloys, the mechanical properties obtained under quasi-static testing conditions are not adequate to
describe or predict the base material properties at the high strain rates and thermal cycles associated
with friction welding operations. The alloy composition, initial temper condition and general
precipitation evolution during the specific thermal cycles resulting from the friction welding
operations were found to determine the material properties at the tested rates of deformation. The
knowledge gained by this fundamental process analysis is key to enabling rapid process optimization
by guiding the appropriate choice of process parameters for a given alloy.
the environmental impact. With advances in technology, the demand for complex parts and
components that must be produced using several processing methods has increased. During
fabrication and service, a wide range of defects can appear in aluminum components and structures,
which could be repaired using a suitable through hole closure method. The search for a friction-based
solid-state keyhole repair technique that fulfills the requirements for high-quality repair welds has
become an important research topic because conventional fusion welding is difficult to apply in many
aluminum alloys. However, many commonly available friction-based welding methods are complex
and multistage processes that require specially designed equipment and are not suitable for sealing
through holes. The development of an adequate keyhole repair process is thus actual necessity.
The present study addresses the development of a suitable keyhole repair procedure of structural
aluminum parts using the refill friction stir spot welding process (RFSSW). For this newly developed
repair method, a plug made of a similar material is applied as a filler element into the keyhole and
RFSSW is used to weld the plug to the surrounding workpiece. To cover a wide range of alloys and
potential applications, the repair method was investigated in different precipitation hardening
aluminum alloys as well as different keyhole diameters and workpiece thicknesses. A fundamental
analysis of the process and resulting material properties considering the alloy-dependent
metallurgical transformations was conducted. Moreover, a knowledge-based process analysis
approach was chosen to study the behavior of the base material during high-shear-rate plastic
deformation and exposure to typical thermal cycles, which are both associated with the conditions
found during friction welding. The influence of the base material composition and properties on the
energy input during friction welding was investigated and a comprehensive analysis of the friction
condition and flow stress development was conducted.
The developed keyhole repair process using RFSSW is a universal through-hole closure method with
advantages such as defect-free welds, high weld efficiencies and superior surface appearance on both
sides of the weld. Within the scope of the present work, processing conditions were defined that lead
to defect free repair welds for all investigated materials and workpiece dimensions. The area of lowest
strength was in all cases found outside of the weld spot, mainly in the heat affected zone. For all
welded precipitation hardening aluminum alloys, metallurgical analysis revealed that the evolution
of the strengthening precipitates during and after the weld primarily determines the final mechanical
properties. Significant differences observed in the response of the base materials to the process were
found to be caused by the alloy composition, specifically by the characteristics of the present
precipitates. The fundamental process analysis revealed that in precipitation hardening aluminum
alloys, the mechanical properties obtained under quasi-static testing conditions are not adequate to
describe or predict the base material properties at the high strain rates and thermal cycles associated
with friction welding operations. The alloy composition, initial temper condition and general
precipitation evolution during the specific thermal cycles resulting from the friction welding
operations were found to determine the material properties at the tested rates of deformation. The
knowledge gained by this fundamental process analysis is key to enabling rapid process optimization
by guiding the appropriate choice of process parameters for a given alloy.
Schlagworte
Refill friction stir spot welding
Aluminum alloy
Friction stir processing
DDC Class
620: Ingenieurwissenschaften
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