Manufacturing technologies

Visualization study on dissimilar Friction Stir Welding (FSW) technology
for lightweight structural manufacturing
Research Division of Materials Joining Assessment, Joining and Welding Research InstituteSpecially Appointed Associate Professor Peihao Geng

https://researchmap.jp/phgeng?lang=en

Professor Ninshu Ma							https://researchmap.jp/ma.ninshu?lang=en

Abstract
Dissimilar friction stir welding has gained significant attention in
the manufacturing industry. Simulation modelling for in-depth understanding of heat generation, thermal conduction and material
flow in dissimilar FSW is of paramount importance. The authors
have sequentially undertaken studies on the FSW of Al alloy/CFRTP
and Al alloy/high-strength steel. Numerical simulation techniques
are employed to elucidate the underlying interaction between joint
formation, microstructure and mechanical property in dissimilar
FSW.

▍ Significance of the research and Future perspective
By conducting fundamental experiments and utilizing numericalsimulations to unveil the joining mechanisms and optimize process conditions, this technology can achieve high-quality joining
between various dissimilar materials. The advancement of thistechnology will drive innovation in the future, particularly in the automotive, aerospace, and other industries, resulting in the creationof more competitive products.

Manufacturing technologies▍ Background & Results
FSW is a revolutionary green manufacturing technology thatfinds extensive applications in lightweight structural manufacturing.
High quality joining of dissimilar materials through FSW stronglydepends on interfacial macro-/micro-structures. However, how theunderlying thermo-mechanical processes affect them and how they
can be controlled remains unclear.
For pin-less friction spot joining of silane-treated Al alloy andCFRTP (Figure 1), authors recently investigated the feasibility oftool modification to enhance joint strength [1], compared the interfacial structures and strength of joints manufactured using 3
different tools. In the case of the flat tool, a significant reduction
in shear strength occurred due to the extensive formation of micro-scale void defects. This issue was mitigated when using theconcave-shaped and ring-shaped tools, as they allowed effective
control of heat generation distribution, resulting in more uniform
temperature distribution and preventing defect occurrence. Therefore, authors recommend the use of the concave-shaped tool in
the friction spot joining process for Al/CFRTP. It offers improved
macro-mechanical interlocking, ensures sufficient chemical reactions, provides an effective joint area, and results in lower residualstresses, ultimately achieving the highest shear strength among thetools.
Concerning with Al/steel friction stir lap welding (Figure. 2) [2],authors discovered that material flow recirculated near the shoulder and pin, primarily originating from the advancing side of theAl and the retreating side of the steel. Increasing the rotational
velocity intensified the overall material flow, resulting in increased
migration on the steel side. At lower rotational velocities, insufficient steel migration around the pin was mainly responsible for theformation of micro-voids and non-bonding defects at the lap interface. At the bottom of the stirring pin, the steel migration flow wasaltered through shearing and squeezing, changing the dimensionsof the hook-like structure and causing steel fragments near theinterface. As the rotational speed increased, the peak temperature
shifted from the Al-stirred zone to the steel-stirred zone. At highrotational speeds, the temperature at the bottom of the stirring pin
approached the solidification temperature of the Al alloy. This led tothe formation of a thicker intermetallic compound layer, significantly
reducing the interfacial strength.

Figure 1 Friction spot joining of Al/CFRTP

Figure 2 Friction stir lap welding of Al/steel

P a t e n t Japanese Patent Application No.2021-179991
Geng, Peihao; Ma, Hong; Li, Weihao et al. Improving bonding strength of Al/CFRTP hybrid joint through modifying friction spot joining tools. Composites
Part B: Engineering. 2023, 254, 110588. doi: 10.1016/j.compositesb.2023.110588
Tr e a t i s eGeng, Peihao; Ma, Yunwu; Ma, Ninshu et al. Effects of rotation tool-induced heat and material flow behaviour on friction stir lapped Al/steel joint formation
and resultant microstructure. International Journal of Machine Tools and Manufacture. 2022, 174, 103858. doi: 10.1016/j.ijmachtools.2022.103858
URL
K e y w o r d friction stir welding, dissimilar joining, lightweight alloy, numerical simulation