This PhD thesis characterizes welded transverse micro-fasteners for joining dissimilar metal and composite materials. Cold metal transfer welding was used to deposit micro-fasteners onto metal substrates. Single lap shear testing of metal-composite joints incorporating these micro-fasteners showed they can efficiently transfer load through adhesive while maintaining damage tolerance. Future work includes quantifying properties like porosity and fiber undulation effects around the welded micro-fasteners.

1. PhD Title: Characterization of welded
transverse micro-fasteners
Student: J. Butler
Supervisors: I.K. Partridge, P. Colegrove
Introduction: Cold Metal Transfer (CMT) welding is a development of gas Table 1: Minimum tensile strength of metal-composite single lap shear
metal arc welding characterized by a low heat input. CMT pin welding coupons incorporating Ø0.8mm 3.0±0.5mm high micro-fasteners
(Stieglbauer & Kazmaier, 2009) allows deposition of micro-fasteners onto the Shear strength per micro fastener
Lay-up (A) and
metal part of complex metal-composite heterogeneous structure. The Composite Brittle Ductile
orientation (B) Pull-out
Fracture Fracture
objective of this work is characterisation of transverse micro-fastened joints
which efficiently transfer load through structural adhesive and yet retain the A=[90, -45, 0, 45, 90, -45, 0, 45]s
damage tolerance required by safety critical applications (Ucsnik et al., 2009 Zero to
T700 / M21 B=22 / 68 / -22 / -68 > 320N* > 320N*
& Nogueira et al., 2011). 320N
Zero to
Methodology: Metal parts are either Stainless Steel (SS) or Titanium alloy B=0/ 45 / 90 / -45
190N
> 320N*
(Ti6Al4V) and composite parts are either quasi-isotropic T700/ M21 laminate
A=[0, -45, 45, -45, 45, 0, 0, -45, 45, -45, 45, 0]
or tri-axial carbon fibre Non-Crimp Fabric vacuum infused with RTM6 resin
NCF / Zero to
(NCF/RTM6). Fig 1 illustrates the CMT pin welding process. Fig 2 shows B=22 / 68 / -22
260N
377N >320N
resin rich zone morphology around embedded micro-fasteners as a function RTM6
Zero to
of composite lay up orientation. Fig 3 shows the geometry of each Single Lap B=45 / 90 / -45 >260N >260N
260N
Shear (SLS) coupon.
* Micro-fasteners coated with
Results: Fig 4 shows a post test section through a SS micro fastener
release agent prior to insertion
inserted in the (cured) composite. Fig 5 shows metallographic sections into composite
through three diameters of welded Ti6Al4V micro-fastener. Table 1
summarizes single and quad micro-fastener SLS coupon results. Tensile test mm mm
results of SLS coupons containing single SS micro-fasteners embedded in Fig 6 SLS tensile test results and observed failure modes
NCF/RTM6 are shown in fig 6. Fig 7 shows a comparison of specific strength
between a transverse micro-fastened and bolted single lap shear joint with
each fastener spaced in an equilateral triangular manner at 4 x diameter. The 400
45/90/-45
0.08
dependency of this analysis on composite bearing strength ( b) is shown. 0.07
The thickness taken for the micro-fastener substrate is 0.4mm. 300 mm mm 0.06
opening
0.05
Fig 1 Manufacture of Ø0.9mm Titanium alloy micro-fasteners on
Load (N)
200 0.04 extension
0.4mm thick Titanium alloy foil; micro-fastener height 3.0±0.5mm 22/68/-22
0.03
Opening (mm)
100 0.02
0.01
0 -
-0.01
-100 -0.02 Ø0.8mm 3.0±0.5mm high
0 0.02 0.04 0.06 0.08 0.1 micro-fastener
500 Extension (mm)
Fig 2 Fluoresce photo-micrographs of micro-fasteners embedded
in composite with various lay-up orientation Ductile Fracture
400
½ mm ½ mm Tensile loading direction
mm mm
Load (N)
300
Ball
200
Orientation:
Pull-out
Orientation: Orientation: Orientation:
0/45/90/-45 22/68/-22/-68 22/68/-22 45/90/-45 100 Cylindrical
T700/M21 Laminate lay-up: NCF/ RTM6 Laminate lay-up:
[90, -45, 0, 45, 90, -45, 0, 45]s [0, -45, 45, -45, 45, 0, 0, -45, 45, -45, 45, 0] 0
0 0.5 1 1.5 2
Extension (mm)
Fig 3 Single and quad SS micro- fastened SLS coupon geometry
Fig 7 Comparison of specific strength of transverse micro-fastened
A: Single micro-fastener coupon
and bolted single lap shear joint
4.0mm thick
EN6114
spacer 8 fastener
Specific strength (MN/Kg)
Single micro-
b=800MPa
ASNA2536
B: Quad micro-fastener coupon fastener nut
6
coupon
4mm rectangular geometry
array of micro- 4 t
Composite ~4mm thick b=400MPa
fasteners
0.5mm thick metal foil 2
Fig 4 Post-test section Fig 5 Typical sections through 0 t
1 2 3 4 5 6 7
through SS micro-fastener Ti6Al4V micro-fasteners Composite thickness, t, (mm)
embedded in T700/M21 EN6114T3 + ASNA2536-3 EN6114T4 + ASNA2536-4
Ø0.8mm Ø1.2mm Ø1.6mm EN6114 3 (steel) + ASNA2536-3 EN6114 4 (steel) + ASNA2536-4
0.8mm SS transverse micro-fastener
Conclusion and Further work
0.8
0.4
mm Future measurement of single micro-fastener mode I and II bridging laws has
mm
been enabled by successful manufacture of Ti6Al4V welded micro-fasteners.
0.4
mm
The use of welded micro-fasteners requires quantification and mitigation of:
1. Porosity in the welded micro-fastener
2. Fiber undulation and consequent reduction of laminate in-plane properties
3. Resin rich zone formation and cracking
mm References
Nogueira, A.C., Drechsler, K., Hombergsmeier, E., Furfari D. and Pacchione M. (2011), “Investigation of a hybrid 3D-
reinforced joining technology for lightweight structures”, 16th International Conference on Composite Structures, Porto
Stieglbauer W. and Kazmaier J. (2009), “Innovative, multifunctional, form-locked joining technology for dissimilar
material combinations”, Welding in the world, Special issue, Vol 53:453-456
Ucsnik S., Scheerer M. , Zaremba S. , Pahr D.H. (2010), “Experimental investigation of a novel hybrid metal–composite
joining technology”, Composites: Part A, Vol 41:369–374