PSI - Issue 33

Ambra Celotto et al. / Procedia Structural Integrity 33 (2021) 887–895 Celotto et al. / Structural Integrity Procedia 00 (2019) 000–000

893

7

a

b

c

AA1070 BM plate

AA6082 FM tip

Zone 2

d

e

f

Crack

Zone 1

g

i

Dispersoids

h

BM

FM

FM

BM

Bonded interface

Cross-section line

10 µm

2 µm

10 µm

Fig. 4. (a,b,c) SEM pictures of preliminary welding setup sequence: (a) Hybrid needle is being pushed on the BM plate and starts to build up a material barrier in front of its flow; (b) The FM tip bends and some cracks are formed; (c) The FM tip suddenly detach from the tungsten base, by breaking the platinum ring (Scale bar: 30 µm). (d,e,f) SEM pictures of improvised welding setup sequence: (d) After detaching from the BM, the FM tip was held vertical by the material accumulation behind it and the micromanipulator has been employed as a punch on the FM tip; (e) Both BM and FM highly deformed under the pressure load; Possible welding zone identified in the squared area, named as Zone1; (f) By subsequent puching on Zone 2, the FM tip broke from Zone 1, where bond occurred (Scale bar: 50 µm). (g) Magnified picture of welded zone after the experiment; The dotted line is referred to the place where the cross section in (h) was cut from; The squared area in (h) is magnified in (i) where the bonded interface is shown in detail and the differences in microstructure between the coupled materials can be appreciated. 4.2. Final setup and welding After detaching from the tungsten base needle, the tip remained vertical supported by the material barrier that was previously raised. Therefore, the pushing experiment proceeded by using the truncated tungsten needle as a punch on the FM wire as visible in Fig.4d. The micromanipulator was moved along its own axis at a speed of 1 µm/s towards the small FM wire piece. The applied pressure resulted to be high enough to highly deform both the workpieces (Fig.4e) and to consequentially to spread the FM onto the BM (Zone 1). After a first pushing, the needle was retracted and repositioned on a remaining protruding end of the FM piece (Zone 2). Another pushing was carried out on that, with the same parameters, to complete the experiment on all the available material. In this second step, the current stressed part of the FM wire (Zone 2) was observed to crack

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