Annex X: Multi-Material Vehicle Lightweight Structures, Materials Joining Technology 2018 Activity and Accomplishments

An assessment on the feasibility of re-fill fraction stir spot welding (RFSSW) on joining of Mg alloy (ZEK 100,1.53 mm thick) and high strength steel sheets (DP 600, 1.0 mm thick) was carried out in 2014. A schematic of refill friction stir welding process is shown in Fig. 1. Strategy for joining of non-ferrous alloys to steel was employed by controlling penetration depth to 0.03 mm on the lower sheet.

Fig. 1. Schematic of refill friction stir welding process, for joining two similar sheets (a) and strategy for joining of non-ferrous alloys to steel by controlling penetration depth to 0.03 mm above the sheet (b)

During welding, the steel sheet placed below, and the plunging depth of the tool into the upper Mg sheet varied from 1.3 to 1.5 mm. The tool operated under a rotation speed of 1600 to 2100 rev/min for 2.5–3.5s welding time. Owing to the severe mechanical deformation imposed by the tool during the refill process, significant grain refinement occurred within the Mg alloy (from 10mm grain size to 1.6 to 6.5 mm in the stir zone). The variation in grain size was symmetrical across the stir zone; with finer grain sizes observed towards the location below the tool sleeve at the outer periphery, while the coarser grains were observed near the centreline of the tool below the pin (see Fig. 2).

Fig. 2 Grain structures in ZEK100 stir zone produced using 1800 rev/min and 3 s (a)1800 rev/min and 3.5 s (b) and 2100 rev/min and 3 s (c)

High concentrations of Zn are dispersed throughout the stir zone and typically at the periphery of the joint directly under the tool sleeve. The Zn coating appears to be displaced from the DP600 steel sheet surface and moved upwards as well as towards the periphery edges of the weld; such movement is consistent with the material flow.

There was evidence that some residual Zn coating on the steel sheet surface remained at the interface of the weld and was not completely displaced by the material flow imposed by the refill welding tool. Some of the Zn has also reacted with the Mg alloy and appears to form a very fine scale Mg–Zn eutectic structure. The presence of a Zn coating on the steel appears to provide a mechanism for bonding. A TEM image of the interface at the centre of the weld is shown in Fig. 3, along with the element mapping. No voids or pores were present at the interface; however, a discontinuous film of oxides could be observed, which likely originated from the original Mg alloy sheet since the surfaces were not brush finished before joining (as expected for a manufacturing scenario). The presence of Zn is only observed in the Mg alloy side, with an increased concentration within 250 nm of the steel interface. However, the most striking observation is the presence of an Al rich film with a thickness of ~100 nm at the interface within the Mg alloy sheet.

Fig. 3. High angular annular dark field image (TEM) of interface with element maps for Al, Mg, Fe, O, C, Zn, Si and Mn

The overlap shear strengths of individual joints reached over 4.9 kN for the Mg ZEK100/HS steel DP600 joints; however, the highest average load was 4.7 kN when using 1800 rev/min, a 3 s welding time and 1.5 mm of tool penetration as shown in Fig. 4. This compares well with the requirements of AWS D8.9M, which recommends an average of 3.8 kN for the equivalent resistance spot welds between the weaker material (ZEK100, which had a tensile strength of 275 MPa). The fracture loads drastically increased when the plunge depth increased above 1.3 mm, which indicates that a critical threshold distance between the tool and steel sheet must be reached in order provide bonding. When the welding time increased to 3.5 s using 1800 rev/min, the loads decreased, to an average of 3.45 kN, and when the tool rotation speed increased to 2100 rev/min, the average fracture load decreased to 3.29 kN. These results suggest that the quality of the bond deteriorates when excess heat is applied during welding. All the fractures during overlap shear testing occurred through the interface.

Fig. 4. Comparison of ZEK100/DP600 joint overlap shear fracture loads a versus plunge depth when using 1800 rev/min and 3.0 s welding time (a) and when revolutions per minute and welding time is varied (b)

These results suggested that re-fill fraction stir spot welding is feasible in joining Mg sheet to high strength steel.