Annex X: Multi-Material Vehicle Lightweight Structures, Materials Joining Technology

Overview of scope:
This Annex was approved by the IEA AMT Executive Committee meeting in London, UK on June 4 2013. The objective of this Annex is to develop joining methods of various lightweight materials to enable the assembly of an optimum light weight vehicle with high energy efficiency

Annex Participants
U.S.: Led by Dr. Zhili Feng, ORNL, USA (Chair)
Canada: Led by Dr. Mark Kozdras, CanmetMATERIALS, Canada
Germany: Led by Dr. Ozlem Ozcan, BAM
Korea: Led by Dr. Mok Young Lee, Research Institute of Industrial Science & Technology
Brazil: Led by Dr. Henera Costa, Universidade Federal Do Rio Grande

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. 10. 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.

Conventional Bolting
In joining two very dissimilar materials such as carbon fiber reinforced polymer (CFRP) composite to magnesium alloys where adhesion and galvanic corrosion are barriers to durable joining. A method for the mitigation of damage in the composites was developed, which involved applying a coating inside the hole with a low viscosity resin, which is compatible with the composite, and has a surface affinity to penetrate into the composite microcracks. This approach is also effective to provide electrochemical isolation of the joint components to inhibit/reduce galvanic corrosion of the Mg, in contact with steel fasteners and CFRP. Immersion corrosion test results of AZ31B Mg alloy bolted to CFRP in 0.1M NaCl immersion revealed that the galvanic corrosion protection/isolation technique employed is effective. As shown in Figure 5, galvanic corrosion polarization potential was greatly reduced in the isolated joints. Compared to initial pH 5.8, ‘Isolated + covered’ had least corrosion of Mg. ‘Isolated’ also decreased Mg corrosion compared to the baseline. The benefits of isolation technique developed at ORNL on the lap shear test strength are shown in Figure 6.

Fig. 5. Corrosion potential of Mg to CFRP joint with different isolation techniques.

Fig. 6. Lap shear failure load comparison between based unprotected joints and 3-step isolation joints

Ultrasonic Welding of Mg Alloy to Steels
Joining of lightweight multi-materials such as magnesium alloys to high strength steels is a challenging because of the highly dissimilar natures of the materials. The joints simply cannot be fusion welded due to the extreme differences in their melt temperatures and joining methods that require a large amount of plastic strain in the magnesium component suffer from magnesium’s poor ductility at room temperature. Ultrasonic welding (UW) technique is under investigation to overcome some of the technical barriers preventing more robust and reliable joining of magnesium to steel. UW involves creating a large degree of plastic deformation at an interface while at the same time delivering heat from frictional and plastic work dissipation mechanisms. This method is solid-state, warm deformation technologies and takes advantage of the extended ductility of magnesium and steel at elevated temperatures.

UW has been researched to join Mg alloys to coated steels with success, where the Zn coating on the steel side acts as a brazing material to form intermetallic bonding with Mg. Joining of Mg alloys to bare steel is much more challenging, as they are immiscible in liquid state and they do not react to form any intermetallic phase to facilitate the metallurgical bonding. The maximum solid solubility of Fe in Mg is 0.00043at%, and solid solubility of Mg in Fe is nil. We developed an in situ high-speed imaging and infrared thermography experimental technique to study interfacial relative motion and heat generation during ultrasonic spot welding of AZ31B magnesium (Mg) alloys, as well as AZ31B to DP590 steels. Such high-speed imaging technique allowed us to gain fundamental understanding of the metallurgical bond formation at the interface, which led to successful development of UW of Mg alloys to bare (uncoated) steels. The experimental setup is shown in Figure 7. An example of interface motion/bonding and temperature evolution from the frictional heat generation is given in Figure 8. The insights gained from such fundamental study made it possible to develop UW process conditions to join different Mg and steel combinations, both coated and bare steels, together, as illustrated in Figure 9.

Fig. 7. Schematic of experimental setup, (right) photograph of a sonotrode, and (left) a high-speed image frame showing part of the two AZ31B (∼3Al-1Zn in wt%) Mg sheets (coated with a speckle pattern) and the sonotrode teeth on the top and bottom of the Mg sheets. Six subsets of the image pixels within the small boxes in the image were assigned to track the motion of sonotrodes and Mg sheets adjacent to three contact interfaces.

Fig. 8. Relative velocity curves across each interface showing sliding (high amplitude) and sticking (low amplitude) motion, as well as the temperature distribution (the inserted color plots surrounding the velocity curves measured at different time frames, t) when sticking relative motion was observed. (a) Sonotrode–Mg interface, (b) Mg–Mg joint interface, and (c) Mg–sonotrode interface. The spikes occurring at 0.26 s in (b) were measurement noise caused by flying metal chips. The blue curves between the inserted infrared images are the line plots of temperature distribution along the cross-sectional centerline. The numbers on the color bars and the horizontal axis of the line plots represent the temperature distribution in Celsius. Interfacial heat generation was observed along with interfacial sliding where the large amplitudes of the relative interfacial velocity were measured. The lack of Mg–Mg interfacial sliding after 0.16 s suggests the formation of a macroscale weld joint.

Fig. 9. UW of different Mg alloy and steel combinations, and their mechanical properties.

Resistant spot welding of Al/Mg with cold spray Zn coating
Resistance spot welding (RSW) is the primary joining method in the manufacturing of automotive assemblies. With the increased use of Al and Mg, there is a pressing need for a technology to produce dissimilar Al/Mg joints, and preferably by RSW since this technology is already prevalent in the industry. In applying RSW to weld Al to Mg, however, the melting of base materials during RSW will lead to the formation of hard and brittle intermetallics, which is harmful to the mechanical properties of the joint. Previous results show that the use of interlayers can facilitate the joining of dissimilar materials. The formation of intermetallics phases can be controlled when a suitable interlayer is utilized. To date, most of the research applying interlayers during RSW welding of Al to Mg is to use metallic foils as an interlayer, such as Zn foil, Cu foil, and Ni foil.

The current work is to explore the feasibility of utilizing a Zn cold sprayed coating as an interlayer for the RSW of Al to Mg. The cold spray was conducted with a gas temperature of 150 ºC and a powder feed rate of 8 g/min. A pressure of less than 200 psi was used during the cold spray to accelerate particles at the substrate. The Zn coating with a thickness of 0.2 mm was produced on the substrate surface of both Al and Mg. The coated coupons are shown in Figure 10. To investigate the effect of Zn coating on the joint formation and strength, RSW of Al to Mg was also carried out without the coating for comparison. A current of 26 to 28 kA with 20 to 40 cycles of pulses were applied during RSW.

Fig. 10. Cold spray machine (left) and Zn coated Mg and Al sheets (right).

The results show that direct welding of Al to Mg is possible, but with a narrow range of parameters.

Significance and Impacts
Automobile fuel economy regulations vary across the globe but there is a unified effort to increase vehicle fuel efficiency. Given that lightweighting can increase fuel economy by 7-9% with each 10% reduction in vehicle weight, the use of lower mass dissimilar materials is increasing. The key barrier is how to meet the required structural performance and durability for dissimilar material joints. Annex X provides integrated, global research collaboration for comparing the myriad of different joining technologies under development and data base to provide guidelines on joining methods.