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

Overview of scope:
The objective of this annex is to develop and evaluate joining methods of various lightweighting materials to enable the assembly of an optimum light weight vehicle with resulting energy efficiency. The focus is on the evaluation and developing new joining methods for assembling structures from dissimilar materials. For this reporting period, two different joining techniques for magnesium alloy to carbon fiber reinforced composite (CFRC) were evaluated.

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:
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 1, 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 2.

Fig. 1. Corrosion potential of Mg to CFRP joint with different isolation techniques

Fig. 2. 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 3. An example of interface motion/bonding and temperature evolution from the frictional heat generation is given in Figure 4. 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 5.

Fig. 3. 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. 4. 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. 5. 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 6. 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. 6. 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. As shown in Figure 7, the nugget size is small with most of the nugget growth into the top Mg sheet. Most samples do not remain intact. The nugget diameter is well below requirement, can only slightly exceed 7 mm if expulsion occurs.

Fig. 7. Optical images showing the cross-section of the joints without Zn coating interlayer welded with (a) 25 kA, (b) 26 kA and (c) 27 kA

On the contrary, after coating with Zn on the surface of both Al and Mg sheets, more controllable and repeatable nugget sizes are achieved. Figure 8 shows the SEM image and EDS result at the interface. No clear formation of the intermetallic phase and dendritic structures can be found between Al and Mg. The Zn coating acts as a barrier layer between the Al and Mg to prevent the formation of the Al-Mg intermetallic phase. EDS result demonstrates the evidence of Zn diffusion into the Al side after RSW welding to produce a metallurgical bond.

Fig. 8. SEM images of the joint interface (a) and (b) as well as EDS result (c)

The single lap shear test was conducted to evaluate the joint strength. Typical load-displacement curves of joints with and without Zn coating interlayer are shown in Figure 9. It can be seen that the maximum load of the weld without Zn interlayer is around 1.2 kN with a very low elongation due to the formation of the intermetallic phase at the interface. Besides, joint samples often crack upon loading. On the other hand, the strength and ductility of the weld produced with Zn coating interlayer are twice as much as those without interlayer.

Fig. 9. Single lap shear test results of (a) joint without Zn coating interlayer and (b) joint with Zn interlay

Preliminary tests have shown that the joint strength can be increased up to 5 kN with a Ni cold sprayed coating interlayer, which will be reported in the next year.

Dissimilar aluminum-polymer linear friction-stir lap welding

In this report, an attempt has been made to join aluminum and carbon fiber reinforced polymer using friction stir welding. It is commonly known that, due to the stark contrast between the properties of aluminum alloys and polymers, this process becomes very challenging. While generally, the difficulty of dissimilar welding varies, joining dissimilar material such as metals to polymer is considered to be the most difficult.

Friction stir welding is a solid-state welding process for dissimilar joining. This process avoids liquid to solid transition while promoting rigorous intermixing which forms mechanical interlocking to enhance the dissimilar weld joints. In this study, friction stir welding using lap configuration (friction stir lap welding, i.e. FSLW) has been adopted to attempt dissimilar welding between Al sheet and polymer sheet.

As shown in Figure 10, three separate tools were utilized. Tool 1: 2mm length M6 threaded, 8° tapered, tri-flats pin with 0.2 mm scribe. The root pin diameter is 6 mm. the shoulder diameter is 15 mm. Tool 2: 2 mm length and 2 mm diameter straight cylindrical pin. The shoulder diameter is 15mm. Tool 3: 2 mm length conical pin with a sharp pin tip. The shoulder diameter is 15mm. The detailed welding parameters are shown in Table 1. Most of the Al sheet used in this work is AA6022 T43 with a thickness of 1 mm. The carbon fiber reinforced polymer is 5 ply T70 thermal set with a thickness of 1 mm.

Fig. 10. Tool geometry

Figure 11 shows the welding results using tool 1 with a rotation speed of 450 rpm and a welding speed of 180 and 250 mm/min. Incomplete consolidation, burnt polymer, and fiber taring can be observed. The main problem for the un-successful welding is material agglomeration on the tool. As shown in Figure 12, severe material agglomeration on the tool pin during welding transforms the sharp pin profile into a blunt/round shape, which significantly degrades the tool capability to transfer material backward. This in turn causes incomplete consolidation of the nugget zone. Upon further inspection, the material agglomeration consists of layers of Al and polymer. Removing this layered structure after each welding attempt has been proven to be very time-consuming. This challenge would render the welding with tool 1 highly impractical.

Table 1. Parameters used for FSLW. Fig. 11. Separated top Al sheet and bottom thermal set T70 sheet when welding using tool 1 with a welding speed of 180 mm/min (left) and 250 mm/min (right)

Fig. 12. Tool appearance before welding (left) and after welding (right)

When welding with tool 2, as shown in Figure 13, incomplete consolidation was found, although superficial bonding was achieved at the joint interface. While no issue of material agglomeration arises (possibly due to significantly smaller pin diameter), the results show that only superficial bonding was achieved where in most cases the material can be separated easily after cooling to room temperature. The main challenge for Tool 2 is the lack of bonding.

Fig. 13. Separated top Al sheet and bottom thermal set T70 sheet when welding using tool 2 with a welding speed of 180 mm/min (left) and 250 mm/min (right)

Welding with tool 3 shows similar results with those with Tool 2. Besides, the issue of material agglomeration occurred as in the case of Tool 3.

The above results demonstrate that FSLW of aluminum to thermal set has proven to be very challenging and only superficial bonding can be achieved. The main reason is due to the lack of intermixing between two materials. Even when partial intermixing is possible, the thermal set polymer is burnt and fiber taring occurs. For future experiments, the utilization of thermoplastics for the polymer counterpart will be investigated.

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.

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