LightBEE : Development of Lightweight Battery Carriers for EV Energy Units

Project description

Within LightBEE, several joining processes are investigated to demonstrate the new battery carrier concepts. The following processes have been identified as very promising to achieve the project goals.

Friction stir welding (FSW) and Friction stir spot welding (FSSW) 

Friction Stir Welding is a friction-based solid-state process allowing spot and line joining of sheets or plain products (extrusions). It uses a rotating tool that is plunged in between two clamped plates, that is traversed along the joint line (see Figure 1 and Figure 3). In friction stir spot welding, similar tools are used for creating a spot weld. Joints are produced with a sinkhole/crater, as shown in Figure 2.

Figure 1: Principle of friction stir welding

 

Figure 2 : Principle of friction stir spot welding

 

Figure 3 : Example of a friction stir weld of a hollow profile (Source: BWI) 

 

The FSW technology has shown great potential for manufacturing panels using double-skin extrusions of aluminium (Figure 5) as well as reinforced structures by overlap welding of 7XXX alloy extrusions (Figure 6). These concepts will be the basis for the developments required for FSW implementation in battery carrier manufacturing in LightBEE. 

Figure 5: Panels produced by FSW of double-skin extrusions (source IK4 LORTEK)

 

Figure 6 : 7XXX extrusion welded to a 7XXX sheet by FSW in the overlap configuration (source IK4 LORTEK)

 

The FSW technology has been investigated for its application in high-strength and lightweight structure manufacturing in the aerospace industry. Proof of that are past projects such as WEL-AIR [1] or the ongoing OASIS [2] or ecoTECH [3]. FSW technology applied to high-strength aluminium alloys (2XXX, 7XXX…) is investigated in these projects in order to create innovative structures showing weight reductions and high structural strength. This type of applications were in some cases implemented in real aeronautic structures, i.e. for fuselage structures by Eclipse Aviation [4] and Embraer [5]. Similar structural and FSW technological concepts will be developed in LightBEE, focusing on materials and requirements relevant for the battery carrier development in automotive. Previous results available from the mentioned projects and applications will be very valuable as reference for LightBEE.

Refill friction stir spot welding (RFSSW)

RFSSW is a spot welding process based on friction. It uses a three-component tool comprising a pin, sleeve and clamping ring to join two or more sheets in the overlap configuration. The process creates spot welds without any crater, leaving a flat surface with minimum material loss. 
RFSSW is highly suitable for welding aluminium alloys, and in particular for welding advanced high-strength alloys, which are not weldable with conventional welding techniques. It is able to join dissimilar material combinations, such as aluminium-steel, aluminium-composites, coated materials and alloys with unequal thicknesses. It also offers a solution to the problems of resistance spot welding of aluminium alloys (electrode degradation and its influence on the weld quality). The process has been investigated for joining of several aluminium alloys [6,7,8], and for joining of aluminium to composites [9,10].

Figure 7: Principle of refill friction stir spot welding

 

Figure 8: Example of a refill friction stir spot weld (Source: Riftec gmbh)

 

Figure 9: Example of a refill friction stir spot weld in aluminium (Source: BWI) 

 

Figure 10: Example of a refill friction stir spot weld of aluminium to a composite (Source: Helmholtz-Zendrum-Geesthacht)  

Electromagnetic pulse welding

Electromagnetic pulse welding is a new, very innovative but nearly unknown production process. The working principle of the welding process is based on the use of electromagnetic forces to deform and weld workpieces. Since this sophisticated welding process doesn't use heat but pressure to realise the weld, it offers important advantages with regard to the conventional welding techniques. The deformation takes places at a very high velocity and is created in a safe way, by using electromagnetic forces. The absence of heat during the weld cycle makes it possible to join dissimilar materials [11].

 

Figure 11: Principle of electromagnetic pulse welding 

 

Figure 12: Example of an electromagnetic pulse weld (Source: BWI)

 

Figure 13: Example of an electromagnetic pulse weld of aluminium [12]

 

Figure 14: Example of an electromagnetic pulse crimp connection of aluminium to composite (Source: BWI [13])

 

The above-mentioned processes are all solid-state joining processes, which have the following advantages compared to fusion welding:

• the produced joints are usually free of various solidification defects such as gas porosity, hot cracking, and non-metallic inclusions, which may be present during fusion welding, 
• excellent mechanical properties are obtained in the weld HAZ, and low distortion and shrinkage is present, due to solid-state nature, 
• ability to join difficult-to-weld aluminium alloys (e.g. materials from the 2000 and 7000 series), dissimilar materials or sheets with big differences in thickness.

Laser beam welding

Laser beam welding is a fusion welding process where radiant energy is used to produce the heat required to melt the materials to be joined. Laser welding of aluminium is widely used in industry, but there are nevertheless limitations which need to be addressed for each application. First, aluminium one of the best reflectors of light. In addition, many aluminium alloys contain magnesium or zinc, which are easily vaporised forming a plasma that blocks the incident beam. Other issues that need to be considered when laser welding aluminium are low power absorption, alloy compositional differences and the importance of surface preparation [14].

Figure 15: Laser welding of aluminium

 

Figure 16: Example of laser welds of aluminium