In advance of his plenary presentation at ILAS 2019, Dr Stefan Kaierle shares how the ability to join dissimilar materials with laser beam welding could be used to decrease the weight of ships and cars, leading to a reduction in their carbon emissions
This article was co-authored by S Nothdurft; O Seffer; R Lahdo; and J Hermsdorf of the Laser Zentrum Hannover.
In this day and age, the awareness of the importance of environmental protection and the conservation of resources is growing in people’s minds, the necessity is undeniable. Therefore, several politic goals and tasks have been defined, for example in the European Energy Strategy 2050.
An important step in reducing Europe’s greenhouse gas emissions will be reducing the emissions produced by the different branches of trade, particularly from the combustion engines of the vehicles involved.
Two strategies are pursuable for reducing the CO2 emissions of vehicles with combustion engines. The first is the complete substitution of a lower-strength material in a component part with material of a higher strength, such as a high-strength steel. This enables a reduction in the thickness of the material being used, and therefore results in a lower weight for the component. The second is the partial substitution of a material in a less loaded area of a component with a lighter material, with the lower strength of the new material still being sufficient to withstand the lower forces of the area.
A material combination which offers both high-strength and low-weight properties, is the combination of steel with an aluminium alloy.
To realise hybrid components using this combination, the two materials have to be connected, however the thermal joining of dissimilar materials, such as steel and aluminium alloys is a challenging task. For one thing, they have very different chemical and physical properties – melting temperature, thermal expansion and thermal conductivity – and for another thing, there is a metallurgic challenge: during solidification, brittle intermetallic phases are formed in the weld metal, which reduce the strength and durability of the joint hugely. Furthermore, the stresses experienced by the welded metals during solidification cause cracking to occur.
Due to its reduced and local heat input, high intensity laser beam welding is an appropriate method for joining steel and aluminium alloys. The smaller weld pool and rapid cooling offer reduced diffusion and lower the mixing ratio between the two materials, enabling a stronger bond to be formed than that previously achievable using conventional thermal joining methods. In addition, its ability to be automated enables this to be done with high productivity in series production.
Figure 1: Micrographs of metallographic cross sections of laser beam welded dissimilar joints of steel and aluminum alloys in shipbuilding (left) and automotive dimension (right). (Image: LZH)
For automotive applications, parts made using aluminium alloys can be used to lower a vehicle’s weight and reduce its fuel consumption. For shipbuilding, deck constructions made of aluminium alloys can be used to not only lower a ship’s weight, but to also lower its centre of gravity, which will increase its stability and driving speed.
A large, oversized adapter construction has previously been necessary to connect the aluminium parts of a deck construction to the steel hull of a ship. Explosion welding has been used to make this adapter in the past, which itself is made of dissimilar materials – the steel part of the adapter is welded to the steel hull of the ship, and the aluminium alloy part of the adapter is welded to the aluminium alloy used in the deck construction. By instead manufacturing the adapter using laser welding rather than explosion welding, a higher strength can be achieved between the dissimilar materials, enabling the size (and therefore weight) of the adapter to be reduced – a smaller connecting area between the materials is required. This, in addition to using the aluminium alloy in the deck constructions, can be used to reduce the weight (and therefore fuel consumption) of a ship.
In the automotive industry, adapter constructions are not used to join dissimilar metals, as sheets of aluminium alloy and steel can instead be welded together directly using lasers.
Forming a strong bond
In both the automotive and shipbuilding industries, laser beam welding can be used to form a lap joint between two metal sheets (see Figure 1) made of steel and aluminium alloys. In this scenario the steel sheet has to be the upper joining partner, as using an aluminium alloy in this position would increase the aluminium content in the weld metal above permitted values, which would weaken the joint.
In addition to controlling the mixing ratio of the two materials in the weld metal, the penetration depth of the weld into the lower aluminium alloy sheet is also an important aspect of this process. By using a penetration depth that is merely a little higher than the thickness of the steel material, a comparatively little amount of aluminium is enabled to be contained in the weld metal, so that the content of the intermetallic phases is limited. Each material and sheet thickness configuration requires a different appropriate penetration depth to achieve the best mechanical properties.
At the Laser Zentrum Hannover, we have successfully formed lap joints using dissimilar materials that are suitable for both the automotive and shipbuilding industries. For shipbuilding, a 5mm steel sheet (S355) was welded to an 8mm aluminium alloy (AA6082) sheet using a high-power diode laser beam source at 10.4kW laser beam power in a welding process with speeds of 1.9m/min. For automotive, a 1.5mm steel (22MnB5+AS150) sheet was welded to a 2mm aluminium alloy (AA6016-T4) sheet using a disk laser beam source at 3.75kW laser beam power in a welding process with speeds of 5.2m/min.
In testing the mechanical properties of the dissimilar joints, we found that for the shipbuilding configuration, a tensile shear force of up to 8.7kN could be withstood by a 25mm-wide sample before breaking, and for the automotive configuration, up to 3.2kN could be withstood by a 22.5mm-wide sample before breaking. Using multiple joints, for example when an adapter configuration with six joints is used in shipbuilding, the tensile shear force that can be withstood can be increased by up to 88 per cent of the yield strength of the aluminium alloy base material.
Despite these promising results, there is a gap remaining, which has to be filled with knowledge about this innovative laser welding process, in order to enable the dissimilar joining of steel to aluminium alloys to be used for practical applications and products. Solving these challenges would provide many opportunities for a wide range of applications and uses. We will be working at the Laser Zentrum Hannover over the coming years in order to fill this gap.
Stefan Kaierle is the executive director of the Laser Zentrum Hannover.
