# Diode Laser Welding/Brazing of Aluminum Alloy to Steel Using a Nickel Coating
## 1 Introduction  
The joining of dissimilar materials offers the potential to utilize the advantages of different materials, often providing a whole structure with unique mechanical properties. Aluminum alloys can reduce the weight of structural parts due to its light weight, and steel has a high strength. Hybrid structures of aluminum alloy and steel are suggested in automotive applications to improve the fuel efficiency and control air pollution through reducing the weight [1; 2; 3].  
The joining of aluminum alloys to steel is a great challenge due to the large differences in the thermophysical properties between these two materials, and especially the formation of inherently hard and brittle Fe-Al intermetallic compounds (IMCs) at elevated temperatures, such as FeAl, FeAl\({}_{2}\), FeAl\({}_{3}\), and Fe\({}_{2}\)Al\({}_{5}\). Early attempts at fusion welding of steel to aluminum alloys suffered from joint brittleness at the dissimilar interface caused by extensive IMC formation. More recently,many advanced welding technologies have been applied in Al/steel joining, such as friction stir welding [4], ultrasonic welding [5], explosive welding [6], laser keyhole welding [7], as well as arc, laser, and laser-hybrid welding/brazing [8; 9; 10; 11; 12]. The welding/brazing offers a great potential for Al/steel joining, owing to the advantage in reducing residual stress and controlling the IMCs. In the welding/brazing of Al to steel, the workpieces and filler metal are heated or melted by a heat source, and the joint has dual-characteristics, i.e., it has a fusion-welded joint at the Al side, and a brazed joint at the steel side. Some related investigations have claimed the observation of Fe-Al IMCs in Al/steel joints by welding/brazing [9; 10; 11].  
The use of materials (interlayer, filler metal, coating, or their combinations) that becomes part of the joint has proven to be sufficient to inhibit the formation of brittle IMCs in dissimilar materials welding/brazing, such as Mg/Ti [13], Al/Mg [14; 15], and NiTi/Ti6 Al4V [16]. For example, in the investigation of laser joining of NiTi to Ti6 Al4V, [PERSON] et al. [16] successfully inhibited the formation of brittle Ni-Ti IMCs by inserting a niobium interlayer. The common features of the materials can be summarized as: (1) a higher melting point than parent metals, so that the alloy mixing of parent materials can be deterred; (2) excellent metallurgical capability with parent metals; (3) no brittle or much less brittle reaction products are formed with either parent material. With these criteria in mind, Ni is considered as a promising candidate for the dissimilar Al/steel joining, because of its good miscibility with Fe and sound metallurgical capability with Al. Although its melting point is 83 \({}^{\circ}\)C lower than Fe, the inhibition of alloy mixing by Ni still can be accomplished by using accurately tailored energy distribution during welding. In fact, [PERSON] et al. [17; 18] made attempts to join Al to steel with a Ni interlayer. However, brittle Fe-Al IMCs were still formed by the direct mixing of Fe and Al, owing to completely melting the Ni interlayer by Nd:YAG laser penetration welding technology.  
In the present study, a diode laser with a more uniform energy distribution is applied. Compared to the keyhole mode in the Nd:YAG laser penetration welding, a conduction mode is figureachieved, which eases the accurate control of energy distribution and thus enables the welding/brazing technology. Ni, in the form of electroplated coating on steel, is used in a lap joint configuration. To highlight the effect of Ni coating, the microstructure and mechanical properties of laser Al/Ni-coated steel joint is drawn as a comparison with that of the laser Al/bare steel joint.