Effects of different powders on the micro-gap laser welding-brazing of an aluminium-steel butt joint using a coaxial feeding method

doi:10.1016/j.matdes.2016.07.011

Materials & Design

Volume 109, 5 November 2016, Pages 10-18

https://doi.org/10.1016/j.matdes.2016.07.011 Get rights and content

Highlights

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Laser butt welding of Al/Fe dissimilar metals was performed using a coaxial powder feeding method.
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Welding process was performed under the micro-gap butt conditions.
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The tensile strength of the joint with AlSi₁₂ powder reached 67.6% of that of the Al base material.
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Only element Si was found in IMC layers with AlSi₁₂, AlCu₅ and ZnAl₁₅ filler powders respectively.

Abstract

Due to the large differences in the physical and chemical properties of aluminium and steel, brittle intermetallic compounds such as Fe-Al form easily when their components are welded together. In this study, a coaxial powder feeding method was used to deliver AlSi₁₂, AlCu₅ and ZnAl₁₅ powders for butt laser welding of aluminium 5251 and TG-1 automotive steel with 1 mm thickness under micro-gap conditions. The butt gap was controlled at 40–80 μm in the experiment, and the focus offset was 400–800 μm. Welds filled with AlSi₁₂ powder had high-quality surfaces and no welding spatter. The fusion zone grain structure consisted of Al-Si eutectic structures, and the intermetallic compound (IMC) layer was 4 μm thick. The average IMC layer thickness was 2–6 μm for three filler powders. Si formed compounds FeAl₃Si and Fe_0·905Si_0.0905 in the IMC layer, which reduced butt's brittleness. However, chemical reactions of Cu and Zn with Fe and Al, respectively, did not exist in the IMC layer. The tensile strength of welding joint filled with AlSi₁₂ powder was the greatest; it reached 67.6% of that of the aluminium alloy base material.

Graphical abstract

Introduction

The substantial increase in car ownership has resulted in many problems, such as energy consumption and environmental pollution. Automotive lightweighting is an important factor in resolving these problems [1], [2]. The dissimilar composite structure of an aluminium-steel alloy combines excellent light weight, corrosion resistance, and ease of processing of an aluminium alloy with high strength, high ductility, and toughness of steel; therefore, this material is important in research on automotive lightweighting [3], [4]. Because of its characteristics, which include heat concentration, ease of automation, and low environmental demand, laser welding is increasingly used to connect dissimilar materials of aluminium and steel [5], [6]. However, the physical and chemical properties of aluminium and steel differ significantly, and both have low solubility. Thus, brittle Fe-Al intermetallic compounds, such as FeAl₃, Fe₂Al, Fe₂Al₇, Fe₂Al₅, FeAl₂ and Fe₄Al₁₃, form easily during the cooling process of welding, reduce the ductility and toughness of the joint and even induce welding cracks [7], [8].

Numerous studies have sought to ensure the welding quality of aluminium-steel alloys. Lap joints have been used to connect aluminium and steel, and brazing components of Si, Cu and Zn has been added between aluminium and steel during welding to reduce the brittleness of joint interface. A lap joint with aluminium on the top and steel on the bottom exhibited a maximum tensile strength of 73% of that of the aluminium alloy base material [9], [10], [11]. Liu et al. [12] conducted an aluminium-steel lap welding experiment by adapting lateral Al powder feeding method to produce a lap joint with an intermetallic compound (IMC) layer thickness of 6–7 μm and a strength of 67% of that of the aluminium alloy base material. The experimental results revealed that a good welding joint can be obtained by using either a wire or a powder as a filler in a lap joint in aluminium-steel welding. However, when a lap joint form is used, the overlap of part of material increases the weight of welded piece, which is not conducive to lightweighting. Therefore, Lin et al. cut grooves in both the aluminium base material and the steel and performed aluminium-steel butt laser welding with wire fillings of Al, AlSi₅, AlSi₁₂ and AlCu₆. The resulting butt weld joint had a maximum tensile strength of 55% of that of the aluminium alloy base material [13], [14], [15], [16]. Regardless of the type of weld created by lap welding with a filler in wire or powder form or by butt groove welding, wire or powder solder serves as a transition layer connecting the aluminium and steel that inhibits the production of a brittle phase at the aluminium-steel interface and creates a high-strength connection between aluminium and steel by melting and wet spreading the base material. Compared to groove welding, the micro-gap butt laser welding technique for aluminium and steel reduces the complexity of process and the amount of solder used. In the micro-gap butt laser welding of aluminium and steel, controlling composition of the aluminium-steel IMC layer to inhibit the brittle phase is the key to increasing the weld strength. Because the characteristics of composition ratio are flexible and easy to control, filler powders are more suitable for aluminium-steel micro-gap laser welding. Studies of butt laser welding with a filler powder have only employed pre-filling on thick, grooved aluminium-steel plates [17], and the micro-gap butt laser welding of aluminium and steel using a synchronous powder feeding method has not been explored. Lateral powder feeding can provide real-time powder delivery during laser welding process; however, due to the directional constraint of the processing track and the poor powder utilization rate, it is difficult for lateral powder feeding to satisfy the requirements of welding formation. Given the shortcomings of lateral powder feeding, we examined micro-gap butt laser welding of aluminium and steel using a coaxial nozzle designed by our research group. Coaxial powder feeding is mainly used in laser cladding techniques and has not been applied in the field of laser welding.

In this paper, automotive aluminium alloy 5251 and third-generation TG-1 automotive steel were used as the study objects, and the micro-gap butt laser welding of aluminium and steel was performed with coaxial powder feeding; three different powders were fed: AlSi₁₂, AlCu₅ and ZnAl₁₅. The surface morphologies, fusion zones, and IMC layers of welding joints filled with three powders were evaluated, and the mechanical properties of the joints were tested to provide technical support for high-quality coaxial powder feeding in micro-gap butt laser welding of aluminium and steel and to promote the development of automotive lightweighting.

Section snippets

Experimental method

The experimental base materials used in this study were automotive aluminium alloy 5251 and third-generation TG-1 automotive steel; both were 1 mm thick. All plates were cut to a size of 80 mm × 60 mm; chemical compositions of two materials are listed in Table 1, Table 2. The test pieces were cleaned with acetone to remove surface oil before welding, and oxide film on the surface of the aluminium alloy was removed with sandpaper. To improve the wettability of solder on the welding interface of

Analysis of the surface morphologies

The surface morphology of the welding seam macroscopically reflects the effectiveness of welding process and reliability of welding seam. In this study, the welding seam's surface morphology was mainly evaluated based on its continuity, smoothness, spatter and cracks.

Fig. 2 presents the surface morphologies of aluminium-steel welds filled with AlSi₁₂, AlCu₅ and ZnAl₁₅ powders. The weld filled with AlSi₁₂ powder exhibited a continuous front formation without obvious collapse, and its back

Conclusions

(1)
In this paper, laser butt welding-brazing of aluminium and steel was performed under micro-gap conditions using the coaxial powder feeding method with AlSi₁₂, AlCu₅ and ZnAl₁₅ filler powders. The highest-quality welding joint was obtained by using AlSi₁₂ filler powder with the following optimal parameters: a laser power of 2250 W, a welding speed of 30 mm/s, a focus offset of 800 μm and a butt gap of 80 μm. The mechanical properties of this joint were superior to those of the joints obtained using

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Nos. 51575175, 51175162).

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Effects of different powders on the micro-gap laser welding-brazing of an aluminium-steel butt joint using a coaxial feeding method

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental method

Analysis of the surface morphologies

Conclusions

Acknowledgements

Study of metallurgic and mechanical properties of laser welded heterogeneous joints between DP600 galvanised steel and aluminium 6082[J]

Materials & Design

Microstructure and properties of laser brazed magnesium to coated steel

Welding Journal

The effect of interface reaction on vibration evolution and performance of aluminium to steel high power ultrasonic spot joints [J]

Materials & Design

Joining of steel-aluminium mixed joints with energy-reduced GMA processes and filler materials on an aluminium and zinc basis

Welding and Cutting

Dissimilar welding of carbon steel to 5754 aluminum alloy by Nd:YAG pulsed laser

Materials & Design

Characterization of intermetallic compound layer thickness at aluminum–steel interface during overlaying [J]

Materials & Design

Application of laser in seam welding of dissimilar steel to aluminium joints for thick structural components [J]

Optics and Lasers in Engineering

Employment of fiber laser technology to weld austenitic stainless steel 304l with aluminum alloy 5083 using pre-placed activating flux [J]

Materials & Design

Development of novel CsF–RbF–AlF 3 flux for brazing aluminum to stainless steel with Zn–Al filler metal

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Dissimilar metal joining of aluminum alloy to galvanized steel with Al–Si, Al–Cu, Al–Si–Cu and Zn–Al filler wires

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Science and Technology of Welding & Joining