Research Article
Multi-scale characterization and simulation of impact welding between immiscible Mg/steel alloys

https://doi.org/10.1016/j.jmst.2020.04.049Get rights and content

Abstract

Vaporizing foil actuator spot welding method is used in this paper to join magnesium alloy AZ31 and uncoated high-strength steel DP590, which are typically considered as un-weldable due to their high physical property disparities, low mutual solubility, and the lack of any intermetallic phases. Characterization results from scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) of the weld interface indicate that the impact creates an Mg nanocrystalline interlayer with abundant Fe particles. The interlayer exhibits intact bonding with both DP590 and AZ31 substrates. To investigate the fundamental bond formation mechanisms at the interface, a finite element (FE)-based process simulation is first performed to calculate the local temperature and deformation at the interface under the given macroscopic experimental condition. Taking the FE results at the interface as inputs, molecular dynamics (MD) simulations are conducted to study the interlayer formation at the Mg/Fe interface during the impact and cooling. The results found a high velocity shearing-induced mechanical mixing mechanism that mixes Mg/Fe atoms at the interface and creates the interlayer, leading to the metallurgical bond between Mg/steel alloys.

Introduction

Magnesium alloys exhibit many unique properties including low density, high specific strength, excellent sound-damping capacities, good electromagnetic shielding performance, and heat conductivity. They are the major contenders for future vehicle weight reduction and energy savings in the global automotive industry. Since steel is still the dominant load-bearing structural material in most vehicles, the development of reliable and cost-effective joining methods for magnesium alloys to steel (Mg/Fe) becomes critically important.

However, Mg alloys and steel are very difficult to join through conventional welding techniques. Typically, the metallurgical joining of dissimilar materials can be divided into reactive and non-reactive types [[1], [2], [3]]. The reactive type forms an intermetallic compound (IMC) layer during the welding process which acts as the bridge to connect the two base materials, such as Fe-Al [4], Ti-Al [5], Mg-Al [6] and Cu-Sn [7]. The non-reactive materials usually rely on high solid solubility, e.g. Cu-Ni. If the mutual diffusivity is low, i.e. the two materials are immiscible, some dissimilar metals can still be bonded by creating a low crystallographic mismatching at the interface (e.g. Cu-Fe [8]). In this case, the closely matched lattices lead to low interfacial energy and join the two immiscible materials with metallic bond. The magnesium-steel system, however, does not satisfy any of the above conditions. Mg and Fe form no compounds according to the phase diagram, and they have extremely low (<0.00041at.%) mutual solid solubility and no matching planes (HCP vs BCC), not to mention the large disparity in their physical properties such as melting temperature, coefficient of thermal expansion (CTE), et al. [1]. In fact, the melting point of Fe (1811 K) is remarkably higher than the boiling temperature of Mg (1363 K), thus Mg will be evaporated before steel is even melted. The large difference between the thermal expansion coefficients (2.6×10-5 K-1 for Mg and 1.4×10-5 K-1 for Fe) will also cause problems including thermal distortion, pores, and cracks in fusion-based welding techniques. Therefore, the Mg/Fe system is conventionally believed to be un-weldable without the aid of coatings or transitional interlayers.

The recent research on creating Mg-steel metallurgical joining has focused on forming an interfacial IMC layer with a third chemical element. This is done by either increasing the alloying element content (e.g. Mg alloy AZ91 welds better than AZ31 [3]) or using direct coating, foil or fillings (Al, Zn, Cu, Ni et al. [1]). These research efforts covered several welding methods including friction stir welding [[9], [10], [11]], laser welding [[12], [13], [14]], arc welding [15], resistance spot welding [16,17] and ultrasonic welding. In these cases, the joint strength is governed solely by the interlayer, and thinner IMC layer is often reported to exhibit better bonding strength [18].

While very few research focuses on direct welding between Mg to uncoated steel, a recent study [2] using DC resistance spot welding method has succeeded in joining pure Mg to pure Fe without interfacial intermetallic transition layer. This is done via locally melting a thin layer of Mg on the Fe surface with the aid of fast cooling. The joining interface exhibits a 100−200 nm thick nano-grain Mg interlayer with an average grain size of 20 nm. The excessive grain boundaries relax the interface residual strain and lead to a metallurgical bond. Although the achieved joint strength (2.08 K N) is lower than that of AZ31/DP600 joint with a Fe3Al IMC layer (5.7 K N), the result indicates that the Mg/Fe interfacial nanostructure can be altered to achieve intrinsic metallurgical bonding between the two systems. It is thus imperative to study the Mg/Fe joint interface formation mechanisms which could lead to the development of new joining techniques for immiscible materials.

In this paper, the authors present the multi-scale experimental and simulation results of an AZ31/uncoated high-strength steel DP590 joint made with vaporizing foil actuator welding (VFAW) process. VFAW is one type of impact welding which has been used to successfully join dissimilar metals with disparate properties, e.g., Al-Cu, Al-Fe, Cu-Ti [19]. Even though it is categorized as a solid-state welding method without external heat sources, impact welding often creates a very thin interlayer with signs of local melting [[20], [21], [22]]. In the case of similar metals, the interlayer consists of ultra-fine grains; and in the case of dissimilar metals, metallic interlocking and/or intermetallic phases are reported [23]. These have been attributed to the rapid increase in local temperature through plastic dissipation of the kinetic energy to heat. While the VFAW method has proven successful for many dissimilar materials welding, using VFAW for immiscible Mg/Fe system has not been reported. Moreover, the underlying bonding mechanism at the interface has not been systematically studied.

Recently, Wang et al. [24] has studied dissimilar Al 6061-O to Cu-T2 lap joints made by electromagnetic pulse welding (EMPW) process which shares the same impact-based welding mechanism but with a different source of driving force. It was found the EMPWed lap joint exhibits an annular (or ring-shaped) morphology welding zone close to the outer edge while the center region is not welded, showing similarity to VFAW joint in this work where the interlayer is formed close to edge and not observed at center.

Recent advances in computational models have afforded researchers to perform in-depth simulations of the VFAW process. Eulerian formulation-based coupled dynamic thermal-mechanical finite element (FE) model has been established in the literature to model impact welding processes for various dissimilar materials [25]. The Eulerian-FE method circumvents the problem of severe element distortion in the Lagrangian formulation and is suitable for modeling the extreme deformation-induced heat generation and the transition from solid to liquid state. The simulation results successfully capture the formation of mechanical interlocking at the joint interface, a critical feature observed in dissimilar material impact weld. An important observation from the simulation is that the deformation is highly heterogenous during the impact, and localization of strain rate and temperature are found near the dissimilar materials interface.

However, the continuum-scale Eulerian FE-based process simulation does not directly predict the formation of IMC or interlayer. This is because the atomic diffusion and reaction at the joint interface cannot be resolved in FE simulation due to the length-scale disparity. Experimentally, an amorphous interlayer is often observed in impact welds, e.g. Fe-Al [20], the formation of which is driven by the atomic diffusion under localized temperature and pressure. In this case, the intrinsic bond strength of impact weld is determined by the strength of interfaces between the base materials and interlayer. To simulate the impact-induced diffusion, molecular dynamics (MD) models have been proposed in the literature [26,27]. In these models, the impact and unloading between base materials are modeled at the atomic scale. The diffusion distance is calculated and validated with experiments for Mg/Al [27], but immiscible material systems such as Mg/Fe has not yet been studied.

In this paper, a thorough multi-scale characterization of Mg/Fe impact weld interface is conducted at the micro- and nanometer length scales using scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), respectively. To understand the formation mechanism of the observed Mg/Fe interface features, two numerical simulations at different length scales are conducted. A process simulation is first conducted using the Eulerian-FE model under the same macroscopic welding condition as the experiments. From the FE analysis results, the spatial distributions of temperature and deformation along the Mg/Fe interface at each time step are obtained. The local thermal-mechanical condition at the Mg/Fe collision front area is passed as boundary condition to a nanoscale MD simulation for analysis of diffusion and solidification. The results are compared with experimental characterization at the interface for validation. This hierarchal analysis aims to gain a fundamental understanding of the intrinsic metallurgical bond formation mechanism between Mg/Fe, which will help to accelerate the development cycle of novel welding techniques for dissimilar metals.

This paper is organized as follows: the VFAW sample between AZ31 magnesium alloy and DP590 high strength steel is first examined using SEM/HRTEM in section 2. Eulerian-FE based process simulation is then conducted and the results are analyzed to extract the local condition at the joint interface in section 3. Informed by the FE results, the MD study of the Mg/Fe interface during impact and cooling is performed and the predictions are corroborated with experiments in section 4. The results are further discussed in section 5 and the paper is concluded in section 6.

Section snippets

Impact welding experiment and interface characterizations

The 1.8 mm thick AZ31 sheet and the 0.8 mm thick uncoated DP590 is joined with vaporizing foil actuator welding (VFAW) method [28] at the Impulse Manufacturing Laboratory in the Ohio State University. Fig. 1 illustrates the VFAW process. In the current work, the DP590 steel sheet is pre-deformed into a concave shape in order to create a standoff gap from the magnesium flyer sheet. The depth (1.8 mm) of the concavity is designed so that the flyer sheet can get to a high enough velocity before

Thermal-mechanical simulation of the impact welding process

Despite the very low solubility and diffusion coefficient, the characterization results of VFAW samples showed significant Fe from the DP590 steel substrate is mixed into Mg in the form of particles, leading to a unique “interlayer” at the AZ31-DP590 interface. This is not only observed at the corner (see region A of Fig. 2) of the weld sample, but also along the interface for a few millimeters. To understand the underlying mechanism behind the interlayer formation, a two-scale hierarchical

Atomic simulation of interlayer formation at impact interface

With the FE model predicted local impact velocity and temperature interpolated as the initial and boundary conditions, the formation of local Mg/Fe interface nanostructure is studied using molecular dynamics (MD) simulations. The MD simulations are performed with the LAMMPS package [38] and modified embedded-atom method (MEAM) interatomic potential [39] to describe the atom interactions. It only considers Fe-Mg bi-material system for simplicity, which is consistent with the experimental

Discussion

The Eulerian-FE and MD simulation results are used to corroborate the HRTEM observations of the Mg/Fe impact weld interface. During the lateral propagation of the oblique impact, the collision front exhibits a sharp increase in the shear velocity and temperature. When the collision propagates through the bonded zone, the very high shear velocity (see Fig. 11(a)) causes a mechanical mixing of the Fe atoms from the substrate into the high-temperature Mg flow. During the cooling stage, Fe atoms

Conclusion

This paper explores the underlying mechanisms of impact welding between magnesium and uncoated steel, the pair which is conventionally believed to be un-weldable owing to their low mutual solubility, lack of intermetallic phases and disparities in physical properties. The vaporizing foil actuator welding experiment demonstrated that the high-velocity impact can form an Mg-based interlayer within the bonded zone which has an intact interface with the uncoated steel substrate. This interlayer

Acknowledgements

This research was sponsored by the US Department of Energy, Office of Vehicle Technology, under a prime contract with Oak Ridge National Laboratory (ORNL). ORNL is managed by UT-Battelle, LLC for the U.S. Department of Energy under Contract DE-AC05 00OR22725. This work was funded by the DOE Vehicle Technologies Office under the Automotive Lightweight Materials Program managed by Ms. Sarah Kleinbaum. Computing support by The Compute and Data Environment for Science (CADES) at Oak Ridge National

References (40)

  • L. Liu et al.

    Mater. Des.

    (2016)
  • L. Liu et al.

    Scr. Mater.

    (2011)
  • G.H.S.F.L. Carvalho et al.

    Mater. Charact.

    (2018)
  • Y.C. Chan et al.

    Mater. Sci. Eng. B

    (1998)
  • Z.K. Zhang et al.

    Trans. Nonferrous Met. Soc. China

    (2014)
  • L. Li et al.

    J. Mater. Process. Technol.

    (2013)
  • L. Liu et al.

    Mater. Lett.

    (2010)
  • D. Ren et al.

    Mater. Des.

    (2014)
  • F. Findik

    Mater. Des.

    (2011)
  • V. Gupta et al.

    J. Mater. Process. Technol.

    (2019)
  • A. Vivek et al.

    J. Mater. Process. Technol.

    (2013)
  • J. Chen et al.

    Mater. Lett.

    (2017)
  • S. Plimpton

    J. Comput. Phys.

    (1995)
  • H. Wang et al.

    Metals

    (2019)
  • G. Song et al.

    Materials

    (2018)
  • Y. Zhao et al.

    Materials

    (2019)
  • D. Dietrich et al.

    J. Mater. Sci.

    (2011)
  • S. Chen et al.

    Metall. Mater. Trans. A

    (2013)
  • C. Schneider et al.

    Sci. Technol. Weld. Join.

    (2011)
  • S. Jana et al.

    Metall. Mater. Trans. A

    (2010)
  • Cited by (0)

    View full text