Effects of heat treatments on laser welded Mg-rare earth alloy NZ30K

doi:10.1016/j.msea.2011.09.052

Materials Science and Engineering: A

Volume 529, 25 November 2011, Pages 401-405

https://doi.org/10.1016/j.msea.2011.09.052 Get rights and content

Abstract

In this study, the effects of heat treatments on the quality of laser welded Mg-rare earth alloy NZ30K were systematically studied. The microstructure and mechanical properties of joints, welded by a 15 kW high power CO₂ laser, under different heat treatments had been tested and analyzed. The results indicated that the heat treatment plays an important role in the mechanical strength of laser welded joint of NZ30K. The microstructure of samples after the solution treatment as well as aging treatment is different from that of the as-received welded joint. For solution treatment, although the microstructure is much different from that of as-received welded joint, the solution strengthening effect is not obvious. There are lots of precipitates in the fusion zone after the aging treatment, which will significantly enhance the ultimate tensile strength (UTS) and the yield tensile strength (YTS) of the welding joint. 79% of YTS is caused by precipitation strengthening. Therefore, the results implied that the UTS and YTS can be greatly improved by proper heat treatment.

Highlights

► Firstly find the tadpole-shape precipitates in the welding joint. ► The precipitation strengthening can account for 79% of the total strength. ► The results can provide some insights on the application of Mg-RE alloy.

Introduction

Energy waste and environmental pollution have become major factors that restricting social development in recent years. Magnesium alloy is applied widely in automotive, electronics, aerospace and other fields because of its low pollution to the environment and energy-saving property [1], [2]. As special properties of rare earth, many new alloys incorporating with rare earth elements have been developed. NZ30K is alloyed by the rare earth element neodymium (Nd), which owns good properties that can be used as components in transportation system, hub of car wheels, and door parts [3], [4], [5], [6].

Proper heat treatment can improve the properties of magnesium alloy significantly. Up to now, many scientific researches are focusing on the welding processing of magnesium alloys [7], [8], [9], [10], while few has reported the effects of heat treatments on laser welded joints of Mg-rare earth alloy.

In the present work, the effects of heat treatments on laser welded joints of the Mg-rare earth alloy NZ30K have been investigated. The relationship between the precipitations and mechanical properties were discussed.

Section snippets

Experimental

Test plates are the hot-rolled Mg-rare earth alloy NZ30K without sequent heat treatment, which is alloyed by the rare earth element Nd, element zinc (Zn) and zirconium (Zr). The chemical compositions of the alloy are listed in Table 1. The dimension of the test plates was 150 mm × 75 mm × 10 mm.

A CO₂ laser system (Trumpf TLF15000T laser) with a maximum output power of 15 kW was used. The diameter of the laser beam focus is 0.8 mm. The pure helium with the flow rate of 25 L/min was used as front side

Optical microstructure of fusion zone

Fig. 1 shows the microstructure of fusion zone under three different heat treatments. Sample 1, sample 2 and sample 3 are exhibited in Fig. 1a, b and c, respectively. It can be seen that the eutectic compounds have almost dissolved into the matrix and the grain boundaries are very clear after T4 treatment. The optical microstructures after T4 and T6 treatment show no obvious differences. Tang et al. [11] reported the kinetics equation of the grain growth during the heat treatment. $\lg [\frac{D^{n} - D_{0}^{n}}{t}] = \lg K_{0}$

Discussion

The most effective elements to improve the strength properties of magnesium alloys are the rare earth metals (RE). The proper heat treatments of the laser welded joint of NZ30K are high temperature solution treatment and then low temperature aging treatment. Here the effects of heat treatments on the mechanical strength of laser welded joint are discussed.

For magnesium alloy, the YTS is mainly depended on five factors: the strength of the matrix (σ_Mg), the strength of secondary phase (eutectic

Conclusions

From the above investigation, the main conclusions can be summarized as follow:

(1)
Heat treatment is an important factor that can affect the microstructure and mechanics properties of laser welded Mg-rare earth alloy NZ30K significantly. After solution treatment, the eutectic compounds located along the grain boundaries dissolve into the matrix.
(2)
After aging treatment, a large amount of precipitates present in the welded joint, which are rod-shape, tadpole-shape and flake-shape. The UTS of the

Acknowledgement

The authors wish to thank Mr. F.H. Wang from National Engineering Research Center of Light Alloys Net Forming for providing the experimental materials.

References (11)

B.L. Mordike et al.
Mater. Sci. Eng. A
(2001)
X.W. Zheng et al.
Mater. Des.
(2010)
J.F. Nie
Scripta Mater.
(2003)
X. Cao et al.
J. Mater. Process. Technol.
(2006)
W.M. Tang et al.
Intermetallics
(2007)

There are more references available in the full text version of this article.

Cited by (17)

Microstructure evolution and strengthening mechanism analysis of novel Mg-RE-Ag alloy during heat treatment
2022, Journal of Materials Research and Technology
Precipitation strengthening is crucial for Mg-RE alloys to achieve excellent mechanical properties. Herein, the high-strength Mg-6Gd-2Y-1.5Ag-1Nd-0.4Zn-0.5Zr alloy was designed and developed by building multi-orientated nano-precipitates. The corresponding microstructure and strengthening mechanism were comprehensively investigated utilizing XRD, OM, SEM, TEM, Brinell hardness and tensile tests under various heat treatment conditions. The results indicated that the as-cast alloy consisted of the primary α-Mg matrix and Mg₅(Gd,Y,Nd,Ag,Zn)-type eutectic phase at the grain boundaries. With the increase of solid solution temperature and time, the grain size of the alloy gradually increased. After solution treatment at 510 °C for 10 h, solute atoms from the eutectic phase fully diffused into the matrix, and the supersaturated solid solution was obtained. The solution-treated alloy was subsequently aged, and multiple types of nanoscale precipitates formed in the matrix. The precipitation evolution can be divided into three distinct stages. At the early aging stage, the fine solute clusters were the primary precipitates. And then a mass of basal γ'' and prismatic β′ phases occurred uniformly in the matrix at the peak-aged stage. After reaching the over-aged stage, the primary precipitates further transformed into coarse γ and β phases. The alloy achieved outstanding mechanical properties at both room and high temperatures after the peak-aged treatment, which was mainly related to the synergistic strengthening of the multi-orientated β′ and γ'' nano-precipitates. The peak-aged alloy's ultimate tensile strength, yield strength, and elongation were 370 MPa, 335 MPa, 2.8% at room temperature, and 258 MPa, 215 MPa, 11.8% at 300 °C, respectively.
Research on the post-weld heat treatment of TIG repair welded joint of sand-cast Mg-Y-RE-Zr alloy
2021, Materials Science and Engineering: A
Citation Excerpt :
It is of great importance to promoting the microstructure through PWHT to improve the mechanical properties of the joint. Dai et al. found that the mechanical properties of laser-welded NZ30K Mg-RE alloy were significantly improved by PWHT and 79% of the yield strength of the joint was attributed to precipitation strengthening derived from T6 treatment [26,27]. Similarly, the T6 treatment (520 °C × 8 h + 225 °C × 12 h) is the most efficient way to improve the tensile properties of Mg-Y-RE-Zr cast alloy [28,29].
The heat affected zone (HAZ) usually leads to the softening of the tungsten inert gas (TIG) welded joint due to the formation of network-shaped eutectics under its high heat input. The current study systematically discussed the microstructural evolution of TIG repair welded joints of the sand-cast Mg-Y-RE-Zr alloy during various progressive solution (T4) treatments. Abnormal grain coarsening (AGC) happened after the conventional T4 treatment at 520 °C for 8 h because of the high welding residual stress in the fusion zone (FZ), leading to extremely low elongation of 1.8% of the repaired joint. To avoid AGC, a two-step progressive T4 treatment was proposed without changing the total T4 treatment time. Neither abnormally coarsened grains nor residual eutectics existed by the progressive T4 treatment of 480 °C × 6 h + 520 °C × 2 h. Excellent comprehensive tensile properties were obtained (YS: 241 MPa, UTS: 323 MPa, and EL: 4.0%) after the aging (T6) treatment. DSC and TEM analyses confirmed that the proposed progressive T4 treatment with a lower treatment time at 520 °C can effectively promote the dissolution of the eutectics. The aging hardening was attributed to the precipitation of the evenly dispersed β′ phase in the matrix.
Effect of solute atom concentration and precipitates on serrated flow in Mg-3Nd-Zn alloy
2018, Journal of Materials Science and Technology
Citation Excerpt :
Magnesium (Mg) alloys containing rare-earth (RE) elements have been widely used in aerospace and aircraft applications due to their low density and high specific strength [1,2]. As a typical heat-resistant Mg-RE alloy, Mg-3Nd-1Zn (wt%) alloy, denoted as NZ31, shows high strength and low cost [3–5]. In applications, NZ31 Mg alloy is usually used in T6 condition, with a standard process of solution treatment at 525 °C and then aging treatment at 200 °C.
Influence of solute atom concentration and precipitates on serrated flow, i.e., Portevin-Le Chatelier effect, was studied in Mg-3Nd-Zn alloy by tensile test at 250 °C with a strain rate of 1 × 10⁻³ s⁻¹. Microstructure and tensile property of the Mg-3Nd-Zn alloy in solution and aging conditions were also investigated. Results indicate that the serrated flow was weakened with aging time, and geometry of the serrations changed from sharp to rounded corner. Through analyzing the mechanism of the interactions between dislocations and solute atoms, it was identified that the precipitates did not only weaken the serrated flow due to the decrease in the concentration of solute atom, but also regulate the serration type by restraining the movement of dislocations during high temperature deformation.
Microstructure and mechanical properties of the laser-welded Mg-3Nd-0.2Zn-0.4Zr (NZ30K) magnesium alloy
2017, Optics and Laser Technology
Citation Excerpt :
They showed that the tensile and yield strength of welded joints were slightly lower than those of base metal (BM). The rare-earth magnesium alloy NZ30K has good performance in terms of high-temperature mechanical properties and corrosion resistance, benefiting from the addition of rare-earth element Nd [22,23]. Rare-earth element Nd exists in the rare-earth magnesium alloy in the form of key solid-solution and precipitation-strengthening element, which provides the rare-earth magnesium alloy better high-temperature tensile and creep properties.
This study aimed to investigate the microstructure and mechanical properties of the laser-welded Mg-3Nd-0.2Zn-0.4Zr (NZ30K) magnesium alloy. A fiber optic laser was used to weld 4-mm-thick Mg-3Nd-0.2Zn-0.4Zr (NZ30K) and AZ31 magnesium alloy plates. The microstructures of NZ30K and AZ31 joints were compared, and the distribution of Nd in NZ30K was emphatically investigated. Besides, microhardness distributions and tensile properties of NZ30K and AZ31 joints at room temperature and high temperature (200 °C) were tested, and fracture features were observed and analyzed. The results indicated that the NZ30K rare-earth magnesium alloy showed better properties especially at a high temperature (200 °C), benefiting from the strengthening phase, compared with the AZ31 joint, such as high-temperature tensile strength, creep resistance at high temperature, and corrosion resistance. The microhardness in the fusion zone of NZ30K changed a little compared with that in base metal, while the microhardness in the fusion zone of AZ31 decreased significantly. The grain of NZ30K in the fusion zone was significantly refined with strengthening-phase distribution on the grain boundary. The tensile strength of NZ30K laser beam–welded joints at a high temperature (200 °C) decreased by 29% compared with that at room temperature, while the tensile strength of AZ31 joint at a high temperature (200 °C) decreased by 53% compared with that at room temperature, which was mainly because of the stable existence of the strengthening-phase Mg₁₂Nd in the NZ30K rare-earth magnesium alloy while the strengthening-phase Mg₁₇Al₁₂ in AZ31 would soften and coarsen with the rise in temperature.
Comprehensive study of phase transformation in age-hardening of Mg-3Nd-0.2Zn by means of scanning transmission electron microscopy
2015, Acta Materialia
Citation Excerpt :
%) and low solubility at room temperature (∼100 ppm), which make it suitable for potential age hardening [8]. The strength of age-hardened Mg–Nd alloys can be further enhanced through solid solution strengthening by controlled addition of a third element such as zinc (Zn) [9,10–14]. Researches have shown that addition of 0.2 wt.
Effects of thermal history on precipitation behavior and precipitation kinetics in Mg–3Nd–0.2Zn (wt.%) alloy were studied. Microstructure analysis of the aged alloys revealed the presence of new phases in the microstructure in addition to the old description: Super saturated solid solution (SSSS) → G. P. zones → β″ → β′/β₁ → β, reported for Mg–Nd system. In this regard, the sequence of precipitation for the studied alloy was identified as: Super saturated solid solution (SSSS) → Clusters of atoms → G. P. zones (I, II, III) → β′ → β₂ → β₁/γ′ → β. The formation of clusters of solute atoms in this system was confirmed by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and atom probe tomography analysis. In addition, new metastable phases designated as G. P. zones (I, II, III) and β₂ were identified in the microstructure of the aged alloys by means of high resolution HAADF-STEM. The β₂ phase was found to be a transition phase in transformation of β′ to β₁. Furthermore, the microstructure analysis revealed that the structure of β₁ phase is indeed a body centered tetragonal structure in spite of face centered cubic structure that has been reported for this phase in the literature.
Precipitation evolution and kinetics in a magnesium-neodymium-zinc alloy
2014, Journal of Alloys and Compounds
Citation Excerpt :
This effect is suitable for texture randomization and age hardening of alloys containing Nd [9–11]. The strengthening of Mg–Nd alloys can be enhanced by controlled addition of third element such as zinc (Zn) [12–15]. It has been reported that smalladdition of Zn to Mg–Nd binary system improves the mechanical properties through solid solution strengthening in the matrix [8,16–18].
In this research, the precipitation sequence investigation and phase identification in Mg–3Nd–0.2Zn–0.46Zr (wt.%) system were performed using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) techniques. The results showed the precipitation sequence to be: Super saturated solid solution (SSSS) → Clustering of atoms/short range ordering → β″ → β′ → β which shows that low zinc content (less than 0.5 %wt) does not influence the precipitation sequence. TEM studies revealed that in addition to prismatic planes, β′′ precipitates lie on pyramidal planes in Mg matrix, which has not been reported previously in this system. Further investigation using TEM and aging experiments showed that these precipitates are responsible for the age hardening of the alloy. Furthermore, the kinetic studies showed that quenched-in vacancies play an important role in the early stage of precipitation and formation of β′′ while formation of β′ is dominantly diffusion controlled process and quenched-in vacancies are no longer effective.

View all citing articles on Scopus

View full text

Effects of heat treatments on laser welded Mg-rare earth alloy NZ30K

Abstract

Highlights

Introduction

Section snippets

Experimental

Optical microstructure of fusion zone

Discussion

Conclusions

Acknowledgement

Mater. Sci. Eng. A

Mater. Des.

Scripta Mater.

J. Mater. Process. Technol.

Intermetallics