Metallurgical Assessment of Novel Mg-Sn-La Alloys Produced by High-Pressure Die Casting

This introduction paper is based on the paper "Metallurgical Assessment of Novel Mg-Sn-La Alloys Produced by High-Pressure Die Casting" published by "The Korean Institute of Metals and Materials".

1. Overview:

• Title: Metallurgical Assessment of Novel Mg-Sn-La Alloys Produced by High-Pressure Die Casting
• Author: Azim Gökçe
• Year of publication: 2019
• Journal/academic society of publication: Metals and Materials International
• Keywords: Magnesium · Microscopy · High-pressure die casting · Light alloys · Mechanical properties

2. Abstract:

Mg alloys containing Al are widely used for industrial applications, but the use of these alloys as an automotive part is limited due to the low melting temperature of the Mg₁₇Al₁₂ intermetallic phase. Therefore, magnesium alloys without aluminum that can withstand higher operating temperatures are of interest to the automotive industry. The objective of this work is to develop Al-free Mg alloys for industrial applications. In the current work, four types of alloys were produced with varying La contents. The high-pressure die casting method was selected to overcome the problems inherent in the gravity casting method with respect to the production of parts with complex shapes and thin walls. X-ray diffraction analysis revealed that the base alloy (Mg-5Sn wt%) comprises of α-Mg and Mg₂Sn phases whereas La containing alloys included intermetallic phases such as LaMg₃, Mg₁₇La₂, and La₅Sn₃. Corresponding grain sizes of the alloys with La are lower than those of the Mg5Sn alloy. Due to this lower grain size and emerging dispersoids, the tensile strength of the Mg5Sn4La alloy (205 MPa) is roughly double that of Mg5Sn. Moreover, the addition of the 4% wt. La to the Mg5Sn alloys led to an increase in yield strength and ductility by 25% and 50%, respectively.

3. Introduction:

The automotive industry is increasingly demanding thin-wall structures and complex shapes, leading to the adoption of alternative manufacturing techniques like High-Pressure Die Casting (HPDC) to overcome the limitations of sand casting. HPDC offers advantages such as higher production speed and lower cost and is highly adaptable to automation and mass production.

Aluminum, magnesium, and titanium alloys, known as "light-alloys", are crucial in various industries. Magnesium stands out due to its low density, high specific strength, good castability, and weldability, making it suitable for aerospace, transportation, defense, electronics, and biomedical applications. Regulations aimed at reducing greenhouse gas emissions drive the need for lighter vehicles, and magnesium, being significantly lighter than steel and aluminum, is a promising material for vehicle weight reduction.

However, pure magnesium's low tensile strength and elongation limit its use in automotive manufacturing. While aluminum and zinc are common alloying elements to enhance magnesium's strength, Mg-Al alloys like AZ91 and AM60, which constitute a majority of magnesium alloys in structural applications, suffer from the low melting temperature of the Mg₁₇Al₁₂ intermetallic phase. This intermetallic phase and the incompatibility of lattice systems between Mg and Mg₁₇Al₁₂ can cause fragility and reduce ductility. Alloys with lower Al content are desired for improved ductility, prompting the search for Al-free alternatives.

Tin (Sn) is considered as a substitute for Aluminum in Mg alloys. Mg₂Sn, the intermetallic phase formed in Mg-Sn alloys, has a higher melting point compared to Mg₁₇Al₁₂, suggesting better high-temperature performance. Mg-Sn alloys also exhibit a narrower solidification range and lower tendency for casting defects compared to Mg-Al alloys. Furthermore, the high solid solubility of Sn in Mg at elevated temperatures allows for strengthening through precipitation hardening.

Rare Earth (RE) elements are known to positively influence Mg alloy properties. Studies have shown that additions of elements like Gadolinium (Gd), Lanthanum (La), and Neodymium (Nd) can improve tensile strength and creep performance. Mg-Sn-RE ternary alloys, particularly with Yttrium (Y) and Cerium (Ce), have also attracted attention for their enhanced mechanical properties through grain refinement and precipitation strengthening. However, there is a lack of research on HPDC processed Mg-Sn-La ternary alloys.

Therefore, this study aims to develop novel HPDC Mg-Sn alloys without aluminum and to assess the metallurgical effects of Lanthanum (La) addition on the properties of these alloys.

4. Summary of the study:

Background of the research topic:

Magnesium alloys containing aluminum (Mg-Al) are widely used, but their application in automotive parts is limited by the low melting temperature of the Mg₁₇Al₁₂ intermetallic phase, which reduces high-temperature performance and ductility. There is a need for aluminum-free magnesium alloys with improved high-temperature properties for automotive applications.

Status of previous research:

Previous research has explored the use of tin (Sn) as an alternative alloying element to aluminum in magnesium alloys (Mg-Sn). Rare earth (RE) elements, including Lanthanum (La), have shown positive effects on the properties of magnesium alloys. Studies on Mg-Sn-RE systems, such as Mg-Sn-Y and Mg-Sn-Ce, have demonstrated enhanced mechanical properties. However, research on high-pressure die casting (HPDC) processed Mg-Sn-La ternary alloys is lacking.

Purpose of the study:

The purpose of this study is to develop novel aluminum-free Mg-Sn alloys with lanthanum (La) additions and to investigate the metallurgical effects of La content on the properties of these alloys produced by high-pressure die casting (HPDC).

Core study:

This study focuses on producing and metallurgically assessing four types of Mg-Sn alloys with varying Lanthanum (La) contents using high-pressure die casting (HPDC). The research investigates the microstructure, phase composition, grain size, and mechanical properties (tensile strength, yield strength, ductility, and hardness) of these novel Mg-Sn-La alloys.

5. Research Methodology

Research Design:

This research employs an experimental design involving the production of four Mg-Sn alloys with varying La contents (Mg5Sn, Mg5Sn1La, Mg5Sn2La, Mg5Sn4La) using high-pressure die casting (HPDC). The study then systematically investigates the metallurgical and mechanical properties of these alloys.

Data Collection and Analysis Methods:

• Material Preparation: Alloys were prepared using an induction furnace and high-pressure die casting.
• Microstructural Characterization: Optical Microscopy, Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD) were used to analyze the microstructure and phase composition. Energy Dispersive Spectroscopy (EDS) was employed for elemental analysis. Grain sizes were measured using Clemex Image Analyzing software.
• Mechanical Property Evaluation: Tensile tests were conducted according to ASTM E8 M standards using an extensometer to measure elongation. Brinell hardness tests were performed to determine hardness values.

Research Topics and Scope:

The research focuses on the following topics:

• Production of novel Mg-Sn-La alloys via HPDC.
• Investigation of the effect of La content on the microstructure and phase composition of Mg-Sn alloys.
• Evaluation of the grain refining effect of La addition.
• Assessment of the mechanical properties (tensile strength, yield strength, ductility, hardness) of Mg-Sn-La alloys.
• Identification of the intermetallic phases formed in Mg-Sn-La alloys.

6. Key Results:

Key Results:

• Grain Refinement: Lanthanum (La) acts as a grain refiner in Mg-Sn alloys, leading to decreased grain size with increasing La content (Fig. 2). The alloy with 4% La (Mg5Sn4La) exhibited the most refined grain structure and the most homogeneous grain size distribution.
• Phase Composition:
  • The base alloy (Mg5Sn) consists of α-Mg and Mg₂Sn phases.
  • La addition leads to the formation of intermetallic phases such as LaMg₃, Mg₁₇La₂, and La₅Sn₃ in La-containing alloys (Fig. 6). The amount of Mg₂Sn phase decreases with increased La content.
• Mechanical Properties Enhancement:
  • Tensile strength, yield strength, and ductility are improved with increasing La content (Fig. 8).
  • The tensile strength of Mg5Sn4La alloy (205 MPa) is approximately double that of Mg5Sn.
  • The addition of 4% La increases yield strength and ductility by 25% and 50%, respectively.
  • Hardness is also increased with La addition, with Mg5Sn4La showing a 30% higher hardness than Mg5Sn.
• Comparison with AZ91 Alloy: The tensile strength of Mg5Sn4La alloy is comparable to that of AZ91 alloy, but Mg5Sn4La exhibits a significantly higher strain rate (300% higher) and improved ductility due to the absence of the brittle Mg₁₇Al₁₂ phase.

Figure Name List:

• Fig. 1 High-pressure die-cast specimens
• Fig. 2 Optical micrographs and grain size values of the alloys as a function of La content. Error bars show the minimum and maximum grain sizes measured
• Fig. 3 SEM images of the investigated alloys. a Mg5Sn, b Mg5Sn1La, c Mg5Sn2La, d Mg5Sn4La
• Fig. 4 SEM image of the Mg5Sn alloy
• Fig. 5 SEM image of a Mg5Sn1La, b Mg5Sn2La alloys
• Fig. 6 XRD patterns of the investigated alloys
• Fig. 7 SEM image of the Mg2Sn4La alloy, and EDS elemental mapping analysis of the area represented with a green rectangle
• Fig. 8 Mechanical properties of the investigated alloys, a yield strength and tensile strength, b elongation and hardness, c stress-strain curves

7. Conclusion:

This research successfully produced novel HPDC Mg-Sn-La alloys and investigated the effects of La addition. The key findings are:

• HPDC production of Mg-Sn-La alloys was successful without macro defects.
• La addition refines grain size and improves grain size distribution due to the formation of La-containing intermetallic phases.
• Mg5Sn alloys consist of α-Mg and Mg₂Sn phases, while La addition leads to the formation of LaMg₃, Mg₁₇La₂, and La₅Sn₃ phases.
• Emerging phases resulting from La addition enhance tensile strength, ductility, and hardness through grain size reduction and dispersion strengthening. The tensile strength of Mg5Sn4La doubled compared to Mg5Sn, and yield strength and ductility also significantly increased.
• Mg5Sn4La alloy exhibits comparable tensile strength to AZ91 alloy but with a 300% higher strain rate and improved ductility due to the absence of the brittle Mg₁₇Al₁₂ phase.

The investigated Mg-Sn-La alloys are promising alternatives to industrial Mg alloys, demonstrating good feasibility for production using the HPDC process.

8. References:

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9. Copyright:

• This material is a paper by "Azim Gökçe". Based on "Metallurgical Assessment of Novel Mg-Sn-La Alloys Produced by High-Pressure Die Casting".
• Source of the paper: https://doi.org/10.1007/s12540-019-00539-1

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