Friction Stir Processing: A Thermomechanical Processing Tool for High Pressure Die Cast Al-Alloys for Vehicle Light-weighting

This introduction paper is based on the paper "Friction Stir Processing: A Thermomechanical Processing Tool for High Pressure Die Cast Al-Alloys for Vehicle Light-weighting" published by "Manufacturing Letters".

1. Overview:

• Title: Friction Stir Processing: A Thermomechanical Processing Tool for High Pressure Die Cast Al-Alloys for Vehicle Light-weighting
• Author: Avik Samanta, Hrishikesh Das, Glenn J. Grant, Saumyadeep Jana
• Year of publication: 2024
• Journal/academic society of publication: Manufacturing Letters
• Keywords: Friction stir processing; High pressure die casting; Tensile properties; Fatigue; Tear toughness

2. Abstract:

This study uses friction stir processing (FSP) for thermomechanical processing of high-pressure die-casting (HPDC) to modify microstructure and improve mechanical properties. FSP is carried out on two different HPDC aluminum alloys: (a) general-purpose, high-iron, HPDC A380 alloy and (b) premium quality, low-iron HPDC Aural-5 alloy in thin wall, flat plate geometry. Subsequent mechanical testing shows ~30% and ~65% enhancement in yield strength and tensile ductility. In addition, FSP leads to ~10 times improvement in fatigue life for A380 alloy and ~70% improvement in fracture toughness for Aural-5 alloy. These findings emphasize the capability of FSP to modify the microstructure of HPDC Al-alloys-based structural components so that they can demonstrate a good combination of strength, ductility, fracture toughness, and high fatigue properties for long-term durability and reliability.

3. Introduction:

The increasing demand for zero carbon emissions drives automotive original equipment manufacturers (OEMs) to seek cost-effective solutions for light-duty (LD) vehicle weight reduction. Replacing heavy structural steel assemblies with lightweight aluminum structural castings is a key strategy. High pressure die-cast (HPDC) aluminum castings offer advantages in design, weight reduction, and quality for critical load-bearing body structures in automotive designs. These include shock towers, pillars, and floor rails. HPDC Al castings also enable intricate structural profiles and effective part unitizing. As electric vehicle technology advances, HPDC aluminum castings become crucial for lightweight enclosures for batteries and powertrain components.

A significant challenge in the Al die-casting industry is balancing die-life extension and ductility of the final casting. Conventional approaches often involve modifying material chemistry. For general-purpose die-casting, iron (Fe) and manganese (Mn) are used to reduce die wear, although high Fe content can reduce ductility and fatigue life. Silicon (Si) is added to improve fluidity, feeding rate, and hot tear resistance. Copper (Cu), zinc (Zn), and magnesium (Mg) contribute to strengthening HPDC alloys. However, HPDC Al alloys often exhibit complex intermetallic compounds, such as needle-shaped β-FeSiAl (FeSiAl5) and polyhedral α-FeSiAl (Al15(MnFe)3Si2), which can detrimentally affect mechanical characteristics, especially ductility. Acicular silicon and second-phase particulates, inherent dendritic microstructure, and gas/shrinkage porosity in HPDC Al alloys further limit their use in structural components.

4. Summary of the study:

Background of the research topic:

HPDC aluminum castings, despite their advantages, suffer from inherent microstructural defects and material chemistry limitations that compromise mechanical properties like ductility, fatigue life, and fracture toughness. These limitations hinder their broader application in vehicle structural components, particularly in the context of increasing demands for lightweighting and durability in the automotive industry.

Status of previous research:

Previous research efforts have focused on modifying alloy chemistry and HPDC processes to mitigate detrimental features. Vacuum-assisted HPDC reduces gas and shrinkage porosity. Premium low-Fe HPDC Al-alloys, such as Silafont, Castasil, and Aural, have been developed to improve ductility by eliminating detrimental beta phases. Strontium (Sr) addition is used to modify acicular silicon into a finer, fibrous structure, enhancing strength and ductility. Sr-modified alloys, like Aural-5, have seen successful structural applications. However, challenges remain, including shrinkage porosity, dendritic microstructure, shear-band formation, externally solidified crystals (ESCs), and second-phase particulates.

Purpose of the study:

The study aims to investigate friction stir processing (FSP) as a thermomechanical post-processing tool to modify the microstructure of thin-wall HPDC Al-alloys and enhance their mechanical properties. This approach offers an alternative to costly alloy composition and HPDC process optimization by directly addressing microstructural defects and limitations responsible for reduced mechanical performance.

Core study:

The core study involves applying FSP to two different HPDC aluminum alloys: (i) high-iron A380 and (ii) low-iron Aural-5, both in thin-wall, flat plate geometries. The research assesses the impact of FSP on microstructure evolution and mechanical properties, including tensile properties, fatigue life, and tear toughness. Mechanical performance is evaluated through coupon-level testing to demonstrate the effectiveness of FSP in improving the overall performance of HPDC Al-alloys for structural applications.

5. Research Methodology

Research Design:

The research employs an experimental design involving FSP of HPDC A380 and Aural-5 alloy plates. The study compares the microstructure and mechanical properties of processed (FSPed) and unprocessed (HPDC) materials. Two HPDC Al-alloys were selected: (i) high-Fe containing A380 and (ii) low-Fe containing Aural-5. FSP trials were conducted on flat plate geometries (3.5 mm thick A380 and 2.5 mm thick Aural-5).

Data Collection and Analysis Methods:

• Microstructure Characterization: Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) were used to examine the microstructure. ImageJ software was used for quantitative analysis of microstructural features, including equivalent circular diameter (ECD) and aspect ratio of phases. SEM-EDS analysis was used for elemental mapping.
• Tensile Testing: Full-thickness subsize ASTM E8 specimens and miniature tensile specimens were used to evaluate tensile properties (Yield Strength (YS), Ultimate Tensile Strength (UTS), and % Elongation).
• Fatigue Testing: High cycle bending fatigue tests were performed on full-thickness rectangular specimens under a 4-point bending configuration at a stress ratio of R=0.1.
• Tear Toughness Testing: Conventional tear tests were conducted according to ASTM standard B871-01 using rectangular specimens with a V-notch to assess tear toughness. Numerical integration was used to calculate unit total energy from force-displacement curves.

Research Topics and Scope:

The research focuses on:

• Investigating the effect of FSP on microstructure modification in HPDC A380 and Aural-5 alloys.
• Evaluating the improvement in tensile properties (yield strength, tensile ductility) after FSP.
• Assessing the enhancement in fatigue life of HPDC A380 alloy due to FSP.
• Determining the impact of FSP on tear toughness of HPDC Aural-5 alloy.
• Comparing the mechanical performance of FSPed material to that of as-cast HPDC material in different regions (die-wall vs. mid-wall).

6. Key Results:

Key Results:

• Microstructure Evolution: FSP effectively eliminated casting porosity and refined the dendritic microstructure in both A380 and Aural-5 alloys, transforming it into a wrought microstructure with uniformly distributed fragmented silicon and second-phase particles. The size and aspect ratio of silicon particles and second-phase particles were significantly reduced after FSP.
• Tensile Property Improvement: FSP led to significant enhancements in tensile properties. Yield strength (YS) improved by ~25% for A380 and ~30% for Aural-5. Tensile ductility (% Elongation) increased by ~65% for A380 and ~35% for Aural-5. Ultimate Tensile Strength (UTS) remained largely unchanged.
• Fatigue Life Enhancement: FSP significantly improved the high cycle fatigue life of A380 alloy in bending mode, showing a ~5-15 times improvement at higher stress levels and more than 15-fold improvement at lower stress levels compared to HPDC A380.
• Tear Toughness Improvement: FSP resulted in a ~70% enhancement in tear toughness for Aural-5 alloy, indicating increased resistance to crack initiation and propagation.

Fig. 2. Low-magnification microstructural overview of (a) HPDC A380; (b) FSPed A380; (c) HPDC Aural-5; (b) FSPed Aural-5.

Fig. 8. Tear toughness test for Aural-5: (a) Sample geometry; and Comparison of (b) the force–displacement curve and (c) unit total energy.

ig. 7. Fatigue life improvement of HPDC A380 alloy after FSP: (a) location of fatigue specimen with the nugget zone, and (b) comparison of no. of cycles to failure for different stress levels.

Fig. 6. Combination of yield strength and % elongation to failure for HPDC and FSPed (a) A380 alloy and (b) Aural-5 alloy.

ig. 5. Comparison of engineering stress vs. percent elongation of bulk tensile specimen.

Fig. 4. EDS Analysis of HPDC A380 alloy at (a) die-wall, (b) mid-wall, and (c) FSPed mid-wall, and HPDC Aural-5 alloy at (d) mid-wall and (e) FSPed mid-wall.

Fig. 3. Comparison of SEM microstructure of HPDC and FSPed material in locations (A) to (E) in Fig. 2.

Figure Name List:

• Fig. 1. Schematic of FSP experimental setting with tool shoulder and pin geometry and position of miniature and bulk E8 tensile specimens with respect to FSP nugget zone.
• Fig. 2. Low-magnification microstructural overview of (a) HPDC A380; (b) FSPed A380; (c) HPDC Aural-5; (b) FSPed Aural-5.
• Fig. 3. Comparison of SEM microstructure of HPDC and FSPed material in locations (A) to (E) in Figure 2.
• Fig. 4. EDS Analysis of HPDC A380 alloy at (a) die-wall, (b) mid-wall, and (c) FSPed mid-wall, and HPDC Aural-5 alloy at (d) mid-wall and (e) FSPed mid-wall.
• Fig. 5. Comparison of engineering stress vs. percent elongation of bulk tensile specimen.
• Fig. 6. Combination of yield strength and % elongation to failure for HPDC and FSPed (a) A380 alloy and (b) Aural-5 alloy
• Fig. 7. Fatigue life improvement of HPDC A380 alloy after FSP: (a) location of fatigue specimen with the nugget zone, and (b) comparison of no. of cycles to failure for different stress levels
• Fig. 8. Tear toughness test for Aural-5: (a) Sample geometry; and Comparison of (b) the force-displacement curve and (c) unit total energy.

7. Conclusion:

This study demonstrates the effectiveness of Friction Stir Processing (FSP) as a thermomechanical tool to enhance the mechanical properties of HPDC aluminum alloys, irrespective of alloy chemistry (high-Fe A380 and low-Fe Aural-5). FSP-driven microstructure modification led to improved tensile ductility (~65% for A380, ~35% for Aural-5) and yield strength (~25% for A380, ~30% for Aural-5). Significant improvements were also observed in high cycle fatigue life for A380 (~5 to >15 times) and tear toughness for Aural-5 (~70%). The enhanced mechanical properties are attributed to the elimination of casting defects like porosity and the refinement of the dendritic microstructure into a homogeneous wrought microstructure with fragmented and uniformly distributed phases.

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9. Copyright:

• This material is a paper by "Avik Samanta, Hrishikesh Das, Glenn J. Grant, Saumyadeep Jana". Based on "Friction Stir Processing: A Thermomechanical Processing Tool for High Pressure Die Cast Al-Alloys for Vehicle Light-weighting".
• Source of the paper: https://doi.org/10.1016/j.mfglett.2024.504512

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