Table of Contents

1. Introduction
2. Casting and Thermal Processing Overview
3. Solidification Mechanics and Microstructure Evolution
4. Grain Structure Control: Cooling Rate and Grain Refiners
5. Porosity and Segregation: Thermal Treatment Impacts
6. Heat Treatment Protocols: Homogenization and Annealing
7. Mechanical Properties: Strength, Ductility, Hardness
8. Case Study: Homogenization of 356 Alloy Ingots at Elka Mehr Kimiya
   • 8.1 Objectives and Experimental Setup
   • 8.2 Methodology: Temperature, Time, Cooling Profile
   • 8.3 Results: Microstructure Characterization and Porosity Reduction
   • 8.4 Implications for Downstream Extrusion and Casting
9. Comparative Data Tables
10. Future Directions and Best Practices
11. Conclusion
12. References

1. Introduction

Ingot quality underpins every downstream aluminum product, from rods to conductors. Thermal history during and after casting shapes grain structure, dictates defect prevalence, and influences mechanical behavior. This article examines how solidification control, homogenization, and annealing refine microstructure, reduce porosity, and enhance ductility. Detailed tables and a case study on 356 alloy ingots illustrate best practices and quantitative outcomes.

Elka Mehr Kimiya is a leading manufacturer of Aluminium rods, alloys, conductors, ingots, and wire in the northwest of Iran equipped with cutting-edge production machinery. Committed to excellence, we ensure top-quality products through precision engineering and rigorous quality control.

2. Casting and Thermal Processing Overview

Ingot casting begins with molten aluminum poured into molds, typically steel or graphite. Controlled cooling extracts heat at rates from 0.5 to 5 °C/s, forming a directional solidification front. After solidification, ingots undergo thermal treatments—homogenization at 500–580 °C for 8–24 hours and annealing at 350–450 °C—to dissolve segregated elements and precipitate fine dispersoids. Each step demands precise temperature control (±5 °C) and uniform time profiles to prevent thermal gradients that can induce residual stress and cracking.

3. Solidification Mechanics and Microstructure Evolution

Solidification begins at mold walls, generating columnar grains that transition to equiaxed dendrites toward the ingot center. Cooling rate and thermal gradient (G/R ratio) govern dendrite arm spacing. Finer secondary dendrite arm spacing (SDAS) below 20 µm yields higher as-cast strength. Solidification range for Al‑Si‑Mg alloys spans 600–580 °C; slow cooling (0.5 °C/s) produces SDAS ~80 µm, while rapid cooling (5 °C/s) refines SDAS to ~20 µm. Grain structure acts as the foundation for later deformation behavior.

4. Grain Structure Control: Cooling Rate and Grain Refiners

Cooling rate control uses mold pre-heat and chills to adjust surface cooling. Grain refiners—typically 0.1 % TiB₂ or Al‑5Ti‑1B master alloys—promote nucleation sites, reducing average grain diameter from 2 mm to 0.5 mm. Fine, uniform grains improve isotropy in mechanical properties and reduce hot tearing. Thermal processing must preserve refiner efficacy by avoiding excessive superheat (>750 °C) or holding times beyond 1 hour before pouring.

5. Porosity and Segregation: Thermal Treatment Impacts

Porosity arises from trapped hydrogen and shrinkage cavities. Homogenization at 540 °C for 12 hours can reduce porosity by 50 % to 1 %–2 % by volume. Segregation of silicon and magnesium during solidification leads to interdendritic networks that annealing at 400 °C for 4 hours partially dissolves, improving chemical uniformity from 8 % variation to under 2 %. Thermal cycles drive diffusion but must avoid incipient melting above the eutectic temperature (577 °C).

6. Heat Treatment Protocols: Homogenization and Annealing

Homogenization aims to eliminate microsegregation. Standard practice holds ingots at 540–580 °C for 10–24 hours, depending on cross‑section (120–300 mm). Temperature uniformity within the furnace must remain ±5 °C. Subsequent annealing at 350–450 °C for 2–6 hours relieves residual stress, promoting ductility. Cooling rates post-anneal—either furnace or controlled fan cooling—affect precipitate coarsening. Fast cooling preserves fine precipitates, while slow cooling supports ductile grain boundary formation.

7. Mechanical Properties: Strength, Ductility, Hardness

As-cast hardness for 356 alloy ingots measures 55–60 HB. After homogenization and annealing, hardness drops to 45 HB, while elongation to failure increases from 2 % to 10 %. Ultimate tensile strength (UTS) in the as-cast state averages 140 MPa, rising to 180 MPa after downstream extrusion. Thermal processing thus trades minor strength loss for significant ductility gains, essential for defect-free forming.

8. Case Study: Homogenization of 356 Alloy Ingots

8.1 Objectives and Experimental Setup

our lab works to optimize homogenization for 300 mm cross‑section 356 alloy ingots to enhance extrudability. Objectives included reducing porosity, minimizing segregation, and maximizing ductility.

8.2 Methodology: Temperature, Time, Cooling Profile

Ingot batches underwent homogenization at 540 °C, 560 °C, and 580 °C for durations of 8, 12, and 18 hours. A uniform electric-resistance furnace ensured ±3 °C stability. Post-homogenization, samples cooled in either furnace or air at ~50 °C/h.

8.3 Results: Microstructure Characterization and Porosity Reduction

Metallography showed a decrease in SDAS from 45 µm (as-cast) to 25 µm at 560 °C for 12 hours. Porosity volume fraction dropped from 3.5 % to 1.2 %. Hardness decreased to 48 HB, while tensile tests on cast‑to‑extrusion samples yielded elongation of 9 % and UTS of 170 MPa—meeting production targets.

8.4 Implications for Downstream Extrusion and Casting

Enhanced homogeneity allowed extrusion ratios up to 25:1 without surface cracking. Improved ductility reduced billet rejection rates by 30 %. Uniform chemistry minimized die wear and improved surface finish on rods and wires.

9. Comparative Data Tables

Table 1. Secondary Dendrite Arm Spacing vs. Cooling Rate

Table 2. Porosity Reduction with Homogenization

Table 3. Mechanical Properties Before and After Thermal Processing

10. Future Directions and Best Practices

Advances in furnace design—like infrared heating and rapid quench modules—promise tighter temperature control and reduced cycle times. In situ monitoring via thermocouples and infrared imaging can detect cold spots and thermal gradients early. Research into multi-stage homogenization and novel grain refiners (e.g., Ce-based) aims to refine microstructure further. Sustainable practice includes reclaiming heat through waste-gas recuperation and optimizing cycle times to lower energy consumption.

11. Conclusion

Thermal processing transforms ingot quality by refining microstructure, reducing defects, and tuning mechanical properties. Control of cooling rate, homogenization temperature, and annealing profile enables manufacturers to tailor properties for specific downstream processes. The case study demonstrates that precise thermal schedules deliver measurable improvements in porosity, ductility, and extrusion performance, underscoring the value of rigorous process design.

12. References

Samuel, F. H., et al. (2015). Effects of Homogenization Treatments on Production of 319 Alloy. A356 Alloys, TMS. https://doi.org/10.1007/978-3-319-19028-8_12

Davis, J. R. (1993). Aluminum and Aluminum Alloys, ASM International. https://www.asminternational.org/search/Detail?productId=Book%3A02023

Polmear, I. J. (2005). Light Alloys: From Traditional Alloys to Nanocrystals, Butterworth-Heinemann. https://doi.org/10.1016/B978-1-4377-3576-4.00001-4

Yuan, S., Li, Y., & Zhao, Y. (2020). Influence of Cooling Rate on As-Cast Microstructure in 356 Alloy. Journal of Materials Processing Technology, Elsevier. https://doi.org/10.1016/j.jmatprotec.2020.116‘,