EROSION OF MOLD-STEEL SURFACE OXIDATION AND NITRIDING LAYERS DUE TO A DIE-CASTING ALUMINUM ALLOY

Paper Title Boosting Mold Life: Why Oxidation Outperforms Nitriding for H11 Die Steel Surface Treatment

This technical summary is based on the academic paper "EROSION OF MOLD-STEEL SURFACE OXIDATION AND NITRIDING LAYERS DUE TO A DIE-CASTING ALUMINUM ALLOY" by Han-xue Cao, Yun-hao Liu, Chao Shen, Hao-xing Tang, Yan-yan Huang, and Jia-le Lei, published in Materiali in tehnologije / Materials and technology (2018).

Keywords

• Primary Keyword: Die Steel Surface Treatment
• Secondary Keywords: erosion resistance, oxidation treatment, nitriding treatment, H11 die steel, aluminum die casting, mold life

Executive Summary

• The Challenge: High-temperature aluminum alloy melts erode the surface of die-casting molds, significantly reducing their service life and increasing operational costs.
• The Method: H11 die steel samples with two different surface treatments (oxidation and nitriding) were immersed in molten A356 aluminum alloy to compare their resistance to erosion.
• The Key Breakthrough: The oxidized H11 steel surface demonstrated significantly better erosion resistance, with lower mass loss and a higher activation energy required for reaction, compared to the nitrided surface.
• The Bottom Line: For aluminum die casting applications, oxidation is a more effective die steel surface treatment than nitriding for protecting H11 molds and prolonging their operational lifespan.

The Challenge: Why This Research Matters for HPDC Professionals

In the high-pressure die casting (HPDC) industry, mold longevity is a critical factor for both productivity and profitability. One of the primary reasons for reduced mold service life is erosion. As high-temperature molten aluminum continuously scours the die surface, it causes both mechanical shock and chemical corrosion. This degradation can lead to parts of the casting sticking to the die, permanent mold failure, and costly downtime. Therefore, finding an effective surface treatment to prevent or slow this erosion is a crucial task for any operation looking to improve mold quality and extend its service life. This research directly addresses this challenge by comparing two common treatments—oxidation and nitriding—to determine which offers superior protection.

The Approach: Unpacking the Methodology

The researchers conducted a rigorous comparative study using materials common in the industry: H11 die steel and A356 die-casting aluminum alloy.

• Sample Preparation: Rectangular H11 steel specimens were heat-treated to a hardness of 48 HRC. They were then divided into two groups for different surface treatments:
  • Oxidation Treatment: Samples underwent vapor oxidation at 550 °C for 1.5 hours under a water-vapor pressure of 150 kPa.
  • Nitriding Treatment: Samples were subjected to a nitrocarburizing process, held at 565 °C for 2.5 hours in a gas mixture of N₂, NH₃, and CO₂.
• Erosion Test: The treated samples were immersed in a molten A356 aluminum alloy bath at 680 °C for varying durations (0.5, 1, and 2 hours) under a protective argon atmosphere.
• Analysis: After the immersion test, the samples were cleaned and analyzed to measure mass loss. A suite of advanced analytical techniques was used to investigate the microstructural and chemical changes, including Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-ray Diffraction (XRD), and Differential Scanning Calorimetry (DSC).

The Breakthrough: Key Findings & Data

The study produced clear, data-driven results demonstrating the superior performance of the oxidation treatment.

Finding 1: Oxidation Treatment Drastically Reduces Mass Loss and Erosion Area

The most direct measure of erosion resistance, mass loss, showed a stark difference between the two treatments. As shown in Figure 1, the oxidized samples consistently lost less mass than the nitrided samples at every time interval.

• After 2 hours of immersion, the mass-loss rate for the oxidized samples was 1.77%, while the rate for the nitrided samples was significantly higher at 2.87%.
• Microscopic analysis confirmed these findings. The scanning electron micrographs in Figure 2 show that the erosion area on the nitrided samples is visibly larger and more severe than on the oxidized samples after the same immersion time. This indicates the oxidation layer provides a more effective physical and chemical barrier against the molten aluminum.

Finding 2: Oxidized Surfaces Create a Higher Energy Barrier Against Chemical Reactions

The research went beyond physical erosion to analyze the chemical reactivity between the treated surfaces and the aluminum alloy. Using Differential Scanning Calorimetry (DSC), the researchers calculated the activation energy—the energy required to initiate a chemical reaction.

• According to the data in Figure 7, the reaction between the oxidized surface and aluminum alloy required a much higher activation energy ( 22.37 eV/atom at peak 2 and 20.58 eV/atom at peak 3) compared to the reaction with the nitrided surface ( 16.10 eV/atom at peak 2 and 6.07 eV/atom at peak 3).
• This high activation energy indicates that the oxidation layer more effectively inhibits the atomic diffusion between iron and aluminum, slowing the formation of harmful intermetallic compounds that lead to die soldering and degradation. The Gibbs free energy analysis further supported this, showing the reaction to form intermetallics on the nitrided surface is thermodynamically easier to occur.

Practical Implications for R&D and Operations

• For Process Engineers: This study suggests that selecting an oxidation surface treatment over nitriding for H11 die steel molds used with aluminum alloys can directly contribute to longer mold life, reduced maintenance cycles, and less downtime associated with mold failure.
• For Quality Control Teams: The data in Figure 5 and the corresponding EDS analysis in Figures 3 and 4 illustrate the specific Fe-Al and Fe-Al-Si intermetallic compounds that form during failure. QC teams can use this information to develop more precise inspection criteria for worn dies, helping to identify the onset of chemical degradation and schedule preventative maintenance before catastrophic failure occurs.
• For Design Engineers: The findings underscore the critical importance of a robust, uniform protective surface layer. This reinforces the need for die designs that minimize high-velocity or turbulent melt flow directly impinging on critical surfaces, as this can accelerate the mechanical and chemical breakdown of even a well-applied surface treatment.

Paper Details

EROSION OF MOLD-STEEL SURFACE OXIDATION AND NITRIDING LAYERS DUE TO A DIE-CASTING ALUMINUM ALLOY

1. Overview:

• Title: EROSION OF MOLD-STEEL SURFACE OXIDATION AND NITRIDING LAYERS DUE TO A DIE-CASTING ALUMINUM ALLOY
• Author: Han-xue Cao, Yun-hao Liu, Chao Shen, Hao-xing Tang, Yan-yan Huang, Jia-le Lei
• Year of publication: 2018
• Journal/academic society of publication: Materiali in tehnologije / Materials and technology
• Keywords: erosion resistance, oxidation treatment, nitriding treatment, immersion test

2. Abstract:

The resistance to erosion due to A356 aluminum-alloy melt of H11 die steel treated with two different surface-treatment technologies (oxidation treatment and nitriding treatment) was investigated by employing a hot-dip-aluminum (immersion) experiment. The microstructure, chemical constituents, phase compositions, heat-flux variation and activation energy were analyzed, using a scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Compared to the nitrided samples, the oxidized samples have a smaller erosion area and a lower mass loss. Moreover, the nitrided samples require lower Gibbs free energy to react with the aluminum-alloy melt. Compared to the samples with the oxidation treatment, the activation energy of the reaction between the nitrided samples and aluminum alloy is smaller. These results indicate that the H11 die steel surface-treated with oxidation tends to exhibit a better erosion resistance to aluminum-alloy melt than the steel surface-treated with nitriding.

3. Introduction:

Aluminum is a widely used material in the automotive industry and the requirement for the quantity and quality of aluminum alloys has been enhancing. Die casting is one of the most efficient forming methods used for aluminum components. A mold affects not only the productivity but also the die-casting performance, such as surface finishing and microstructure refinement. So, many researchers focus on improving the mold quality and prolonging the service life of a mold. It is well known that one of the main reasons for a reduced service life of a mold is the fact that the die surface becomes eroded due to the high temperature of aluminum-alloy melts. During a die-casting process, erosion occurs because high-temperature melt continuously scours the die surface and produces a mechanical shock and chemical corrosion. When the die is drawn, a part of the casting may be left on the die surface, or the mold may be torn apart, causing a permanent failure of the mold. Therefore, preventing the erosion of a mold is a very important task taken to prolong its life. A suitable surface treatment of a mold is one of the most effective technologies.

4. Summary of the study:

Background of the research topic:

The service life of die-casting molds is often reduced by erosion from high-temperature aluminum alloy melts. Effective surface treatments are critical for prolonging mold life.

Status of previous research:

Various surface treatments like shot peening, nitrocarburizing, and PVD coatings have been studied. G. H. Farrahi et al. found that nitrocarburizing decreased the fatigue life of D3 steel. Bo Wang et al. investigated nitrided-layer microstructures on H13 steel. Jose Mario Paiva et al. studied AlCrN/Si₃N₄ nanocomposite PVD coatings. However, the specific erosion resistance and mechanisms of oxidation and nitriding treatments on H11 die steel for aluminum casting have not received sufficient attention.

Purpose of the study:

To investigate and compare the erosion resistance of H11 die steel after surface-oxidation and nitriding treatments when exposed to A356 aluminum alloy melt, and to understand the underlying mechanisms to provide a basis for prolonging the service life of a die.

Core study:

The core of the study involved subjecting H11 die steel samples with oxidation and nitriding surface treatments to a hot-dip immersion test in molten A356 aluminum alloy. The study then analyzed mass loss, microstructure, phase composition, and reaction kinetics (heat flow and activation energy) to compare the performance of the two treatments.

5. Research Methodology

Research Design:

A comparative experimental study was designed. Two groups of H11 die steel samples were prepared with different surface treatments (oxidation and nitriding). These samples were then subjected to an erosion test via immersion in molten A356 aluminum alloy for set periods (0.5, 1, and 2 h).

Data Collection and Analysis Methods:

• Mass Loss: Samples were weighed before and after the immersion test to calculate the mass-loss rate.
• Microstructural Analysis: Scanning electron microscopy (SEM) was used to observe the surface morphology and erosion.
• Chemical Analysis: Energy dispersive spectroscopy (EDS) was used to determine the chemical composition of the reaction layers and intermetallic compounds.
• Phase Analysis: X-ray diffraction (XRD) was employed to identify the specific phases formed on the sample surfaces after the reaction.
• Thermal Analysis: Differential scanning calorimetry (DSC) was used to measure the heat flow and calculate the activation energy of the reaction between the treated steel and aluminum alloy powder.

Research Topics and Scope:

The research focused on the erosion resistance of surface-treated H11 die steel against A356 aluminum alloy melt. The scope included evaluating the effects of oxidation and nitriding treatments on mass loss, microstructure evolution, intermetallic phase formation, and the thermodynamics and kinetics of the steel-aluminum reaction.

6. Key Results:

Key Results:

• Oxidized samples exhibited lower mass loss compared to nitrided samples. After 2 hours, the mass-loss rates were 1.77% for oxidized and 2.87% for nitrided samples.
• The erosion area on nitrided samples was larger than on oxidized samples.
• The reaction between nitrided samples and aluminum melt required lower Gibbs free energy, indicating the reaction is easier to occur.
• The activation energy for the reaction between the oxidized surface and aluminum alloy was significantly higher (e.g., 22.37 eV/atom) than for the nitrided surface (e.g., 16.10 eV/atom), indicating the oxidation layer is a more effective reaction inhibitor.
• Different intermetallic compounds formed on the surfaces: the oxidized surface formed Fe₂Al₅, Fe₃Al₂Si₃, and FeAl₃Si₂, while the nitrided surface formed Fe₃Al and FeAl.

Figure Name List:

• Figure 1: Mass-loss rate after the immersion
• Figure 2: Scanning electron micrographs of oxidation samples after immersion for: a) 0.5 h, b) 1 h, c) 2 h; scanning electron micrographs of nitriding samples after immersion for d) 0.5 h, e) 1 h, f) 2 h
• Figure 3: Chemical composition of sample A3 at S1 and S2
• Figure 4: Chemical composition of sample B3 at S3 and S4
• Figure 5: X-ray diffraction diagrams for samples: a) A3 and b) B3
• Figure 6: DSC results of the thermal analysis of the samples with two different surface treatments: a) 10 K/min; b) 15 K/min; c) 20 K/min
• Figure 7: Activation energy (unit: eV/atom) of the reaction between A356 aluminum alloy and die steel with two different surface treatments at a) peak 1, b) peak 2 and c) peak 3 (the positions of peaks 1, 2 and 3 are shown in Figure 6)

7. Conclusion:

The study concluded that H11 die steel surface-treated with oxidation exhibits better erosion resistance to molten A356 aluminum alloy than steel treated with nitriding. The oxidized samples showed lower mass loss and smaller erosion areas. The reaction between the nitrided samples and the aluminum melt is easier to occur, as indicated by a lower Gibbs free energy. Furthermore, the reaction with the oxidized surface requires much more activation energy, showing that the oxidation layer is more effective at inhibiting the reaction. Thus, an oxidation treatment is the superior choice for prolonging the service life of H11 dies used in aluminum die casting.

8. References:

• ¹J. Hirsch, T. Al-Samman, Superior light metals by texture engineering: Optimized aluminum and magnesium alloys for automotive applications, Acta Mater., 61 (2013) 3, 818-843, doi:10.1016/j.actamat.2012.10.044
• ²F. Wang, Q. Ma, W. Meng, Z. Han, Experimental study on the heat transfer behavior and contact pressure at the casting-mold interface in squeeze casting of aluminum alloy, Int. J. Heat Mass Tran., 112 (2017), 1032-1043, doi:10.1016/j.ijheatmasstransfer.2017.05.051
• ³J. M. Paiva, G. Fox-Rabinovich, E. Locks Junior, P. Stolf, Y. Seid Ahmed, M. Matos Martins, C. Bork, S. Veldhuis, Tribological and wear performance of nanocomposite PVD hard coatings deposited on aluminum die casting tool, Materials, 11 (2018) 3, 358, doi:10.3390/ma11030358
• ⁴P. Terek, L. Kovačević, A. Miletić, P. Panjan, S. Baloš, B. Škorić, D. Kakaš, Effects of die core treatments and surface finishes on the sticking and galling tendency of Al-Si alloy casting during ejection, Wear., 356-357 (2016), 122-134, doi:10.1016/j.wear.2016.03.016
• ⁵Q.-Y. Han, Mechanism of die soldering during aluminum die casting, China Foundry, (2015) 2, 136-143
• ⁷M. Ariati, D. M. Nurjaya, R. Aldila, Die soldering phenomenon on the H13 tool steel with shot peening and nitriding surface treatment, Advanced Materials Research, 1101 (2015), 157-163, doi:10.4028/www.scientific.net/AMR.1101.157
• ⁷G. H. Farrahi, H. Ghadbeigi, An investigation into the effect of various surface treatments on fatigue life of a tool steel, J. Mater. Process. Technol., 174 (2006) 1, 318-324, doi:10.1016/j.jmatprotec.2006.01.014
• ⁸B. Wang, X. Zhao, W. Li, M. Qin, J. Gu, Effect of nitrided-layer microstructure control on wear behavior of AISI H13 hot work die steel, Appl. Surf. Sci., 431 (2018), 39-43, doi:10.1016/j.apsusc.2017.03.185
• ⁹I. Peter, M. Rosso, F. S. Gobber, Study of protective coatings for aluminum die casting molds, Appl. Surf. Sci., 358 (2015), 563-571, doi:10.1016/j.apsusc.2015.08.013
• ¹⁰M. Adamiak, L. A. Dobrzański, Microstructure and selected properties of hot-work tool steel with PVD coatings after laser surface treatment, Appl. Surf. Sci., 254 (2008) 15, 4552-4556, doi:10.1016/j.apsusc.2008.01.091
• ¹¹V. Joshi, A. Srivastava, R. Shivpuri, Intermetallic formation and its relation to interface mass loss and tribology in die casting dies, Wear, 256 (2004) 11, 1232-1235, doi:10.1016/j.wear. 2003.08.001
• ¹²M. Fang, Q. H. Li, F. X. Gan, Kinetic crystallization behavior of SbOx thin films, Physica B, 352 (2004) 1-4, 206-209, doi:10.1016/j.physb.2004.07.012
• ¹³D. O. Ovono, I. Guillot, D. Massinon, Determination of the activation energy in a cast aluminium alloy by TEM and DSC, J. Alloys Compd., 432 (2007) 1-2, 241-246, doi:10.1016/j.jallcom.2006.05.132
• ¹⁴E. K. Tentardini, A. O. Kunrath, C. Aguzzoli, M. Castro, J. J. Moore, I. J. R. Baumvol, Soldering mechanisms in materials and coatings for aluminum die casting, Surf. Coat. Technol., 202 (2008) 16, 3764-3771, doi:10.1016/j.surfcoat.2008.01.019

Expert Q&A: Your Top Questions Answered

Q1: Why was a hot-dip immersion test chosen for this study instead of simulating an actual die-casting process?

A1: The hot-dip immersion test was employed to isolate and investigate the fundamental chemical and diffusion-based erosion mechanisms. By removing the variables of high pressure and melt velocity present in actual die casting, the researchers could get a clearer, more direct comparison of the inherent chemical resistance provided by the oxidation and nitriding layers against the molten aluminum alloy.

Q2: Figure 7 shows a much higher activation energy for the oxidized samples. What is the practical significance of this finding?

A2: The higher activation energy (e.g., 22.37 eV/atom for oxidized vs. 16.10 eV/atom for nitrided at peak 2) is highly significant. It means that the reaction between the oxidized steel surface and the aluminum alloy is much more difficult to initiate and sustain. This high energy barrier effectively limits the atomic diffusion that leads to the formation of harmful intermetallic compounds, which is the primary reason the oxidation layer provides superior protection and prolongs die life.

Q3: The paper suggests the nitriding layer may not be uniform. Could this have contributed to its poorer performance?

A3: Yes, the discussion section explains this as a likely factor. The paper suggests that a non-uniform or less compact nitriding layer could allow the molten aluminum to wet the mold surface more easily and permeate through it. This would accelerate the mutual diffusion of iron and aluminum atoms, leading to faster degradation and erosion compared to the more inhibitive and likely more uniform oxidation layer.

Q4: What specific intermetallic compounds were found on each type of treated surface after the erosion test?

A4: The XRD analysis in Figure 5 revealed different reaction products. The oxidized sample surface (A3) contained Fe₂Al₅ binary alloy, as well as Fe₃Al₂Si₃ and FeAl₃Si₂ ternary alloys. In contrast, the nitrided sample surface (B3) only showed the formation of Fe₃Al and FeAl binary alloys.

Q5: The paper mentions that the Gibbs free energy for FeAl formation is significantly lower than for Fe₂Al₅. Why is this important?

A5: This is a key thermodynamic insight. FeAl was found on the nitrided samples, while Fe₂Al₅ was on the oxidized samples. A lower Gibbs free energy for FeAl formation means that the reaction on the nitrided surface is more spontaneous and has a greater chemical driving force. This indicates that the reaction between the nitriding layer and the aluminum melt is thermodynamically easier to occur, which helps explain its poorer overall erosion resistance.

Conclusion: Paving the Way for Higher Quality and Productivity

The erosion of mold surfaces remains a significant hurdle in aluminum die casting, directly impacting service life, maintenance costs, and part quality. This research provides compelling evidence that the choice of Die Steel Surface Treatment is not trivial. The findings clearly demonstrate that an oxidation treatment provides a more robust and chemically stable barrier for H11 die steel against molten aluminum alloy compared to a standard nitriding treatment. By significantly reducing mass loss and creating a high energy barrier against harmful chemical reactions, oxidation stands out as the superior strategy for prolonging mold life.

At CASTMAN, we are committed to applying the latest industry research to help our customers achieve higher productivity and quality. If the challenges discussed in this paper align with your operational goals, contact our engineering team to explore how these principles can be implemented in your components.

Copyright Information

• This content is a summary and analysis based on the paper "EROSION OF MOLD-STEEL SURFACE OXIDATION AND NITRIDING LAYERS DUE TO A DIE-CASTING ALUMINUM ALLOY" by "Han-xue Cao, et al.".
• Source: doi:10.17222/mit.2018.035

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