Date of Award

3-11-2024

Publication Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Additive manufacture 3D;Computer simulation;Die insert;Failure mechanism;H13 steel;Stress corrosion cracking

Supervisor

Henry Hu

Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

Abstract

The application of additive manufacturing (AM) 3D printed die inserts with conformal cooling channel has become popular in the industry of high pressure die casting (HPDC). The AM technology enables die inserts to be constructed with conformal cooling channels that significantly improve tooling performance through high cooling uniformity and reduced cycle times. In this study, the chemical composition of the as-printed AM 3D H13 steel was analyzed by inductively coupled plasma-optical emission spectrometry (ICP-OES) firstly. The surface profile and roughness were evaluated by optical microscopy and a surface measuring station. The defects were identified by scanning electron microscopy. The Rockwell hardness of the as-printed AM H13 steel was measured. The generic properties of the as-printed AM H13 steel meet the ASTM and NADCA standards. The corrosion resistance of the steel determined by the electrochemical method varied significantly with the concentration of salt solutions. Second, the observation on the inner surface of the AM 3D printed H13 die insert revealed that the residual gas existed at crevices of conformal cooling channel surface and a thinner steam layer formed between the cooling water and cooling channel surface during HPDC production cycle. As steam was much more corrosive than the cooling water to the printed AM H13 steel tools, corrosion pits were produced at the surface crevice formed on the inner surface of the inserts. Thirdly, a computer simulation software (Magmasoft®) was employed to numerically understand the failure characteristics for optimization and process control for the AM 3D H13 insert. Thermo-mechanical analyses were conducted to investigate temperature and thermal stress development. The failure area was predicted by the maximum principal stress at the cooling channel surface. Finally, the reality check was used to verify the predicted results. For the internal cooling channel, the failure should be directly related to the corrosion pits or crevices on the inner surface of the AM 3D printed H13 inserts where the maximum principal stress was present. The corrosion pits or crevices were the origin of cracking and the HPDC thermal cycles facilitated the crack growth. The energy dispersive X-ray spectroscopy (EDS) revealed the effect of the Cr enrichment along the grain boundaries to be more sensitive to corrosion in the presence of aggressive agents, e.g., the localized H+, HO-, and O2- ions in the vaporized steam on the inner surface of the AM 3D printed H13 inserts. Additionally, the high-level contents of the harmful components, such as sulfur and phosphorus segregated at the grain boundaries, severely degraded the corrosion performance of the AM 3D printed H13 insert. As a result, the cracks followed an inter-granular path propagating through the bulk material. The failure mechanism should be considered stress corrosion cracking (SCC) originated from corrosion pits on the inner surface of the AM 3D printed H13 steel insert.

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