Here we discuss electro-spark plating applied to metal alloys to repair defects in injection molding tooling and casting moulds.
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What is electro-sparking deposit?
Electro-Spark Deposition (ESD) is used for precision repair of high-value worn components on a small scale using pulsed micro-welding. Electrospark deposition is also known as pulsed electrode surfacing, pulsed fusion surfacing, electrospark alloying, electrospark quenching, and spark hardening. In industry, it is used to repair defects in injection molding tools and foundry moulds.
ESD systems have a capacitor-based power source producing pulses through a rotating wire consumable electrode. These pulses have a high current in a very short duration. The material of the consumable electrode (anode) is deposited on the part (cathode) by an electric spark.
ESD applied to metal alloys
A high temperature plasma arc is generated between the tip of the electrode and the metal alloy part by the direct current when the capacitor energy is released. This high temperature varies between 8000 and 25000°C. The plasma arc ionizes the anode, and this molten material is then quickly transferred to the part.
This ionized anode is transferred to the substrate by short pulses. The high temperature arc consists of particles of anode, heat flow (heat jet) and plasma (developed by the decomposition of gases and active atoms of nitrogen, oxygen and carbon). Most of the heat is transported by the heat jet and the plasma.
Since the pulses are short, heat transfer via heat jet and other gases is minimal, and the only heat transfer to the substrate is through the small number of anode microparticles deposited on the substrate. These pulses therefore give a low heat input to the substrate, which does not lead to any modification of its microstructure. This method may be more beneficial than fusion welding processes typically used to repair alloys that have poor heat affected zone properties (such as low toughness, high hardness, liquefaction cracking).
Additionally, this process creates a strong metallurgical bond between the substrate and the coating. The microalloy between the molten electrode and the base materials initiates plasma formation by the decomposition of air, carbonatites, carbides and nitrides.
The quality of the deposition depends on specific discharge parameters such as pulse time, current and voltage, which depend on the chemical and physical properties of the device electrode materials. Therefore, the parameters depend on the type of electrode deposit, temperature dependence, fluidity, electrical resistance, density, diffusivity, chemical reactivity of anodic elements, thermal conductivity and melting temperatures.
There are several advantages associated with ESD applied to metal alloys. For example, no special preparation of the part surface is required. The deposited layer achieves specific amorphous properties due to the high solidification rate and may not require further finishing. ESD can be easily applied to the surface of the complex part, allowing deposition in strictly indicated places. Metal alloys and pure metals can be used as the electrode. ESD allows the use of hard fusible compounds and metallic ceramics for deposition.
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Another advantage of ESD applied to metal alloys is that there is a lack of sample heating in the deposition process. Additionally, ESD provides pollution-free procedures by eliminating the use of toxic non-metallic materials like cyanide in the coating process. In addition, the equipment is very simple and the quality of the additive materials determines the costs associated with ESD.
Another advantage of the ESD process is that the thermal density of the part is very low, which allows the properties of the base material and its chemical composition to be maintained. The thin layers formed by the ESD process are divided into two sub-layers. The diffused inner layer has the resultant properties of both the base material and the added material, while the surface or outer layer has a heavily modified structure. ESD also increases the erosion and wear resistance of small surfaces.
ESD is ideal for repairing shallow and minor defects; however, it is not suitable for large defects due to the slowness of the process and the maximum thickness of coating being 2mm.
Sometimes, the geometry of the substrate limits the coating. The contact between electrode and surface of the substrate is necessary for ESD, but some surface geometries do not allow such contact; therefore, the coating is only applied to adjacent areas.
The characteristics of deposited layers are governed by process parameters like pressure on the electrode tip, the number of passes over each area of substrate, temperature, frequency, spark duration, spark energy etc. Surface waviness with a period less than the diameter of the electrode can cause the coating to build only on the peaks. Some deposition parameters might cause initial covering to be insufficient, leaving some parts uncovered. As the arc goes to the closest surface, only the high points will continue to build up with successive passes and layers. The solution for this issue is choosing an optimum work regime through suitable parameters.
Recently there has been a lot of research conducted on applying ESD simultaneously with other methods to make specific alloys. For example, in 2021, a paper was published by researchers at the University Politehnica of Bucharest, Romania, investigating a groundbreaking high corrosion-resistant coating deposited via ESD.
In this research, an alloy with high entropy was fabricated through the dust of Zr, Ti, Ta, Nb and Hf by mechanical alloying and spark plasma sintering. A thin layer was coated on a stainless steel sample through ESD. This coating had improved hardness compared to the substrate, and when they tested for corrosion resistance it showed a very low corrosion rate of 0.00024 mm/year.
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References and Further Studies
Johnson, R. N. (2002). Alternative coatings for wear and corrosion: the electrospark deposition process. published in Proceedings, American Electroplaters and Surface Finishers Society. https://www.nmfrc.org/pdf/awk02/awk02d06.pdf
Manea, C. A., Sohaciu, M., Stefănoiu, R., Petrescu, M. I., Lungu, M. V., & Csaki, I. (2021). New HfNbTaTiZr High-Entropy Alloy Coatings Produced by Electro-spark Deposition with High Corrosion Resistance. Materials, 14(15), 4333. https://www.mdpi.com/1996-1944/14/15/4333
Twi-global, What is electro-spark deposition (ESD)?. [online] Available on :
Vizureanu, P., Perju, M., & Nejneru, DAC (2018). Advanced electro-sparking process on metallic alloys. In (Ed.), Advanced Surface Engineering Research. Intech Open. https://www.intechopen.com/chapters/62514