In an article recently published in the journal Additive Manufacturing, researchers discussed an innovative post-processing method to improve casting molds made by binder jetting additive manufacturing (AM).
Study: Novel post-processing procedure to enhance casting molds manufactured by binder jetting AM. Image Credit: Vasyl S/Shutterstock.com
AM is of great interest to various leading industrial sectors such as automotive, aerospace, and medical. One of the AM techniques that received the most attention recently is Binder Jetting (BJ), which is based on the conglomeration of powder particles by means of a binding substance.
Although Binder Jetting is successfully applied in sectors such as aeronautics and biomedicine, substantial effort is being devoted to applying it to the foundry industry owing to the cost and time reductions it can enable. Despite the success of the Binger Jetting application for metal casting, there are still several challenges that limit its application in this field.
Developing techniques to lower the volatile load of Binger Jetting molds, which enhances their applicability to metal casting, is interesting given the narrow range of the permissible variation of binder level.
Some studies have made marginal advancements in utilizing infiltration remedies like diluted magnesium sulfate in distilled water. Unfortunately, these initiatives do not concentrate on the reduction of volatiles, so it is still necessary to address the issue of developing particular post-processing techniques for producing casting-focused parts to increase the technique's application.
About the Study
In this study, the authors presented a revolutionary component post-processing method. This approach combined several heat treatments with a vacuum infiltration step that used an Epsom salt solution. With minor component design changes, the process lowered the volatile content of Binger Jetting additive manufacturing parts and increased compression strength. This additional processing greatly enhanced the application of Binger Jetting additive manufacturing components as disposable casting components.
The team showed a case study with an expendable mold for casting aluminum after outlining the proposed process and examining the significant improvement of the AM calcium sulfate parts' attributes, such as permeability, the decrease in compressive strength, and the decrease in volatile content. This process made casting safer, enhanced the quality of the part's surface, and lessened the internal porosity of the cast parts.
The researchers created a novel post-processing method for calcium sulfate parts using calcium sulfate and additive manufacturing binder jetting techniques. This method relied on an infiltration process in conjunction with various thermal treatments to increase the strength and decrease the volatile content of the parts. This significantly expanded the use of this kind of part as disposable components, such as cores, molds, etc., in the casting process. Following a description of the process, its application to various cubic specimens was discussed to analyze how the material changed throughout the post-processing steps.
A case study involving an aluminum casting mold was used to apply the approach. The quality of the component produced using the additive manufacturing mold and post-treated using the suggested method was contrasted with that of a part produced using a different method with a similar mold being utilized after production without using the suggested post-processing method.
The impact of the proposed post-processing was such that the compressive strength was increased and moisture content was decreased, which enhanced the application of the material as disposable components in metal casting operations. The material developed the 1DHT condition as a result of this treatment.
Gypsum was converted to a dihydrated condition during infiltration, which allowed magnesium sulfate heptahydrate to permeate deeply inside the material. Maximum solubility magnesium sulfate solution was infused into pure water at this pressure for one minute. The second drying HT involved heating the material to between 150 and 200°C for six hours to eliminate moisture and stop the growth of γ-anhydrite.
The amount of removed moisture increased with temperature, and the amount of produced γ-anhydrite weakened the material. Due to mass gain during infiltration and moisture loss during heat treatments, weight and volume fluctuation were dependent on the situation. The density varied slightly across the various states, but after the procedure, it returned to a value that resembled the origin.
The results could not be correlated to the specimens' porosity because of the little variation and measurement errors. In all situations throughout the post-processing method, 1DHT, IHR, and 2DHT, the material's strength was lower in the starting state, AB, than it was in any other condition. In the IHR condition, 335% more strength than the original was attained.
A 71% improvement in strength over the initial state, AB, was possible at the temperature of 200°C that eliminated the greatest moisture. Ra and Rt metrics both showed improvement, with Rt showing the most improvement. This was connected to the smaller degree of roughness seen in the mold material. The volumetric and internal porosities of the pieces made using the treated mold were decreased.
In conclusion, this study discussed a brand-new post-processing method for calcium sulfate additive manufacturing parts employing binder jetting. This approach involved applying various thermal treatments in conjunction with a vacuum infiltration process using an Epsom salt solution made of magnesium sulfate heptahydrate.
The post-process was applied in four steps to the parts made by Binder Jetting from their as-built state, AB. Gypsum was first dried by heating it to 80 °C for two hours, returning it to its hemihydrated state, and preparing it for the subsequent infiltration process without losing its shape.
The presented post-processing was used in a case study of casting AlSi7Mg aluminum alloy using an impromptu mold design after studying the effect of the post-processing on the material. The quality of the part produced with the same mold design but without using the described process was compared to that produced with the same mold design but treated with it. This comparison led to the conclusion that the treated mold produced parts with finer and more uniform roughness.
The authors mentioned that the suggested method lowers the moisture content of the Binder Jetting parts, increasing their permeability and lowering the amount of volatiles retained in the parts during the casting process. They also stated that the proposed method enables the increase in the compressive strength and reduction in volatile content of parts made of gypsum by using the AM binder jetting technology without materially changing their geometry.
Rodríguez-González, P., Zapico, P., Robles-Valero, P. E., et al. Novel post-processing procedure to enhance casting molds manufactured by binder jetting AM. Additive Manufacturing, 103142 (2022). https://www.sciencedirect.com/science/article/pii/S2214860422005310