A Series of Test Shows the Impact of Carbon-Based Coatings in Aluminum Diecasting Operations.
Die castings produced with as-cast or nearly as-cast surfaces are becoming more common, where near-surface layers of the castings are machined. As a result, various problems related to the boundary between chill layers and general internal structures are more apparent. Controlling heat insulation during molten metal infusion, heat transfer during pressurization and thermal diffusion to the mold are critical to diecasting quality. Proper control of these variables will lead to dense cast structures free of chill boundaries.
In the paper “Key Issues for Quality Stabilization of Aluminum Die Castings,” Yuichi Furukawa, Toyota Motor Corp., Toyota, Japan, and Yoshiki Tsunekawa, Toyota Technical Institute, Nagoya, Japan, investigated aluminum diecasting processes. The report includes the results of a durability test of various carbon-based coatings, the effects on solidification structure and a coating’s ability to stabilize diecasting quality.
The typical microstructures of a diecast oil control valve (a) without carbon-fullerene coating and (b) with carbon-fullerene coating are shown.
How does a carbon-based coating in an aluminum diecasting operation affect surface roughness, molten metal flow, solidification structure and casting quality?
Background
In the diecasting process, molten metal fills permanent molds at high speeds and is unlikely to penetrate delicate concave shapes inside the mold surface. Once the mold is filled, high pressure is applied to the molten metal, at which time it fills the concaved areas. The concept of the casting interface with both heat insulation and transfer properties is shown in Fig.1.
In a cold chamber diecasting method, metal is poured into cold sleeves, which create initial solidification layers before cavities are filled entirely. The molten metal becomes solid as soon as it touches a mold, forming a chill layer that has better mechanical properties than the casting’s internal structure. Solidification layers, which can become evident during near-surface machining, degrade mechanical properties and impact fluidity.
Conversely, proper hardness cannot be achieved in a heat-insulated condition (which does not create chill layers) or when castings are removed from a mold that is still at a high temperature. Galling (i.e., wear caused by adhesion between sliding surfaces) also might occur in these situations.
The boundaries between chill layers and the casting’s internal structure must be eliminated to produce consistent, high quality castings. To do this, insulation is necessary during the infusion of molten metal to create a casting interface, which transfers heat during solidification.
The study included a method that allows the metal to remain non-wetting during infusion, but then becomes wetting when pressure is added to create a casting interface that transfers heat. Carbon has poor wettability with molten aluminum but has high lubricity. It long has been used as a parting agent. However, because the application of carbon can negatively affect its working environment, it is used only as a plunger lubricant.
For this study, a carbon-based coating resilient in an aluminum diecasting environment, where casting pressure is added, was tested. The report includes durability tests of carbon nanofiber, carbon-graphite and carbon-fullerene coatings and their effects on solidification structure and casting quality.
Procedure
When the unevenness of a mold’s surface is significant, aluminum can flow into concave parts and lead to galling defects. Fig. 2 shows a testing setup to evaluate metal flow by mold surface roughness and carbon materials. It consisted of an electric furnace and a 7.9 × 7.9 × 1.2 in. (200 × 200 × 30 mm) sample mold. The test piece mold was tilted 20 degrees to create a slope for metal flow. All coated surfaces of the molds were recorded with a thermo camera to study the heat transfer condition. The condition of metal flow was classified into four grades, and each was indexed. The sum of indexes, obtained by pouring molten metal three times, was defined as the metal flow index.
The metal flow test results are shown in Fig. 3. For all types of carbon coatings, metal flow was not smooth on a mirror surface, that is, Ra<1.0 μm. Any type of carbon coating showed a good result at Ra=1.5 μm or above. Infrared images taken by a thermo camera indicated that immediately after metal was infused, the temperature of the test piece surface was within the range of 167F (75C) and 181F (83C) for the case of a metal flow index score of 3 or less; between 143F (63C) and 158F (70C) for a score between 5 to 7; and 122F (50C) or less for a score of 8 or over. In this way, a relationship between the heat transfer at metal flow and the metal flow property was confirmed. The indicated temperatures were corrected using the infrared emissivity that had been measured in advance. The emissivity of the carbon nanofiber, carbon-graphite and carbon-fullerene coatings were 0.9 or higher, which proved the temperature measurements were accurate.
Results and Conclusions
Differential thermal analysis was conducted for carbon nanofiber, fullerene and carbon-fullerene coating in air and nitrogen atmospheres. The thermogravimetry scans of coatings are shown in Fig. 6. In an air atmosphere, the carbon nanofiber coating suddenly dropped in the temperature range of 752F (400C) and 1,112F (600C). It kept reducing until 50%wt. This is because cutting powder of the carbon nanofiber coating was used as a differential thermal analysis sample, and it contained the compound layer that was chemically combined with the test piece mold.
The weights of the fullerene and carbon-fullerene coatings do not decrease until 752F (400C) but then show sharp declines. The fullerene coating starts resolving at 752F (400C) and easily vaporizes into CO and CO2. Compared to other types of carbon materials, fullerene oxidizes easily. However, based on the analysis results, at 752F (400C) or lower, the fullerene coating can control the high-temperature degradation of the carbon nanofiber coating. In a nitrogen atmosphere, the fullerene coating remains stable and is not dissolved until 1,652F (900C). The thermogravimetry scan of the carbon nanofiber coating in a nitrogen atmosphere shows a gradual decrease in weight as the temperature increases. Conversely, fullerene and carbon-fullerene coatings demonstrate lower weight decline ratios compared to that of the carbon nanofiber coating up to 1,202F (650C). It is possible that the fullerene coating has a protective effect on the carbon nanofiber coating as observed in an air atmosphere.
