Technical Article Continued from pg 27


1200 lb Crucible 357 Alloy


1330º F melt temperature (*)


60% relative humidity (*) Table 3: Parameters for Degas Modeling


photograph of the TA unit is shown in Figure 2. The TA algorithm analyzes the sample


curve liquidus and computes a score on a scale from 1-9 for evaluating grain fineness (GF). A score of 1 references a curve that compares perfectly with curves exhibiting no grain refining. In contrast, a GF score of 9 is achieved when the sample curve compares with those curves know to have produced “perfect” grain refining of melts with the same alloy composition.


A pictorial representation


of the subject grain refinement levels is provided in Figure 3. The results of the TA evaluation of the incumbent process are presented in Table 2. The results of the TA analysis clearly


show an opportunity for improved grain refining as the maximum level of 9.0 was not achieved in any of the three alloy (355, 356, 357) melts benchmarked.


Salt Form Grain Refining Before there was grain refining with metallic additives such as TiBor, grain refining was predominantly achieved via salt pucks.


decompose to form nuclei (e.g. TiAl3 TiB2


are formed from metallic TiBor. Ultimately salt form grain refining


and AlB2


with pucks fell out of favor since they needed to be plunged with steel bell jars causing elemental contamination of Fe in the melt. Metallic form TiBor did not


28 ❘ May 2020 ®


The salt pucks would ,


) in situ. These same nuclei


require steel jars so it gained widespread adaptation as a resolution to the Fe contamination issue of salt form pucks. More recently, a novel granular salt


flux form grain refiner was developed as an improvement over metallic form TiBor. The salt form grain refiner can be applied without steel tools so it overcomes the Fe contamination issues associated with the pucks. The salt form grain refining flux has the additional benefit of being a very strong cleaning flux capable of reacting with oxides to chemically separate them from aluminum.


Degassing and Degas Modelling The predominant way for reacting the salt form grain refining flux is via a metal treatment station (MTS). In a MTS, a vortex is temporarily created by disengaging a vortex breaker baffle board and increasing RPM’s of the graphite shaft and rotor used in the rotary impellor degassing process.


PLC controlled


additions of the treatment flux are added into a vortex and mixed to complete reaction prior to the vortex breaker baffle board re-engaging the melt, effectively stopping the vortex.


XSR 220 rotor


0.50 ml H2 / 100 g Al starting level


600 s minimum treatment time (*) 95° F ambient temperature (*)


Alloy


355 356 357 355 355 357


Grain Fineness (GF)


9.0 9.0 9.0 9.0 9.0 9.0


Table 4: Results of TA Evaluation with Grain Refining Flux


the extremes where degassing is most difficult, e.g., high temperature and humidity. The modeled parameters are listed in Table 3. The results of the model for the newly proposed procedure is presented in Figure 4. The results of the degas modelling


confirmed that the new rotor design and unit combination should degas the melt in approximately 6-8 minutes under non- optimum


environmental conditions. After the vortex is


stopped, the MTS completes a standard rotary degassing cycle. A degassing model was utilized to determine a minimum cycle time achievable. 357 alloy was used for the model since it is generally the toughest of the aforementioned alloys to degas. The modeled conditions tended towards


A 13-15 minute average cycle should be sufficient for when rotor wear, belt wear and perhaps baffle plate wear lead to a slight reduction in degassing performance. The model confirmed (not pictured) that 25 minutes were required to degas the same melt conditions using the historic degassing method. A dimensional schematic of the MTS degassing unit designed for trials appears in Figure 5. The MTS unit can be moved into place on top of the melt furnace by either a fork truck or an overhead hoist. The unit employs a retractable carriage that will automatically lower the degassing shaft, rotor and baffle plate into the melt during treatment and automatically withdraw them at the completion of the treatment. The unit, which is designed to properly withstand the heat of the melt, is outfitted with a hopper to feed the grain refining flux through an auger drive system to deliver precise amounts of flux each treatment.


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