Analyzing Aluminum Melt Quality from Furnace to Mold.
As global competition intensifies, foundries face myriad demanding issues that affect profitability. Improvements in design, lead time, scrap and production directly impact future market possibilities. Viable improvement strategies aim to utilize the most economical method with the least capital investment.
A particular constraint to foundry performance is the lack of real-time melt quality assessment. Because they slow production, quality control processes for monitoring metal cleanliness create performance strains. In addition, virtual designs using computer modeling ultimately require physical validation, and this can cause problems if upstream processing (hydrogen gas, inclusions, metal temperature, grain refinement and modification) is not measured accurately.
Faced with these challenges, Ford Alloy Wheel Plant (AWP), a low-pressure permanent mold (LPPM) foundry in New Zealand, investigated the sensitivities of melt quality to process variables (such as delivery, treatment and handling). This article describes AWP's experience with hydrogen (H) gas analysis and inclusion/oxide film levels at six key melt processing sites.
AWP manufacturers aluminum alloy wheels and cross-members at a rate of 2 million components/year. A 50:50 mix of A356 primary aluminum ingot and recirculating processed scrap is melted in two reverberatory furnaces and are charged directly to the dry hearths, while as-cast scrap wheels are melted in the open charge wells. A metal pump circulates metal from the main body of the furnace, through the charge well and back. This aids melt rate and homogenizes melt temperature and chemistry.
Molten metal is rotary degassed before passing through a 30-ppi sintered alumina filter and tapped out from the furnace into a 1.5-ton transfer ladle. Ladle treatment includes a magnesium trimming addition, titanium boride (TiB) grain refinement, strontium modification, fluxing and rotary degassing to reduce levels of H gas and non-metallic inclusions. Metal then is transported via forklift and poured at heights ranging from 8-20 in. into casting/holding furnaces located below each LPPM machine.
Tests were performed at six melt stages (Fig. 1) considered essential to achieving sustained control of melt cleanliness, which ultimately leads to normalization of casting quality performance.
Testing of H levels during metal treatment and transfer processes for experimental cast aluminum rotors was meant to:
* calibrate a standard H gas measurement technique (initial bubble test);
* determine H levels and propensity for pickup throughout the process;
* establish strategies for process improvement.
Three instruments/techniques were used. One instrument works on the initial bubble test (IBT) method, in which a small sample of liquid aluminum is poured into a pressure-tight chamber. As the sample solidifies under low pressure, it is observed for bubbles (arising from H gas evolution). The H content in the melt then is calculated from the recorded temperature and pressure by an onboard computer.
Although this technique is most commonly used in foundries, the results can be very subjective. Hence, multiple measurements were performed at each stage by the same experienced inspector to minimize error. Two other techniques were used to calibrate and verify the initial bubble measurements.
H levels recorded before and after casting machine refills increased from degassed levels of 0.12 ml/100 g to 0.16 ml/100 g and 0.19 ml/100 g, respectively. This is attributed to various unfavorable conditions during metal transfer:
* exposure of metal surface to atmospheric/moist air during the delivery process;
* unfiltered and exposed pouring into casting machine furnaces at heights of up to 0.3 m;
* significant disturbance at machine fill, introduction and re-distribution of inclusions due to the continuous in-flow of metal.
Ladle degassing resulted in lower levels of H with all three types of rotors.
Two experimental rotors (a castellated rotor/cap and a rectangular rotor) were initially run to establish degassing parameters that resulted in optimum bubble distribution and minimum vortexing. The rotors then were tested under optimum conditions and compared with a standard-use castellated rotor. The castellated rotor cap demonstrated the lowest gas levels with a 30% longer use than the standard rotor. The rectangular rotor also produced lower H gas levels than the standard, consumed half as much circulating gas, and lasted more than twice as long.
There was a minimal difference in gas levels between the rotors. Consequently, AWP adopted the rectangular rotor into the standard degassing process.
Metal temperature at casting is another important factor in product quality, and metal supply temperature must be controlled within narrow limits to avoid defects. Historically, to achieve the correct ladle temperature at casting machine refill, AWP has superheated melt furnaces to 1436F (780C). However, both oxide formation and H absorption rates increase rapidly at these temperatures. A temperature profile of the upstream melt process identified ladle filling and degas stages as the main contributors to temperature loss. The temperature of the metal supply from the scrap charging practice also varied widely.
Modifications have been made to the filter box heater setting, and, coupled with a controlled scrap loading system, the furnace control temperatures have been reduced by 68F (20C), giving a considerable saving in fuel costs with no loss in metal quality. The degas machines also are being fitted with insulating lids to reduce temperature loss during the 8-min degas cycle, and this will allow a further 50F (10C) drop in melt furnace control temperature and more removal of H with the improved rotor/degas system.
One of the most serious problems in aluminum castings is the presence of non-metallic inclusions, which can affect mechanical properties, machinability, porosity, corrosion resistance and even cosmetic appearance. Inclusion particle sizes range from a few microns (MgO and [Al.sub.4][C.sub.3]) to several millimeters ([Al.sub.2][O.sub.3] and Mg[Al.sub.2][O.sub.4] films and clusters). In measuring inclusions and oxide film, AWP sought to map characteristics and levels of inclusions in the six key stages of metal treatment and transfer and identify melt-damaging processes to establish strategies for process improvement.
Fracture Test Results
Five K-mold fracture tests were performed.
AWP used K-factor and K-value to interpret fracture surfaces containing inclusions. The K-factor is a ratio of total independent inclusion counts in 20 fractured surfaces (5 bais). While this is the most quantitative and sensitive interpretation, it is time-consuming. The K-value is more commonly used because it simply expresses the total dirty fractures out of 20 as a ratio (a dirty fracture would contain at least one inclusion). The disadvantage is reduced sensitivity to inclusion levels. Results are shown in Fig. 2.
Inclusion levels increased significantly from the degas well to the transfer ladle (before degas). This was expected since the maximum drop height of metal during filling of the ladle is 5 ft. This is far greater than the critical fall height for liquid aluminum (0.04 ft), above which the melt surface has enough energy and velocity ([greater than]0.5 m/s) to overcome surface tension and continuously expose fresh layers of the melt to the atmosphere. At drop heights of up to 5 ft, the maximum impact velocity is elevated to almost 11 times the maximum allowable velocity. This not only creates excessive oxides throughout the melt, but it also entrains further moisture and air during the splashes. This also explains a similar rise in inclusions after casting machine refills, even though the drop height is less severe (0.66-0.98 ft).
A computerized commercial pressure filtration system was used to analyze melt inclusions, and a commercial non-computer-based system was used to cross-analyze results.
Several 3-lb molten aluminum samples were taken at each melt location to develop a working range for typical tests (Fig. 3).
Based on the computerized system alone, degraded metal displayed relatively lower gradients than "base" levels in the charge well. Also, results show benefits of filtration at the tap-out stage, indicating poorer quality metal without filters. Refinement using rotary degassing with nitrogen gas appears to generate lower gradients (higher inclusion levels) than online curves before degas. There is a similar result with no filter at tap out, although this is not as extreme.
At pouring into the casting machines, the metal quality has been further damaged. Also, there are similar trends of degrading metal quality from the charge well to casting machines. However, the interpretation from K-bar tests actually reveals a decrease in inclusion levels during degas.
The solidified filtered samples were sectioned and analyzed to quantify levels of impurities remaining in a cross-section of each filter. This technique generated information on types of inclusions while also giving a quantitative measurement of inclusion and oxide film content (Fig. 4). While some results conflicted with the pressure filtration system results, possibly due to the different types fo inclusions present, some interesting points were seen.
Charge Well and Degas Well--Inclusion types in the charge and degas wells are predominately aluminum carbides ([AI.sub.4][C.sub.3]) that are smaller than 3 microns.
Transfer Ladle Before Degas--The next stage involves tapping out metal through a 30-ppi sintered aluminum filter and into a 1.5-ton ladle. TiB particles introduced during grain refinement become the dominant inclusions in the melt. The metallographic samples from trials with and without the tap-out filter did not confirm the improvement indicated by the pressure filtration curves.
Trials with and without the tap-out filter indicated a significant filtration effect.
Transfer Ladle After Degas--Degassing is shown to be effective in further removal of [A.sub.4][C.sub.3].
Casting Machine Before and After Refill--Inclusion types in this category are mainly TiB, magnesium oxides and spinels. The formation of these compounds would be expected in casting furnaces where conditions of long-term exposure to air and high temperatures [up to 1328F (720C)] prevail.
The results demonstrated a good correlation between the initial bubble test method and the more sophisticated systems. In all three measurement systems, the results showed significant sensitivities of gas levels to melt processes in the foundry. In particular, there was a higher gas level in casting machines, which was attributed to the turbulent filling process that allows moisture and further oxides to become entrained and deeply embedded in the melt. Trials are underway to reduce the H levels in the casting machine through an improved metal delivery technique that minimizes the vertical drop height and decreases velocity surges in the melt during filling.
Results for inclusion measurement using a standard K-bar test showed a general increasing trend in inclusion levels throughout the foundry. In particular, a significant increase was evident after furnace metal tap-off and casting machine refills. A pressure filtration system was employed to further quantify and assess inclusion characteristics at each process.
Based on metallographic data, AWP has directed efforts to minimize TiB and magnesium oxides in the casting furnace, partially through inert gas pressurization lower casting temperatures, and improved cleaning process.
While there are increasing resources for R&D in computer prediction capabilities for downstream casting processes, the impetus must equally exist upstream to develop real-time metal quality feedback.
This article was adapted from a presentation (01-028) at the 2001 AFS Casting Congress. Conference Proceedings are available through AFS Publications at 800/53 7-4237 or at the AFS E-Store at www.afsinc.org.
For a free copy of this article circle No. 341 on the Reader Action Card.
|Printer friendly Cite/link Email Feedback|
|Comment:||Analyzing Aluminum Melt Quality from Furnace to Mold.|
|Author:||Mitchell, Dave J.|
|Date:||May 1, 2001|
|Previous Article:||Pouring stream shrouding at Harrison steel castings.|
|Next Article:||Pouring Concept Extends Hold Times for Treated Ductile Iron.|