The Effects of Rotary Degassing Treatments on the Melt Quality of an Al–Si Casting Alloy

Abstract

In order to produce cast components, which meet the quality requirements of the automotive and aerospace industries, the control of liquid metal quality prior to the casting process is essential. Rotary degassing is the most commonly used melt treatment method in the foundry industry, which can effectively reduce the inclusion and solute hydrogen content of the metal. This procedure is often combined with fluxing, which allows more efficient inclusion removal during melt processing. In this study, the effects of rotary degassing treatments executed with and without flux addition on the melt cleanliness were compared. The quality of the molten metal was characterized by the microscopic inspection of K-mold specimens, X-ray computed tomography of reduced pressure test samples, and by Density-Index evaluation. The inclusions found on the fracture surfaces of K-mold samples were analyzed with energy-dispersive X-ray spectroscopy. Based on the results, rotary degassing coupled with flux addition can be an effective inclusion and solute hydrogen removal technique that can significantly improve melt quality. On the other hand, rotary degassing executed without flux addition can increase the inclusion content of the melts. This can be attributed to the chemical reaction between the liquid alloy and the N 2 purging gas during the degassing process. It was also found that inclusion content highly influences the tendency to porosity formation.

The Effect of Rotary Degassing Treatments with Different Purging Gases on the Double Oxide- and Nitride Film Content of Liquid Aluminum Alloys

“…On the other hand, no significant improvement was achieved by degassing with N 2 . The number of bifilms present in the melt was even increased by the treatments with N 2 , which is in accordance with the findings of Gyarmati et al [39] and the LiMCA data reported by Tremblay and Maltais. [50] During the investigation of K-mold samples, in all cases, film-like discontinuities were found, which comprised of two distinct layers that could be clearly seen on the opposing fracture surfaces (Figure 9).…”

“…[32][33][34][35] According to Campbell, [3] degassing treatments are only effective in removing relatively large-sized bifilms, while numerous small-sized bifilms can be introduced into the liquid metal during the treatments. This statement is in accordance with the findings of Lazaro-Nebreda et al, [36] Yorulmaz et al, [37] Dispinar and Campbell, [38] as well as Gyarmati et al [39] As was highlighted by Cao and Campbell, [40] the purging gases, which are often considered inert, could contain trace oxygen and water vapor which can lead to the oxidation of the inner surface of the purging gas bubbles. N 2 readily reacts with liquid Al at temperatures above 700 °C and forms AlN, which reaction can even be utilized for manufacturing Al-AlN composites.…”

“…N 2 readily reacts with liquid Al at temperatures above 700 °C and forms AlN, which reaction can even be utilized for manufacturing Al-AlN composites. [41] Gyarmati et al [39] suggested that by the consumption of N 2 , which is caused by the nitridation at the gas bubble/ melt interface, purging gas bubbles can lose buoyancy which can prevent their rise to the melt surface. In this way, rotary degassing with N 2 can cause the formation of numerous small-sized nitride bifilms.…”

Experimental and numerical investigations of oxide-related defects in Al alloy gravity die castings

“…The decrease in the mechanical properties due to the presence of double oxide defects is well known [12][13][14], and it affects the components produced by gravity casting [15] and low-pressure casting [16,17]. An accurate preparation of the melt with degassing and filter systems is effective for preventing the entrainment of thick bifilms (thickness = 10 μm ÷ 1 mm) inside the casting [8,12,18]. In contrast, thinner bifilms, generated by the action of turbulence during the pouring and filling phases, are difficult to eliminate, due to their small size (thickness = 1 nm ÷ 1 μm) [5].…”

Possibilities of degassing of AlSi7Mg0.3 alloy with argon and nitrogen

“…For example, one study [5] applies ultrasonic vibration with a combination of argon blowing. However, according to another sources [6][7][8] the most used degassing methods are rotor devices, which reduce the hydrogen content to the desired values within a few minutes. The principle of the machine consists in blowing an inert gas into the molten alloy bath.…”