Hydrogen Content Of Low Pressure Casting A356 Aluminum Alloy Melt And Its Influence On Casting Pinholes

A356 aluminum alloy has the characteristics of low density, excellent formability and corrosion resistance, and has important application value in the field of automobile lightweight. Due to the hydrogen absorption characteristics of aluminum melt, it is easy to mix with water vapor in the air during smelting and casting production The reaction increases the hydrogen content of the melt. The solubility of hydrogen in liquid aluminum is about 19.1 times that in solid aluminum. During the solidification process, excess hydrogen will be precipitated in the form of gas to form defects such as pinholes and pores, resulting in rough casting surface As the degree increases, the casting performance and serviceability will deteriorate. Therefore, in order to obtain high-quality aluminum alloy castings, the hydrogen content in the aluminum melt must be strictly controlled.

At present, the research on the control of hydrogen content in aluminum melt is mainly focused on the degassing refining process, which mainly includes gas purging, rotary jet degassing, vacuum degassing and ultrasonic degassing, among which the most widely used is rotary jetting degassing. There are many new technologies in the purification and degassing of melts at home and abroad, such as RID method, Alpur method and RDU method, which greatly improve the degassing efficiency of melts and can reduce the hydrogen content in aluminum melts to 0.10mL/(100gAl). However, compared with the degassing process that has been extensively studied, less attention has been paid to the hydrogen absorption behavior of the aluminum melt in the subsequent process of degassing and refining, especially for the fact that there are many There are few reports on the hydrogen absorption behavior of aluminum melts under the influence of factors.

Associate Professor Zhang Huarui of Ningbo Innovation Research Institute of Beihang University and others published a paper entitled "The Hydrogen Content of A356 Aluminum Alloy Melt in Low Pressure Casting and Its Effect on Casting Pinhole The article analyzes the changes of hydrogen content of A356 aluminum alloy melt during standing, transfer and casting, and explores the influence of factors such as absolute water content of air, dew point temperature of compressed air, Sr content and other factors on the hydrogen absorption of aluminum melt Influenced by the analysis of casting pinhole defects under different hydrogen content of the melt. The results show that the transfer link is a relatively serious part of the hydrogen absorption, and the absolute water content of the air and the Sr content are the main factors affecting the hydrogen absorption of the melt. At different hydrogen contents Under these conditions, the casting contains both point-like pinholes and reticular pinholes distributed between dendrites, and the porosity increases with the increase of hydrogen content in the melt. Under the premise of ensuring the modification effect, reducing the addition of Sr is beneficial to Reduce the hydrogen content of the melt and reduce the generation of pinholes in castings.

【Test materials and scheme】

The raw materials mainly include A356 aluminum alloy (see Table 1 for composition), Al-5Ti-1B refiner, Al-10Sr modifier, solid refining agent (mainly composed of KCl and NaCl) and high-purity nitrogen (purity ≥ 99.99%).
After the A356 aluminum ingot is melted, it is transferred to a transfer ladle for degassing refining (refining agent + rotary blowing nitrogen gas), as well as refinement and modification treatment; The transfer bag is injected into the holding furnace of the low-pressure casting machine. According to the actual production cycle requirements, it is transferred once every 2 hours, and the injection time is 30s each time. At the same time, compressed air is introduced into the holding furnace, and the aluminum melt is under pressure from bottom to top Enter the cavity, and finally cool and solidify to form a casting. The aluminum melt is sampled and analyzed in the static, transfer and low-pressure casting links after the degassing and refining are completed, and the absolute water content of the air, Sr content, and the flow rate of each sample are recorded. Parameters such as the dew point temperature of the compressed air entering the holding furnace.
The hydrogen content of the aluminum melt is measured by the decompression solidification method. The specific steps are: put the crucible containing the aluminum melt into the vacuum chamber of the decompression solidification hydrogen detector for decompression. Weigh and calculate the density of the sample after decompression and solidification.

Direct reading spectrometer (ARLeasySpark1160) high-frequency inductively coupled plasma spectrometry was used to measure Sr content in aluminum melt. Metallographic microscope was used to observe the microstructure of castings, and samples were prepared by standard metallographic techniques. The surface of the sample is polished to a mirror surface with sandpaper, and the metallographic structure is observed in a non-corroded state. The pinhole rate and the average diameter of the pinhole are calculated using ImageJ software, and no less than 10 metallographic photos are randomly taken from each sample for statistics ｡

【Graphic results】
Fig.1 Changes of hydrogen content in aluminum melt under different air absolute water contents in the static stage

Fig.2 Changes of hydrogen content in aluminum melt under different Sr content conditions in the static process


Fig.3 Changes of hydrogen content in aluminum melt under different air absolute moisture content in the transfer process
Fig.4 Changes of hydrogen content in aluminum melt under different Sr content conditions in the transfer link

(a) 12:00~24:00 time period

(b) 0:00~12:00 time period

Fig.5 Effect of dew point temperature of compressed air on hydrogen content of melt in casting process
(a) 0.15mL/(100gAl), low power

(b) 0.15mL/(100gAl), high power

(c) 0.26mL/(100gAl), low power

(d) 0.26mL/(100gAl), high power

(e) 0.52mL/(100gAl), low power

(f) 0.52mL/(100gAl), high power

(g)0.70mL/(100gAl), low power

(h) 0.70mL/(100gAl), high power

(i) 0.76mL/(100gAl), low power

(j) 0.76mL/(100gAl), high power


When the absolute water content of the air was (17.21±2.26) g/m3 and the Sr content was 0.01%, the hydrogen content of the aluminum melt in the transfer process increased from 0.15 mL/(100 gAl) to 0.76 mL/(100 gAl), an increase of 4.07 times. During the transfer process, the aluminum melt can be fully in contact with the air is the premise of hydrogen absorption. The high absolute water content of the air and the Sr content of the melt are the main reasons for the large amount of hydrogen absorption of the melt. The premise of ensuring the metamorphic effect Under these conditions, appropriately reducing the amount of Sr added is beneficial to reducing the hydrogen content of the aluminum melt.

There are mainly two types of pinholes in castings: point shape and network shape. The pinhole rate and average pore size increase with the increase of the hydrogen content of the melt as a whole. When the hydrogen content of the melt increases from 0.15mL/(100gAl) to 0.76mL /(100gAl), the pinhole rate increased from 0.68% to 2.48%, and the average pore diameter increased from 60μm to about 100μm.

Citation format for this article:

Zhang Huarui, Ma Hanyu, Du Ming, et al. Hydrogen content of low-pressure casting A356 aluminum alloy melt and its influence on casting pinholes[J]. Special Casting and Nonferrous Alloys, 2022,42(11):1 321-1 325 .

ZHANG H R,MA H Y,DU M,et al.Hydrogen content of low pressure casting A356 aluminum alloy melt and its influence on pinhole sin castings[J].Special Casting & Nonferrous Alloys,2022,42(11):1 321-1 325.