A380 aluminum alloy is widely used worldwide because of its physical, chemical, casting, and mechanical properties. During the machining process of a batch of A380 aluminum alloy parts produced by the die-casting method, it was found that some parts on the surface of the parts were difficult to machine, the surface machining quality was poor, and the tool was seriously worn. The eye can be observed after processing the surface. There is an uneven distribution of bright spots and less than 5 mm × 2 mm of several surface flaking, but not find common aluminum alloy die-casting pinhole defects were present.
For this reason, the quality of the aluminum alloy parts was inspected, and the die-casting defects were detected and analyzed by scanning electron microscope and X-ray spectrometer.
The chemical composition of aluminum alloy parts was analyzed by a German MAX type direct reading spectrometer according to GB/T7999-2007 “Aluminum and aluminum alloy photoelectric direct reading emission spectroscopy analysis method.”
The pinhole degree of aluminum alloy parts is tested according to GB 10851-1989 “Casting Aluminum Alloy Pinhole.”
Appearance quality, fracture organization
The appearance quality and fracture organization of aluminum alloy parts are tested according to GB/T8733-2007 “Casting Aluminum Alloy Ingot.”
Scanning electron microscopy and X-ray energy spectrum analysis
Scanning electron microscopy and X-ray energy spectrum analysis of the defective parts of aluminum alloy parts were performed by Hitachi S3400N scanning electron microscope and HORIBA EX-250 X-ray energy spectrometer, respectively.
The total weight of this batch of aluminum alloy parts is about 21,000 kg, divided into 4 oven batches. The test results are shown in Table 1. the test results show that the chemical composition of this batch of aluminum alloy ingots fully conforms to the ASTM B179 and GB/T 8733-2007 standards on the chemical composition of A380.1 aluminum alloy ingots.
The results of the pinhole degree test of the aluminum alloy ingot are 1~2; see Figure 1, which shows that the content of gas (main hydrogen) in the ingot is low. It is by the regulations of GB10851-1989 “Casting Aluminum Alloy Pinhole.”
The appearance of aluminum alloy parts has a clean surface, no mold corrosion, foreign inclusions, delamination, and cracks, in accordance with GB/T8733-2007 “Casting Aluminum Alloy Ingot.”
Aluminum alloy parts fracture organization is dense, silvery-white, with no slag and foreign inclusions, in line with GB/T8733-2007 “casting aluminum alloy ingot.”
Sample surface spalling defects analysis
Selected sample surface spalling area under the electron microscope observation found that the defective organization is not dense, with the characteristics of loose defects, see Figure 2, 3; fracture is along the characteristics of the crystal fracture, see Figure 4.
The sample is broken, the fracture organization is observed by scanning electron microscope, and the morphology is shown in Figure 5. From Figure 5, about 1 mm below the die-casting surface, there is a common aluminum alloy die-casting “cold bean” defect; there is an aggregation of slag under the die-casting surface. The X-ray spectrometer point analysis results of the two cold beans are shown in Table 2 and Figures 6 and 7; the composition is not the same. The results of the energy spectrum analysis of the aggregated slag are shown in Figure 8, which shows that the impurities, such as C, O, Fe, Ca, etc. are very high.
Die casting defects cause analysis
Hard spot, bright spot, and matrix spalling causes
For Al-Si-Cu type aluminum alloy, due to the die-casting process, alloy solidification, and other factors, it will cause the composition and organization of the segregation. The eutectic silicon concentration is the hard spot (HV 1 320), the part with less eutectic silicon and more an (Al) solid solution has lower hardness and sticky tool. The a(Al) deformation under the cutting force makes the white spot appear on the processing surface. The area of Cu element deviation is easy to form the coarse Al2Cu phase (HV 560), which is also a kind of hard phase and bright white color. The impurity elements, especially Fe, mixed into the melt, Fe and Al to generate the FeAl3 phase is a hard point (HV960), or generate AlFeSi phase, hardness HV578. These harder phases will make the substrate organization brittle, causing tool wear, machining difficulties, and even making the substrate spalling.
loosening and a small amount of porosity generated causes
The die-casting surface did not find the common pinhole defects in the aluminum alloy; aluminum casting alloy ingot pinhole degree of re-examination indicates that the ingot in the gas content is low, and die-casting produce loose condition than sand much less, a small amount of loose and porous mainly related to the casting process.
Inclusions, slag generation reasons
Oxide inclusions are due to the reaction between the aluminum liquid and furnace gas O2, CO, CO2, H2O, etc., generating Al2O3 inclusions and improper pouring operation caused. And slag is due to the melt in the deterioration treatment; the melt refining cover process does not let the melt and slag fully up to exclude, melting tools are not clean, the coating on the mold is not firm, brought into the melt or cavity will make the casting produce inclusions and slag defects.
Cold beans generated causes
Cold beans are one of the main defects of die casting; cold beans are usually located under the skin of the casting or embedded inside the casting; it is related to the poor design of the pouring system, die-casting speed is too fast. In liquid metal injection cavity spatter, metal beads quickly condensed crystallization due to the surface of the oxide film so that it can not be fused with the surrounding liquid but embedded in the matrix, and retain a clear interface and small gaps. Its fast cooling speed and hardness will cause uneven cutting force and reduce tool life. The cold bean causes uneven local stress inside the substrate, and it will become the source of cracks when stressed, as shown in Figure 9, which is the secondary crack caused by the cold bean as the source cracks. The batch of aluminum alloy castings very close to the two cold bean compositions is very different; one of the cold beans in the C, O, Pb, Fe, Zn, Ca
The content of impurity elements such as C, O, Pb, Fe, Zn, and Ca in one of the cold beans was very high, indicating that the aluminum alloy melt was contaminated.
The defects such as bright spots, holes, surface flaking, and cold beans that appear in the A380 aluminum alloy parts after machining are caused by the die-casting process, alloy solidification, improper pouring operation, or contamination of the aluminum al