1. Macroscopic Factors Contributing to Crack Formation
1.1 During semi-continuous casting, cooling water is directly sprayed onto the ingot surface, creating a steep temperature gradient within the ingot. This results in uneven contraction among different regions, causing mutual restraint and generating thermal stresses. Under certain stress fields, these stresses can lead to ingot cracking.
1.2 In industrial production, ingot cracking often occurs at the initial casting stage or originates as microcracks that later propagate during cooling, potentially spreading throughout the entire ingot. In addition to cracking, other defects such as cold shuts, warping, and hanging may also occur during the initial casting stage, making it a critical phase in the entire casting process.
1.3 The susceptibility of direct chill casting to hot cracking is significantly influenced by chemical composition, master alloy additions, and the quantity of grain refiners used.
1.4 The hot cracking sensitivity of alloys is mainly due to internal stresses that induce the formation of voids and cracks. Their formation and distribution are determined by alloying elements, melt metallurgical quality, and semi-continuous casting parameters. Specifically, large-sized ingots of 7xxx series aluminum alloys are particularly prone to hot cracking due to multiple alloying elements, wide solidification ranges, high casting stresses, oxidation segregation of alloy elements, relatively poor metallurgical quality, and low formability at room temperature.
1.5 Studies have shown that electromagnetic fields and alloying elements (including grain refiners, major alloying elements, and trace elements) significantly affect the microstructure and hot cracking susceptibility of semi-continuously cast 7xxx series alloys.
1.6 Additionally, due to the complex composition of 7050 aluminum alloy and the presence of easily oxidized elements, the melt tends to absorb more hydrogen. This, combined with oxide inclusions, leads to the coexistence of gas and inclusions, resulting in a high hydrogen content in the melt. Hydrogen content has become a key factor affecting inspection results, fracture behavior, and fatigue performance of processed ingot materials. Therefore, based on the mechanism of hydrogen presence in the melt, it is necessary to use adsorption media and filtration-refining equipment to remove hydrogen and other inclusions from the melt to obtain a highly purified alloy melt.
2. Microscopic Causes of Crack Formation
2.1 Ingot hot cracking is primarily determined by the rate of solidification shrinkage, feeding rate, and the critical size of the mushy zone. If the size of the mushy zone exceeds a critical threshold, hot cracking will occur.
2.2 Generally, the solidification process of alloys can be divided into several stages: bulk feeding, interdendritic feeding, dendrite separation, and dendrite bridging.
2.3 During the dendrite separation stage, dendrite arms become more closely packed and liquid flow is restricted by surface tension. The permeability of the mushy zone is reduced, and sufficient solidification shrinkage and thermal stress may lead to microporosity or even hot cracks.
2.4 In the dendrite bridging stage, only a small amount of liquid remains at triple junctions. At this point, the semi-solid material has considerable strength and plasticity, and solid-state creep is the only mechanism to compensate for solidification shrinkage and thermal stress. These two stages are the most likely to form shrinkage voids or hot cracks.
3. Preparation of High-Quality Slab Ingots Based on Crack Formation Mechanisms
3.1 Large-sized slab ingots often exhibit surface cracks, internal porosity, and inclusions, which severely impact the mechanical behavior during alloy solidification.
3.2 The mechanical properties of the alloy during solidification largely depend on internal structural features, including grain size, hydrogen content, and inclusion levels.
3.3 For aluminum alloys with dendritic structures, the secondary dendrite arm spacing (SDAS) significantly affects both mechanical properties and the solidification process. Finer SDAS leads to earlier porosity formation and higher porosity fractions, reducing the critical stress for hot cracking.
3.4 Defects such as interdendritic shrinkage voids and inclusions severely weaken the toughness of the solid skeleton and significantly reduce the critical stress required for hot cracking.
3.5 Grain morphology is another critical microstructural factor influencing hot cracking behavior. When grains transition from columnar dendrites to globular equiaxed grains, the alloy exhibits a lower rigidity temperature and improved interdendritic liquid permeability, which suppresses pore growth. Additionally, finer grains can accommodate larger strain and strain rates and present more complex crack propagation paths, thereby reducing the overall hot cracking tendency.
3.6 In practical production, optimizing melt handling and casting techniques—such as strictly controlling inclusion and hydrogen content, as well as grain structure—can improve the internal resistance of slab ingots to hot cracking. Combined with optimized tooling design and processing methods, these measures can lead to the production of high-yield, large-scale, high-quality slab ingots.
4. Grain Refinement of Ingot
7050 aluminum alloy primarily uses two types of grain refiners: Al-5Ti-1B and Al-3Ti-0.15C. Comparative studies on the in-line application of these refiners show:
4.1 Ingots refined with Al-5Ti-1B exhibit significantly smaller grain sizes and a more uniform transition from the ingot edge to the center. The coarse-grained layer is thinner, and the overall grain refinement effect is stronger across the ingot.
4.2 When raw materials previously refined with Al-3Ti-0.15C are used, the grain refinement effect of Al-5Ti-1B is diminished. Furthermore, increasing the Al-Ti-B addition beyond a certain point does not proportionally enhance grain refinement. Therefore, Al-Ti-B additions should be limited to no more than 2 kg/t.
4.3 Ingots refined with Al-3Ti-0.15C consist mainly of fine, globular equiaxed grains. Grain size is relatively uniform across the width of the slab. An addition of 3–4 kg/t of Al-3Ti-0.15C is effective in stabilizing product quality.
4.4 Notably, when Al-5Ti-1B is used in 7050 alloy, TiB₂ particles tend to segregate toward the oxide film on the ingot surface under rapid cooling conditions, forming clusters that lead to slag formation. During ingot solidification, these clusters shrink inward to form groove-like folds, altering the surface tension of the melt. This increases melt viscosity and reduces fluidity, which in turn promotes crack formation at the base of the mold and the corners of the broad and narrow faces of the ingot. This significantly raises the cracking tendency and negatively impacts the ingot yield.
4.5 Considering the forming behavior of 7050 alloy, the grain structure of similar domestic and international ingots, and the quality of the final processed products, Al-3Ti-0.15C is preferred as the in-line grain refiner for casting 7050 alloy—unless specific conditions require otherwise.
5. Grain Refinement Behavior of Al-3Ti-0.15C
5.1 When the grain refiner is added at 720 °C, the grains consist primarily of equiaxed structures with some substructures and are the finest in size.
5.2 If the melt is held too long after adding the refiner (e.g., beyond 10 minutes), coarse dendritic growth dominates, resulting in coarser grains.
5.3 When the addition amount of grain refiner is 0.010% to 0.015%, fine equiaxed grains are achieved.
5.4 Based on the industrial process of 7050 alloy, the optimal grain refinement conditions are: addition temperature around 720 °C, time from addition to final solidification controlled within 20 minutes, and refiner amount at approximately 0.01–0.015% (3–4 kg/t of Al-3Ti-0.15C).
5.5 Despite variations in ingot size, the total time from adding the grain refiner after melt exit, through the in-line system, trough, and mold, to final solidification is typically 15–20 minutes.
5.6 In industrial settings, increasing the amount of grain refiner beyond a Ti content of 0.01% does not significantly improve grain refinement. Instead, excessive addition leads to Ti and C enrichment, increasing the likelihood of material defects.
5.7 Tests at different points—degas inlet, degas outlet, and casting trough—show minimal differences in grain size. However, adding the refiner directly at the casting trough without filtration increases the risk of defects during ultrasonic inspection of processed materials.
5.8 To ensure uniform grain refinement and prevent refiner accumulation, the grain refiner should be added at the inlet of the degassing system.
Post time: Jul-16-2025
