Fig. 1 Sand mold (a: casting mold; b: frozen sand mold; c: sprue; d: melt flow pipe)

Fig. 2 Rapid freezer (a: export of liquid nitrogen; b: frozen sand mold; c: temperature control panel; d: liquid nitrogen flow control valve; e: liquid nitrogen tank)

Fig. 3 A Ablation-vibration devices (a: frozen sand mold; b: water inlet pipe; c: movable guides; d: ablation coolant outlet; f: thermocouple temperature measurement location); B vibration device (a: frozen sand mold; b: mold fixed wall panel; c: clamp; d: vibration spring; e: motor)

Fig. 4 Freeze ablation process flow diagram

Fig. 5 Schematic of NbB2 particle extraction (a: Al-NbB2 intermediate alloy sample, b: NbB2 powder)

Fig. 6 Cooling rate and solidification time of Al-NbB2 intermetallic alloy

Fig. 7 a XRD pattern of Al-NbB2 intermetallic alloy; b metallographic micrograph of Al-NbB2 intermetallic alloy at VB; c XRD pattern, d SEM image and EDS results of extracted NbB2 powder

Fig. 8 Microstructure of the intermediate alloy “V” in Al-NbB2 and the size distribution of NbB2 particles

Fig. 9 Refinement effect of Al-NbB2 intermediate alloys with different contents on Al-Cu alloys

Fig. 10 Comparison of grain size at different positions of sample W after solution treatment

Fig. 11 TEM images of NbB2 particles in their standard morphology (taken from location W + VC). b Enlarged image of the red box in a; c-e EDS maps of a; f high-magnification image of the interface between the Al matrix and the NbB2 particles in region A; g localized enlarged photograph of region A in f; h FFT micrograph of the NbB2 particles; i SAED pattern of Al

Fig. 12 TEM images of NbB2 particles in aggregated morphology (from W + VA). a NbB2 particles in aggregated state; b enlarged image of the red box in a; c-e EDS maps of a; f high-magnification image of the interface between the Al matrix and the NbB2 particles in region A; g FFT micrograph of NbB2 particles; h localized magnified photograph of region A in f

Fig. 13 SEM images of cast sample W (+ 0.8 wt% VA/VB/VC). a SEM image at W + 0.8 wt% VA; b SEM image at W + 0.8 wt% VB; c SEM image at W + 0.8 wt% VC; d1, d2, and d3 EDS maps of d

Fig. 14 a Metallic fluidity test mold; b spiral fluidity casting samples; c plot of variation of Al-NbB2 intermediate alloy on Al-Cu alloy at different sites

Fig. 15 Schematic of alloy thermal tear test mold and related parameters for HCS calculation

Fig. 16 Morphologies of hot cracking test with the addition of NbB2 particles of different sizes and variation of HCS: a Al-Cu; b Al-Cu + VA; c Al-Cu + VB; d Al-Cu + VC; e chart of HCS changes

Fig. 17 Mechanical properties of Al-Cu alloys containing NbB2 particles of different sizes

Fig. 18 Solidification mode of Al-Cu alloys: a columnar growth; b equiaxial growth with addition of NbB2 particles

Fig. 19 Simulation results of effective stress in castings based on Procast software: a solidification time of the casting; b1-b2 distribution of the effective stress in the casting; c variation of the effective stress in the casting

Fig. 20 Schematic of grain refinement and force changes at grain boundaries

Fig. 21 Simulation chart of the solid phase fraction in the hot tearing formation region and solid phase fraction curves for various points