High-pressure die casting (HPDC) has advantages such as high efficiency and complex forming capabilities, and is widely used for complex aluminum alloy housing parts, such as engine blocks and gearbox housings. To improve as-cast strength of die-cast aluminum alloys, alloy composition optimization and heat treatment are commonly used as strengthening methods. Considering impact of alloy composition changes on fluidity, actual range of alloy composition adjustment is limited. Furthermore, due to inherent internal gas entrapment problem in conventional high-pressure die-cast aluminum parts, solution aging heat treatment is not sauitable for strengthening. Vacuum die casting can solve surface bubbling problem during heat treatment process to some extent, but it incurs additional time and production costs. High-speed filling process of castings is conducive to uniform distribution of reinforcing phases in casting. Preparing reinforcing phase-strengthened aluminum alloys through high-pressure die casting, thereby improving strength of die-cast parts, is a worthwhile approach.
To ensure uniform distribution of reinforcing phase within aluminum matrix and its reinforcement, two conditions must be met: ① reinforcing phase must be wetted at interface with molten aluminum; ② there must be no brittle intermediate phase between reinforcing phase interface and molten aluminum. Commonly used particulate reinforcing phases include SiC, Al2O3, B4C, and TiB2. Compared to other ceramic reinforcing particles, TiB2 possesses advantages such as high melting point and hardness, and good chemical stability, making it considered an excellent reinforcing phase for aluminum-based composites. In recent years, research on TiB2-reinforced aluminum-based materials has mainly focused on 7000 series aluminum alloys. Researchers have prepared TiB2-reinforced aluminum-based composites using an in-situ mixed salt method, finding significant improvements in both elastic modulus and strength. TiB2-reinforced aluminum-based composites were also prepared using a high-energy ball milling + discharge plasma sintering + hot extrusion process, resulting in significantly improved performance. Furthermore, there are a few research reports on preparation of TiB2 particulate-reinforced die-cast aluminum alloy composites using a high-pressure die-casting process. Studies have found that directly adding TiB2 nanoparticles to AlSi9Cu2Mg die-casting alloy significantly refines grain size, resulting in a substantial increase in both yield strength and tensile strength. However, there are no reports on preparation of TiB2-reinforced die-casting aluminum alloy composites using intermediate alloys. Existing research indicates that Al-5Ti-B grain refiners include Al3Ti and TiB2 particles. Al3Ti phase is unstable in high-temperature melts (dissolving into molten aluminum at a rate of 40 μm/min at 720 ℃), while TiB2 particles are almost insoluble. This study uses industrially common ADC12 die-casting aluminum alloy as matrix alloy and Al-5Ti-1B grain refiner as source of TiB2 particle reinforcement. TiB2/ADC12 aluminum alloy materials were prepared using conventional high-pressure die casting processes. Influence of Al-5Ti-1B content on microstructure and mechanical properties of ADC12 die-casting alloy was investigated, aiming to provide a reference for its application.
A certain mass of pure Al (99.7% by mass), K2TiF6 powder (chemically pure), and KBF4 powder (chemically pure) were weighed out in a mass ratio of 3.9:1:2.4. Weighed K2TiF6 and KBF4 powders were mixed and thoroughly ground to obtain a mixed powder, which was then dried in a 200 ℃ oven for 2 h. After pure aluminum melted in a resistance furnace, mixed powder was added to pure aluminum melt at 850 ℃ in several batches while continuously stirring. Byproducts from molten salt reaction were removed to obtain an aluminum melt, which was then allowed to stand in resistance furnace for 30 min. Slag was removed, and mixed melt was poured into a rod-shaped metal mold (preheated to 200 ℃) to obtain Al-5Ti-1B master alloy.
ADC12 matrix alloy was divided into four groups and placed in graphite crucibles, then melted in a resistance furnace at 720 ℃. After alloy was completely melted, Al-5Ti-1B master alloys with mass fractions of 1%, 3%, 5% were added to ADC12 melt, respectively, and mixture was continuously stirred to form TiB2/ADC12 aluminum alloys. A slag remover and degassing agent was added, slag was skimmed off, and mixture was allowed to stand in resistance furnace for 20–30 min. Finally, resulting aluminum melt was die-cast using a cold chamber die-casting machine (Toshiba Machine DC-350J-MSmodel). Die-casting mold and die-cast sample are shown in Figure 1.
Microstructure of material was characterized using a Zeiss Axio Imager 2 optical microscope. Microstructure and fracture morphology were observed, compositional analysis was performed using a NANO SEM430 scanning electron microscope and energy dispersive spectroscopy. Phase analysis was conducted using a D8 ADVANCE X-ray diffractometer with a CuKα radian source, scanning angle range of 10°–80° [45 kV, step size 2 (°)/min, 2θ = 100°]. Hardness testing was performed using a Huayin HBRVS-187.5 digital display Brinell hardness tester, with a loading load of 20 N and a loading time of 5 s. Five points were measured for each sample, and average value was taken. Room temperature tensile tests were conducted using a WDW-100G microcomputer-controlled electronic universal testing machine at a tensile rate of 1 mm/min. Tensile specimens were lath-shaped, and specimen dimensions are shown in Figure 2.
 
Figure 1. Die-casting mold and die-casting sample
 
Figure 2. Schematic diagram of tensile sample dimensions
Figures 3 and 4 show the XRD patterns and microstructures of Al-5Ti-1B master alloy prepared by molten salt method, respectively. It can be seen that Al-5Ti-1B master alloy contains three phases: Al, Al3Ti, and TiB2. Based on relevant studies, Al3Ti phase is mainly distributed in a blocky form in matrix; nanoscale TiB2 particles exist in aggregate form, mainly distributed along α-Al grain boundaries.
 
Figure 3. XRD pattern of Al-5Ti-1B master alloy
 
Figure 4. Microstructure of Al-5Ti-1B master alloy
Figure 5 shows typical metallographic structures of TiB2/ADC12 aluminum alloys with different contents of Al-5Ti-1B master alloy. Figure 6 shows α-Al grain size statistics. Figure 7 shows SEM morphology of TiB2/ADC12 aluminum alloys with different contents of Al-5Ti-1B master alloy. Figure 8 shows magnified morphology and EDS spectrum of a local area corresponding to Figure 7. EDS analysis reveals that coarse bulk phase mainly contains Fe, Mn, and Si, while acicular phase mainly contains Al and Ti. It is speculated that acicular phase is primarily Al3Ti phase formed during solidification. Combined with relevant research, bulk phase is mainly α-AlFeMnSi phase. These results indicate that Al-5Ti-1B can nucleate and refine α-AlFeMnSi phase.
 
Figure 5: Metallographic images of TiB2/ADC12 aluminum alloys with different Al-5Ti-1B contents.
 
Figure 6: Average grain size of α1-Al and α2-Al in TiB2/ADC12 aluminum alloys with different Al-5Ti-1B contents.
 
Figure 7: SEM morphology of TiB2/ADC12 aluminum alloys with different Al-5Ti-1B contents.
 
Figure 8: Magnified morphology and EDS spectrum of area corresponding to Figure 7.
Figure 9: XRD pattern of TiB2/ADC12 aluminum alloy. It can be seen that phases in ADC12 matrix alloy are mainly composed of Al and Si phases. With increase of Al-5Ti-1B content, Al3Ti diffraction peaks appear in diffraction pattern, and these peaks gradually increase in intensity with increasing content, consistent with SEM analysis results. Furthermore, due to low TiB2 content, no diffraction peaks were observed in alloy material. Figure 10 shows hardness changes of TiB2/ADC12 aluminum alloy samples with different Al-5Ti-1B contents.
Figure 11 shows typical tensile curves and mechanical properties of TiB2/ADC12 aluminum alloy. Figure 12 shows room temperature tensile fracture morphology of TiB2/ADC12 aluminum alloys with different Al-5Ti-1B master alloy contents. It can be seen that fracture surface is dominated by mixed fracture, including a mixture of dimple and intergranular fracture structures.
 
Figure 9. XRD patterns of TiB2/ADC12 aluminum alloys with different Al-5Ti-1B addition amounts.
 
Figure 10. Hardness of TiB2/ADC12 composite samples with different Al-5Ti-1B addition amounts.
 
Figure 11. Mechanical properties of TiB2/ADC12 aluminum alloys with different Al-5Ti-1B addition amounts.
 
Figure 12. Room temperature tensile fracture morphology of TiB2/ADC12 aluminum alloys with different Al-5Ti-1B contents.
Conclusions:
(1) Adding Al-5Ti-1B master alloy to ADC12 die-cast aluminum alloy can effectively refine coarse primary crystalline phase (α1-Al) and α-AlFeMnSi intermetallic compounds, but has no significant refining effect on fine secondary crystalline phase (α2-Al).
(2) When amount of added Al-5Ti-1B exceeds 5%, α1-Al no longer refines, and needle-like coarse Al3Ti phases appear in microstructure.
(3) When amount of Al-5Ti-1B added is 3%, hardness (HBW), tensile strength, yield strength and elongation reach optimal values of 110, 290 MPa, 179 MPa and 6.2%, respectively.