Semi-Solid Forging Process of Aluminum Alloy Connecting Rods for the Hydrogen Internal Combustion Engine  
[PERSON]  
1 School of Mechanical Engineering, Xi'an Jiaotong University, Xi'an 710049, China; [EMAIL_ADDRESS]. As one of the important force transmission parts for the internal combustion engine, the connecting rod is often machined and manufactured with steel, cast iron, iron-based powder, and other materials [14; 15; 16; 17]. With the increasing demand for lightweight automotive components in the market, high-strength aluminum alloy materials have attracted extensive attention from many scholars and businessmen in the connecting-rod-manufacturing field [18; 19; 20]. As a representative of high-strength aluminum alloy, the 7075 aluminum alloy has numerous merits, such as low density, lightweight, and high strength. It is, therefore, gradually applied to manufacturing various automobile parts and is regarded as a promising manufacturing material for the connecting rod of a HICE. The mechanical properties of connecting rods for HICEs not only depend on the material itself but are also affected by the manufacturing process. Traditional manufacturing processes for connecting rods include hot forging [21; 22; 23], powder metallurgy [24], liquid-die forging [25], and die casting [26]. Although the above methods can obtain connecting rods with high dimensional accuracy and qualified mechanical properties, these methods have some shortcomings as follows. The forging force for the hot-forging process is large, requiring special large equipment and high equipment costs [27]. The die-casting process is prone to shrinkage porosity, shrinkage cavity, insufficient filling, and other defects within the connecting rod. The costs of molds and samples for the powder metallurgy process in small-batch production are high. In terms of liquid-die forging, it is risky to premature solidification at thin-walled and fine structures, which affects the quality of the connecting rods. It is a challenge for the liquid-die forging to produce the connecting role with a reduced scrap rate. Therefore, there is an urgent need to explore a new connecting-rod forming process for providing an improved solution for the manufacturing of 7075 aluminum alloy connecting rod applications in the HICE. Semi-solid forming of metal (SSFM) is an innovative metal-forming technique initially introduced by Professor [PERSON] and his team at MIT [28]. This method has since been extensively applied and explored by various researchers [29; 30; 31]. In the SSFM process, semi-solid materials with fine and spherical grains are first produced. Subsequently, components with refined microstructures and superior performance can be fabricated through techniques such as extrusion, forging, rolling, or die casting [32; 33]. The SSFM process can also operate through stages, including the first cooling or solidifying the semi-solid blank into a solid blank and then reheating and forming the solid blank with suitable dimensions according to the production demands [34; 35; 36]. The SSFM process combines the advantages of liquid casting forming and solid plastic forming, as it has excellent flow mobility in the material and high physical properties in the product, which is regarded as one of the most potential green forming technologies [37; 38; 39]. The SSFM process mainly contains procedures, which are the preparation of the semi-solid blank and the formation of a part, which has received extensive attention in recent years [40; 41; 42; 43]. [PERSON] et al. [44] investigated the influences of treatment temperature and  
Figure 1: Shape and dimension of connecting rod for the HICE. maintaining time on the microstructures of hot extruded 7075 aluminum alloy during the isothermal heat treatment (IHT).