A Study on Semi Solid Squeeze Forging of High Strength Brass. K.H. Choe*, G.S.Cho*, K.W. Lee*, Y.J. Choi**, K.Y. Kim*** and M.H.

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1 A Study on Semi Solid Squeeze Forging of High Strength Brass K.H. Choe*, G.S.Cho*, K.W. Lee*, Y.J. Choi**, K.Y. Kim*** and M.H. Kim**** *Advanced Material R/D Center, KITECH, Dongchun-dong, Yeonsu-gu, Incheon, , Korea **Hanbat National University, San16-1, DuckMyoung-dong, Yuseong-gu, Daejeon, , Korea *** Hankuk Aviation University, 200-1, Hwajeon-dong, Deogyang-gu, Goyang-city, Gyeonggi-do, , Korea ****Inha University, 253, Yonghyun-dong, Nam-gu, Incheon, , Korea Abstract The microstructures and mechanical properties of high strength brass made by semi solid squeeze forging were investigated and compared with those of conventionally hot forged product and gravity die castings. No shrinkage or gas hole was found in squeeze forgings. Fine equiaxed crystals developed at the center of squeeze forgings, while grains in the corner of squeeze forgings were elongated perpendicular to the pressure direction. The grains of squeeze forgings were smaller than those of hot forgings and gravity die castings. It is suggested that a rapid heat transfer condition due to applied pressure is responsible for grain refinement. Tensile and yield strengths of squeeze forgings were as high as those of hot forgings but elongation was positioned between that of hot forgings and gravity die castings. Key words: Semi solid squeeze forging, High strength brass 77/1

2 Introduction The name high strength brass is given to the wrought and cast alloys indicating their particular virtue of high strength, which can be achieved by additions of Al, Fe, Mn and Sn. These alloys were formerly known, and are still sometimes referred to, as "manganese bronzes." For many applications, their extra strength is needed for parts to be used in corrosive environment[1]. Forgings made from copper base alloys offer a number of advantages over products made by other processes. Dimensional accuracy is greater than by casting, working the alloys develops improved strength, and overall cost is modest. However, because for forging more heat must be applied to the ingot which was solidified once, there are some disadvanteges in the economy of energy and time. Semi Solid Squeeze Forging is a hybrid process which the casting and the forging are done consecutively. This process is operated as follows. Firstly, the molten material is poured into the metal mold and starts to be solidified. When the thickness of solidified shell reaches at a certain level, semi-solidified material is removed from the die and transported to a hydaulic press machine. Then, the pressure was applied on the semi-solid alloys to solidify and shape final products. Becuase of the consecutive operation of casting and forging, this process has some advantages such as simple process and no need of additive heat for forging. In addition, the grain refinement is expected due to rapid cooling by forging during solidification. In this study, we investigated the microstructures and mechanical properties of high strength brass made by semi solid squeeze forging and compared them with those of conventionally hot forged product and gravity die casting. Experimental In this experiment, KS CACIn304 copper alloy(similiar with JIS HBsC4, UNS C86300) was used and its chemical composition was shown in Table 1. The alloy was molten using induction furnace. The molten material was poured into the die which was preheated at 573K. Pouring temperature was 1273K. When the ratio of solid reaches at 0.5, semi-solid material was removed from the die and transferred to a hydraulic press. In a hydraulic press, one-dimensional pressure is applied to shape final products. The applied pressure was 600MPa and the pressing time was 10 seconds. The microstructures and mechanical properties of this forgings (squeeze forging) were investigated and compared to those of hot forged product (hot forgings), and as-cast ingot (gravity die casting). Samples were etched by nitric acid(50%) and their macrostructures were observed. Microstructures were also observed. For the observation of mechanical properties, mechanical tests were done and their fracture surfaces were observed by SEM. 77/2

3 Results and Discussion Macrostructure The macrostructures of semi solid squeeze forgings, hot forgings and gravity castings are shown in Fig. 3. In squeeze forgings, the area labeled as A is the portion that remained at liquid phase before being forged and fine equiaxed crystals were found. The area labeled as C was previously solidified in the die, so the grains of these areas were deformed by applied pressure. No shrinkage or gas hole was found. Among area A, area B and area C, there were the differences in the charactersitic of solidification, so it was expected that there was discontinuity in the macrostructure, However, no distinct difference of macrostructures was found. It is considered that because the ingot was not fully solidified, grains are not deformed severely by applied pressure and previously solidified shell was annealed by the latent heat from unsolidified areas. In hot forgings, the all areas were severely elongated during forging process. In die castings, shrinkage was found at the top of castings and numbers of gas holes were also found. Microstructure Fig. 4 shows the microstructures of semi solid squeeze forgings and hot forgings. A, B, and C in Fig. 4 correspond to the areas labeled as A, B and C in Fig. 3. It was found that very little α solid solution was present in a matrix of β phase. In copper alloy, β phase is stable in binary alloys containing more than 39.5% Zn, but strong β stabilizers such as Al promote its presence at lower zinc contents[2]. Guillert[3] suggested Zn equivalent of Cu alloying elements. From this relation, Zn equivalent of the alloy used in this study is 48.4%Zn. Therefore, the matrix consists of β phase. In squeeze forging, though the area A was pressurized directly, equiaxed crystals developed. At the area B, equiaxed grains were found as well. However, the grains in the area B had facets, but in the area A grain hadn t. This means that in the area B solid state transformation and diffusion reaction occurred during cooling but grains of the area A had insufficient time for this reaction. On the other hand, grains in the area C, which were solidified in the die, were deformed perpendicular to pressure direction. Compared with the grains in same position of hot forgings, the grains in squeeze forgings have different shape. The grains in area B of hot forgings were deformed by forging pressure. On the other hand, forging pressure didn t affect the shape of grains in same position of squeeze forgings. It is considered that the area A remain as liquid phase until forging, so it acted as buffer to forging pressure. In hot forgings, all grains were deformed perpendicular to forging pressure direction and grain size was increased by applied heat during hot forging process. 77/3

4 Compared with gravity die castings, particles of α solid solution and grains of β phase matrix were finer. In the case of squeeze castings, the applied pressure and the instant contact of the molten metal with the mold surface produce a rapid heat transfer condition that yields a fine-grain casting.[4] Similarly, in the case of squeeze forgings, finer grains was found at the area whose heat transfer rate is high. Mechanical properties Mechanical properties of high strength brass made by various processes are shown in Fig. 5. In the case of squeeze forging, test results showed that its mechanical properties are equivalent to those of sand casting. Except for block, all specimens showed similar values of tensile and yield strength. The elongation of squeeze casting fell between that of forging and gravity die casting. Fig. 6 shows the fracture surface of high strength brass made by various processes. In semi solid squeeze forgings, typical ductile fracture surface was found dissimilar to in gravity casting. Similiar structure was also found in hot forgings. Conclusions The microstructures and mechanical properties of high strength brass made by semi solid squeeze forging were investigated and the following conclusions were drawn from this study. 1. No shrinkage or gas hole was found in squeeze forgings. 2. Fine equiaxed crystals developed at the center of squeeze forgings, while grains in the corner were elongated perpendicular to pressure direction. 3. Grain size of squeeze casting was smaller than that of hot forgings and gravity die castings. It is due to a rapid heat transfer condition by applied pressure. 4. Tensile and yield strengths of squeeze forgings were as high as those of hot forgings but elongation fell between that of forgings and gravity die casting References ASM International, Copper and Copper alloy, (2001), 91 Campbell J, Castings, 2nd Ed, Butterworth-Heinemann Ltd, Oxford 2003, ISBN S.Oya, Nonferrous Metals Casting, Nikan Industry News, (1968), ASM, Metals Handbook vol.15 Casting, (1998), 323 Acknowledgements Authors would like to acknowledge Gabsan Metal Co. for technical support and help with squeeze forging. 77/4

5 Tables Table 1 Chemical composition of copper alloy Cu Al Sn Zn Pb Fe Ni Mn Si Figures Fig. 1 Semi solid state before squeeze forging Fig. 2 Final product after squeeze forging (a) C (b) C A B A B 77/5

6 (c) Fig. 3 The macrostructures of semi solid squeeze forgings(a), hot forgings(b) and gravity casting(c). (a) Squeeze forging (b) Hot forging A B 200 μm C Fig. 4 The microstructures of semi solid squeeze forgings(a) and hot forgings(b) 77/6

7 Strength (MPa) Yield Strength Tensile Strength Elongation Squeeze forging 0.25 Block Forging As Cast Fig. 5 Mechanical properties of high strength brass made by various processes. 5 0 (a) Squeeze Forging (b) Hot Forging 77/7

8 (c) Gravity Casting Fig. 6 The fracture surfaces of semi solid squeeze forgings(a), hot forgings(b) and gravity casting(c) 77/8