CHEMICAL COMPOSITION IMPROVEMENT OF ALUMINUM-IRON BRONZE INDUSTRIAL CASTING

Introduction. Interstate standard GOST 493 provides for the maximum allowable zinc content in bronze BrA9Zh3L not more than 1 (wt.) %. Zinc in aluminum-iron bronze composition has controversial influence on casting technological and mechanical properties. The specified element improves material castability, but, in certain amount, leads to product embrittlement. Problem statement. In the present work the problem of effective amount of zinc determination for casting bronze BrA9Zh3L properties improving without negative affecting the plastic and impact characteristics of foundry products has been solved. Purpose. The purpose of this study was an evaluation of Zn content influence on BrА9Zh3L bronze structure and mechanical properties and determination of its rational doping. Materials and Methods. Aluminum-iron bronze BrА9Ж3Л according to GOST 493, alloyed with zinc in an amount 0...4 (wt.) %. Fractography of destroyed impact samples surfaces and the products microstructure have been studied according to ASTM E3 − 11 (2017) requirements. Mechanical static tensile tests were carried out according to GOST 1497, impact toughness according to GOST 9454. Results. It has been established that the zinc content increasing over then 0,2 % by weight in bronze BrA9Zh3L composition effects not only on significant strength decreasing, but also on sharp ductility dropping. The reason for such regularity is the number of eutectoid component in BrA9Zh3L structure increasing. Conclusions. Active loosing of BrA9Zh3L bronze plasticity and ductility, associated with alloy structural state changing, has been recorded in the range of 0,2...0,6 (wt.) % Zn. For industrial cast products maximum Zn content in bronze BrA9Zh3L has been recommended to limit by 0,2 (wt.) % against normatively stipulated 1,0 (wt.) % as per GOST 493.

Problem Statement. At present, data on the effect of Zn on BrA9Zh3L bronze structure and mechanical properties are missing or fragmentary. Therefore, investigation aimed to discovering the reasons of bronze BrA9Zh3L embrittlement with Zn content increasing is relevant [7].
Purpose. The aim of this study was an evaluation of Zn content (up to 4, 0 (wt.) %) influence on BrА9Zh3L bronze structure and mechanical properties and determination of its rational doping.
Materials and Methods. In this study castings of cylindrical from BrA9Zh3L bronze as per GOST 493 [1] without heat treatment have been investigated. Castings have been obtained as a result of pre-deoxidized melt of bronze (the melt overheating 100 ± 15 °C), pouring into single steel chill mold (without protective surface coating) with an initial temperature 200 ± 10 °C. Size of chill mold cavity -16100 mm.
Subsequently, samples have been prepared from these cylindrical castings for mechanical tensile testing [8]. For impact toughness determination samples were cast "in size" into quartz ceramic shell forms (CFS) with an initial temperature 950 ± 15° C. Mesnager-type U-notched specimens for impact testing have been mechanically manufactured [9] after castings separation from pouring gating system. Table 1 demonstrates content of chemical elements in bronze test pieces. Results and Discussion. Results of the study of Zn effect (up to 4, 0 (wt.) %) on the properties of bronze BrA9Zh3L indicate that with Zn content in bronze BrA9Zh3L increasing (Figure 1, a) the level of tensile strength ( В ) monotonously decreases. But with Zn content increasing up to 2, 5 (wt.) % the values of yield strength ( 0,2 ) and elasticity ( 0,05 ) increase. At the same time the difference between  В and  0,2 values continuously decreases with zinc content increasing in bronze reaching a minimum at content Zn = 4 (wt.) %. This regularity suggests that Zn content in bronze increasing leads to its fragility.
Appearance of bronze samples containing more than 0,2 (wt.) %Zn fracture surfaces shows signs of brittle fracture and have cleavage zones in fragile structural component areas. This indicates a confident tendency for cold cracks in castings of such alloys occurrence.
Microstructural investigation results [11] of tested bronzes show amount of eutectoid structural component continuous increasing with Zn content in bronze rising and corresponding decreasing in -phase amount with simultaneous changing its morphology to dendritic.
Therefore, results of metallographic analysis (see Fig. 2) indicate that exactly amount of eutectoid component and α-Сu crystals morphology are the main factors for BrA9Zh3L bronze ductile and viscous properties levels sharp decreasing when changing chemical composition in Zn. It follows from above that level of BrA9Zh3L bronze mechanical properties are fundamentally dependent on zinc content in it and, as a result, on structural state (see Fig. 2). In this regard, maximum content of Zn, according to results of the present research, has been recommended to limit by 0,2 (wt.) % instead of normatively stipulated 1,0 (wt.) % as per GOST 493.
2. When Zn content is 0, 2 ... 0, 6 (wt.) % sharp decreasing of BrA9Zh3L bronze plastic properties levels has been discovered. 3. It has been shown that the reason for bronze BrA9Zh3L fragile destruction is increasing in its structure eutectoid component amount. 4. In order to avoid castings and, accordingly, cast parts for technical purposes embrittlement the maximum Zn content in bronze BrА9Ж3Л limitation to 0,2 (wt.) % has been recommended against normatively stipulated 1,0 (wt.) % as per GOST 493.