Improving the physical, mechanical and operational characteristics of cast internal combustion engine pistons made of aluminum alloy using secondary charge

The article is devoted to improving the physical, mechanical and operational characteristics of cast internal combustion engine pistons made of aluminum alloy using secondary charge. The advantage of the proposed approach is that these parts are manufactured from any aluminum alloy using the remelting of failed internal combustion engine parts of the same name. The paper shows that while using a secondary charge in the form of remelting failed parts of the same name, the quality of cast products not only does not deteriorate, but, on the contrary, their physical and mechanical characteristics even increase. It should be noted that the high density of the alloy obtained from remelting failed pistons and the high degree of its modification confirmed the authors’ assumption that in this case refining and modification operations can be avoided. The presented approach using the remelting of failed parts makes it possible not only to save fresh charge materials, but also to preserve the chemical composition of the primary parts, and, consequently, the level of their physical, mechanical and operational characteristics.


Introduction
When operating various machines and mechanisms that include pistons, emergency cases of failure of this part are possible.Such emergency situations are especially critical when operating an internal combustion engine (ICE).For various reasons, it is not always possible to quickly replace this part (cost, complete sale, discontinuation of production of a given standard size, distance from suppliers, long delivery time, etc.) To solve this problem quickly, the authors manufactured original equipment and developed an operational technology for manufacturing pistons in both single and small-scale versions.This technology allows producing cast pistons of the required quality of almost any standard size with the required physical, mechanical and operational characteristics within just a few work shifts.Manufacturing is carried out from any aluminum alloy using remelting of failed parts of the same name.It enables to preserve their factory chemical composition, save primary charge materials and reduce cost.
The internal combustion engine piston is a part, the shape of which is shown in Figure 1.Moreover, its dimensions and the ratio (H/D) of height (H) to diameter (D) are determined by the power required for each specific engine.The power range of modern automobile engines ranges from 1 to 1001 hp.[1]) According to literature data, for engines of different types and power, the H/D ratio can have different values, for example, from 0.463 (H = 13.2 mm, D = 28.5 mm) [2]) to 1.349 (H = 47.9 mm, D = 35.5 mm) [3].Currently, internal combustion engine pistons are made mainly from aluminum-silicon alloys.It should be noted that the choice of alloys of the indicated composition is not accidental.
As a result of dilatometric studies with heating of alloys of the Al-Si system (in the range of silicon content from 5 to 25%) to 600 0 C, minima of the linear expansion coefficient were discovered in the vicinity of the compositions Al-9% Si [4] and Al-18%Si [5], which is of particular importance for ensuring stable operation of the piston when heated.

Materials and methods
In the mass production of pistons, they are usually produced by casting into metal molds (molds) with a vertical split, with the part positioned bottom down.The inner surface of the piston is usually formed by a metal three-or five-section detachable removable rod.Filling and feeding of the casting during the crystallization process is carried out using a vertical slotted gating-feeding system (GFS) when metal is supplied along the side cylindrical surface of the piston wall.Moreover, the mass of such a gatingfeeding system reaches 50% of the casting mass [6].
To cast pistons, a rather complex mold is used, which is mounted on a mechanized machine.For example, equipment for casting pistons in mass production (up to 100 castings per hour) [7] is a machine that includes: • two metal mold halves, detachable in a vertical plane, the inner surfaces of which are intended to form the outer cylindrical surface of the piston; • a metal removable rod, which serves to shape the inner surface of the piston, consisting of two side rods, with the help of which the piston bosses are formed; • central wedge rod installed between the side rods; • two metal side cylindrical rods, located horizontally and used to form holes in the bosses for the finger; • a mechanism for removing the rods after the casting has hardened; • mechanism for closing and opening the mold.
It should be noted that the labor intensity of production and the cost of such a mold are quite high and its production pays off only in large-scale or mass production.
In contrast to existing approaches, in this paper, to design the outer surface of the piston, it is proposed to produce a one-piece chill mold in the form of a cylinder with a bottom (shaker chill mold) and a core box consisting of two halves with a vertical connector between them, intended for the manufacture of a disposable sand core used for design of the inner surface of the piston, including bosses with holes in them for the finger, in which the upper gating-feeding system is arranged, which does not come into contact with the wall of the mold, which enhances its feeding characteristics; wherein the core box is made from a failed piston by cutting it in a vertical plane into two symmetrical halves, each of which contains a boss, and placing between these halves when molding the core a shaped compensating plate, which serves to compensate for the reduction in diameter by the thickness of the cut of the piston into two halves.
The simplicity of the equipment makes it possible to speed up the manufacturing process and reduce its cost; reduce the time for manufacturing a piston by reducing the duration of tooling manufacturing; ensure the production of pistons in single or small-scale production with the required geometry and quality.

Experimental results
The equipment and casting technology were tested on pistons of one of the modifications of a bus internal combustion engine with a ratio H/D = 0.78 (H = 60 mm; D = 76.5 mm) with a bottom thickness of 6.3 mm.Previously, the failed pistons were measured, and the cleanliness of the surface was determined in order to comply with them during machining.
The Brinell hardness of the pistons (HB), measured at the bottom, averaged 1000 MPa.Determination of the chemical composition of the pistons showed that they were cast from an aluminum alloy of the Al-Si system with the composition: Si is 9.46%; Cu is 0.64%; Mg is 0.62%; Mn is 0.28%; Ti is 0.04%; admixture of Fe is1.3%, the rest is Al.
The required mechanical properties in the heat-treated state according to the T6 mode usually used for castings from this alloy (hardening followed by artificial aging) must be no less than: Due to the fact that the dimensions, configuration and thickness of the cylindrical wall and bottom of the pistons did not allow turning them into standard samples for testing mechanical properties, and also due to the fact that, according to standard technical documentation, the mechanical properties of piston alloys are assessed only by strength indicators, namely tensile strength v and hardness HB, only hardness was measured on the pistons, and v was determined by calculation using a generally accepted method, which is based on the existence of a correlation between the values of hardness and tensile strength for any alloys, including cast aluminum, as well as for the system which includes the alloys from which failed pistons are cast.The presence of such a correlation was confirmed when working with an alloy similar in composition to the alloy from which the failed pistons were made [8].The essence of this paper is that at the beginning, these samples were cut out and tested from a part which dimensions made it possible to cut standard tensile test samples.Then, based on the experimentally measured and averaged hardness of the part, its average value was divided by a certain coefficient K, the value of which is determined preliminary experimentally by dividing the average experimental hardness by the average experimental values of tensile strength.In this case, coefficient K was found by dividing the hardness value of the HB alloy AK9M2 according to GOST 1583-89 (950 MPa), as indicated above, a piston similar in composition to the alloy, by the value of tensile strength v (195 MPa), as a result of which the coefficient was obtained K = 4.87.Next, by dividing the experimentally determined and averaged hardness of failed pistons (НВsr exsp = 1000 MPa) by the value of the coefficient K (4.87), the calculated value of temporary resistance vras equal to 205 MPa was obtained.Thus, the guideline for the required strength indicators of the mechanical properties of pistons intended for casting is the values of HB, not less than 1000 MPa and not less than 205 MPa.
Using the technology described above, a core box was made from a failed piston and, with its help, rods were made from a chemical-hardening mixture.The working alloy was prepared in an electric resistance furnace according to the standard technology for foundry aluminum-silicon alloys of hypoeutectic composition by remelting failed pistons with the addition of a calculated amount of magnesium, taking into account its losses due to waste [9].
It should be noted that among foundry specialists there is an opinion that high-quality castings with the required physical and mechanical characteristics can only be obtained using primary, fresh charge.Moreover, in some cases, for cast parts for critical purposes, this point of view is enshrined in the technological documentation in the form of a ban on the use in the charge of production waste, remelting, scrap and rejected parts cast in our own workshop.
However, in numerous studies we have conducted, it has been established that when such components are used in the charge, the quality of cast products not only does not deteriorate, but, on the contrary, their physical and mechanical characteristics even increase.This has been installed on various alloys: • AL4 (automotive casting: includes 60% secondary charge) [10], • AL27-1 (aircraft: includes up to 50% secondary charge), • AL5, AL9 and AL34 (aircraft: includes up to 40...70% of secondary charge) [11], • ML4hfmagnesium alloy with special properties (marine technology: includes up to 80% of secondary charge) [12], • heat-resistant alloys (flight technology: includes up to 40...80% of secondary charge) [13].
The explanation for this effect lies in the mechanism supported by a lot of researchers [14,15] of transferring the structural and physical-mechanical characteristics of charge materials to the castings obtained from them.During the process of repeated melting in the cycle "melting → crystallization → melting," the structural components of the charge become increasingly refined.Since during the melting process the melt usually does not overheat above the temperature of complete destruction/dissolution of these structural components, during crystallization they facilitate the nucleation process.This ultimately leads to the production of castings with a fine-crystalline structure, and, as a consequence, with increased mechanical properties.
In connection with the above, when casting these pistons, the use of a secondary charge in the form of remelting failed parts of the same name was not in doubt.It should be noted that the use of secondary charge reduces the cost of castings.
After carrying out the required metallurgical operations and reaching the required temperature, the alloy was poured into a chill mold with a sand core installed in it.After the metal had solidified and the casting had been removed from the die, the sand rods were removed from the piston.The heat treatment of the pistons was carried out according to the T6 mode (heating for hardening to a temperature of 5155°C, holding for 6 hours, cooling in water; subsequent aging for 4 hours at a temperature of 2005°C, cooling in air).Chemical analysis of simultaneously cast standard witness samples showed that the content of elements in the alloy (9.68% Si; 0.63% Cu; 0.56% Mg; 0.24% Mn; 0.05% Ti; 0.9% Fe, the rest -Al) differed little from their amount in the original pistons.
Measurement of hardness along the bottom of heat-treated pistons showed an average value of НВexsp = 1210 MPa, which is 21.0% more than the hardness of the original parts and 27.0% more than the requirements of GOST 1583-89 for the analogue alloy AK9M2.
The calculated value of v ras was 248 MPa (v ras = НВexsp: K = 1210 : 4.87 = 428 MPa), which is 20.9% more than v of the failed piston and 27.2% more than the requirements of GOST 1583 -89 for analogue alloy AK9M2.
Processing by cutting the pistons to the required dimensions did not cause any difficulties in obtaining the required surface cleanliness.

Discussion
When examining the treated surfaces, a significant advantage was found in the quality of the alloy of cast pistons over failed ones, namely, the complete absence of gas porosity on them, and, consequently, in the volume of the casting, while the treated surfaces of failed pistons turned out to be affected by porosity, especially the bottom, where its value is estimated at 2 points on the VIAM scale, with a predominant pore size in the range of 0.3...0.5 mm, which qualifies as coarse porosity.It is known that when castings made of aluminum alloys are damaged by porosity, the mechanical properties are reduced, and in the case of work under pressure, the tightness is also reduced.The fracture of the wall and bottom of the pistons showed a typical pattern of ductile fracture for an alloy specially treated with a sodiumcontaining modifier, and the bending angle of the fragment of the piston wall before its destruction was 65 0 , which indicates a fairly high fracture toughness and plasticity associated with a high degree of grinding of the eutectic -the latter was confirmed during the study microstructures.
The high density of the alloy obtained from remelting failed pistons and the high degree of its modification confirmed the authors' assumption that in this case refining and modification operations can be avoided.
Unlike the classic slotted LPS, usually used for casting pistons, in which the slot feeder is located vertically along the entire height of the side surface of the part and whose weight is up to 50% over the mass of the casting itself, according to the developed technology, metal is poured through the "skirt" of the piston.At the same time, the rough mass of the cast piston together with LPS, casting tolerances and allowances for machining (0.13 kg) is only 11% higher than the mass of the part in the machined state (0.28 kg).Elimination of the slotted metal supply significantly reduces the complexity of machining (the operation of cutting off the slotted supply from one piston and the same slot from its opposite side is eliminated) of the casting.

Conclusion
The entire cycle of manufacturing equipment, sand rods, examining failed pistons, their casting, thermal and mechanical treatment, assessing their quality fits into 6 seven-hour work shifts, with a significant portion of the time spent on heat treatment -up to 16 hours), due to the fact that part of work is carried out in parallel, which reduces the production time of pistons to almost three shifts from the moment the order is received until the finished pistons are delivered.
The use of fairly simple and quickly manufactured equipment, reducing the labor intensity of manufacturing the piston, significantly reduces its cost.
The principles of the developed technology can be easily applied for the production of small-scale and piece castings, not only from aluminum, but also from any other alloys and, what is especially important, using the remelting of failed parts, which allows not only to save fresh charge materials, but also to preserve the chemical composition of the primary parts, and, consequently, the level of their physical, mechanical and operational characteristics.