The usage potantiel of modified B390 aluminum alloys instead of cast iron on bearing parts of refrigerator compressors

Nowadays, due to rapid depletion of energy resources, operating performance of the machine's equipment come into prominence. One of the main parameters that affect the efficiency is the tribological properties of the machine elements. Several studies show that approximately one third of the produced energy is consumed by friction. Abrasion that occurs because of friction, leads to energy, materials and money loses. When selecting materials for bearing system, the tribological properties of these materials should be taken into consideration. This tribological properties of materials can be improved by surface modifications or different coatings that can be applied on the surface of bearings. Compressors is the main element of the refrigerator's cooling system is compressors that are consisted of several bearings. It is known that wear resistance of this bearings are directly related to compressor efficiency and dissipation. In the current system, cast iron is used as the crankshafts and body materials at typical hermetic type compressors. Manganese phosphate coating is applied on the cast iron crankshafts due to improve the tribological properties of the bearings. This coating that behave as oil absorber, protect the surface against the wear for a limited period. However, the friction induced a wear on manganese phosphate layer overs time and causes base material loss. Depending on the material loss, the compressor's cooling efficiency decreases and that leads increased energy consumption of the refrigerator. In this study, usage potential of hypereutectic Al-Si alloy B390 instead of cast iron for refrigerator compressors' body material is investigated. Due to the properties of lightness, excellent castability , high wear resistance, high corrosion resistance, Al-Si alloys are the most preferred materials among other aluminum casting alloys. Considering these properties, this type of aluminum alloys takes cast iron's place in the field of automotive and aeronautics industries. The main reason being preferred is major silicon element existing in the structure that provides high strength and high wear resistance. Additionally, density of aluminum is 2.7 g/m3, three times lighter than iron. If large mass production is taken into consideration, the number of the parts that are produced using aluminum much more than the number of parts that are produced using iron based materials having same mass value. Furthermore, another advantage of Al-Si alloys is to be suitable for pressure die casting, so most of the machining process will be removed compared to cast iron bearings. Because in this production method for aluminum, it is possible to produce complex shapes generally in one step or using less machining process. In this study, primary investigations were about exploring tribological behaviour of cast iron pin/cast iron disc couple. After that, different tribo-couples of cast iron pin/B390 are tested applying various modifications to B390 aluminum alloy. These modifications were about to adding phosphorus to aluminum alloy, cooling rate after casting process, heat treatment (precipitation hardening), chemical etching of surface with NaOH. General purpose of these modifications is to investigate the effect of size of primary silicon particles, their homogeneity in the structure, hardness and protruded primary silicon particles to surface by preferentially etching of aluminum on wear resistance in detail. First of all, Stribeck curves are found out in order to discover lubrication regimes (hydrodynamic, mixed and boundry lubrication regime) of manganese coated and uncoated samples. In hydrodynamic lubrication regime, there is a stable oil film layer between tribo-couples, coefficient of friction value is low and it is possible to say that there is no real wear between the surfaces. Mixed lubrication regime is a transition region from hydrodynamic to boundary regime. The most aggressive wear conditions are observed in boundary lubrication regime, because stability of oil film between surfaces spoils and tribo-couples begin to be directly in contact. That's why coefficient of friction value gives a rise in this area. Boundary regime is the most significant region that shows high wear rate. To simulate the conditions of crankshaft and main body bearings, tribological experiments of pin on disc samples are conducted by using reciprocating motion at 80 °C in oil bath. To discover the boundary lubrication regime, Stribeck curves are obtained for long wear experiments (8 hours) under constant load (50N) and same temperature (80 °C) in order to determine coefficient of friction values. Because boundary lubrication regime is related to start-stop motion and running-in-period process of refrigerator compressors and that regime is very critical for wear due to inadequate oil on the surfaces of bearing parts. To prevent material loss from the surfaces, heat treatment of precipitation hardening and chemical etching with NaOH solution (10 %) applied in different durations (30 sec-120 sec- 180 sec- 240 sec). After heat treatment process, the most remarkable change occurs on eutectic silicon particles. Their needle-like shapes are converted into spherical shape and that prevents crack propagation during wear experiments. After this heat treatment process, aluminum samples are exposed to chemical etching with NaOH and alkali oxides occurred during etching process are removed by using nitric acid (10 %). After precipitation hardening, the highest hardness value is obtained in 5 hours aged sample, and this sample is chemically etched and tribologically tested for 8 hours with the same conditions that is explained above. The main effect of chemical etching of aluminum discs with NaOH is preferential dissolution of aluminum that creates a difference on height of primary silicon particles and their positive effects during wear by protecting the surface are observed. After tribological experiments, wear track of all pin and disc samples are analyzed using optical profilometer to measure wear depth of disc and track diameter of pin. Wear characteristics of the samples are analyzed by XRD and SEM-EDS. Addition to these characterization methods, wear volumes of samples are calculated depending on previous geometry using wear track after tribological experiments, then specific wear rate coefficients are found out. If wear characteristics of the tribo-couples are investigated from SEM images after tribological experiments. Tribo-couple of cast iron pin and B390 aluminum disc is exposed to abrasive wear, while tribo-couple of cast iron pin and cast iron disc has adhesive wear, it means due to micro-welding of similar materials, this couple has broken particles that separate from surface. That's why tribo-couple of cast iron pin and cast iron disc face the situation of unexpectable wear rate. Specific wear rate coefficients are calculated by using Archard Equation. The specific wear rate coefficient of cast iron pin/ NaOH-etched (for 30 sec) aluminum disc tribo-couple is 0.6 m3/Nm, while cast iron pin/ aluminum disc (no treatment) is 2.23 m3/Nm and cast iron pin/ heat treated aluminum disc is 0.84 m3/Nm. Amount of wear depending on time also investigated. 8 hours wear experiment of cast iron pin and NaOH-etched (for 30 sec) aluminum disc which has the lowest specific wear rate coeffient is divided into four same sub-durations ( 2 hours- 4 hours- 6 hours- 8 hours). In every duration step, test conditions experiment was stopped and wear depth of the disc, track diameter of pin were measured using optical profilometer. It is observed that wear track diameter that occurs on cast iron pin increases as time progresses; however on the NaOH-etched (for 30 sec) aluminum disc, it can be said that amount of wearing decreasingly increases.