Effect of different coatings applied to injection casting molds on mold life

It is known as the most abundant element aluminum, after oxygen and silicon, which is about 8% on the earth's surface. The density of aluminum an atomic weight of 26.89 in the group 3A of the periodic table at 20 oC is 2.7 gr / cm3, melting point is 659.8 oC, boiling point is 2450 oC. Lightness, sufficient strength when alloyed, reusability, light and heat reflectivity, machinability, high corrosion resistance, formability, high heat and electrical conductivity, drawability and forming distinguish aluminum from other metals. This element, which is three times lighter than iron, can be alloyed with other elements and gain strength close to demire. There are two types of aluminum production, primary type and secondary type. Primary aluminum production is made in electrolysis cells, which have high investment costs. Secondary aluminum production is melted and recovered from the lower cost scrap. Today, it is widely used to obtain aluminum from scrap, which is more economical than the production of ore. Aluminum and its alloys are the most commonly produced metals among non-ferrous metals. The tendency to use lighter materials has accelerated the production of aluminum and its alloys. Aluminum and its alloys, which are easy to recycle and harmless to the environment, are used in construction, aerospace, automotive, machinery, decoration and many other fields. Aluminum-silicon alloys, which exhibit high mechanical properties with their alloys, are one of the most used alloys in the aluminum casting industry. Silicium alumina provides high flowability. Al-Si alloys also facilitate the casting of parts with fine microstructure and complicated geometries. While the elements such as Cu and Mg improve the aging properties of Al-Si alloys, the grain strength and modifying properties increase the strength properties. Aluminum alloys are usually obtained by casting. Sand casting and gravity die casting is the oldest production method of casting. However, with increasing technological developments, aluminum and alloys are produced by pressure die casting and investmnet casting methods. Especially the use of pressure die casting method is increasing rapidly. High pressure die casting and low pressure die casting methods are the most common pressuredie casting methods. It is possible to produce aluminum and its alloys in a serial production by high pressure die casting method. With this casting method, more parts are obtained at the unit time. High-pressure die casting consists of five steps. The first one is the closing of the mold. It is then filled with molten aluminum at high pressure and speed. The triple stage mold is filled with liquid metal, then the mold is opened and the part is removed. Lastly, the mold is to be reprocessed in the casting cycle by spraying oil and air. In the high-pressure die casting method, obtaining the solid parts depends on three main effects. The first is the casting machine, the second is the casting parameters and the third is the mold design. In high pressure die casting, two types of casting machines are used, one with a cold chamber and one with a hot chamber. In the hot-chamber casting process, the metal pump is immersed in liquid metal and at the same temperature as the metal. In the cold chamber casting process, the metal pump is independent of the melting pot. In this method, the liquid metal is conveyed to the casting system by scoop or robotic systems. Due to the interest of aluminum and its alloys in iron, it is preferred to produce in cold chamber casting machines. The cold chamber casting machine consists of injection cylinder, piston rod, piston, mine cylinder, fixed vise, moving vise, pusher mechanism box, toggle lever, lever bushes, screw closure equipment and closing cylinder. In high pressure casting, the desired casting parts are obtained with ideal casting parameters. The first phase, second phase and third phase values must be entered in the casting machine. The nitrogen pressure and the pump pressure must also be adjusted. In addition to this, knowing the parameters such as solidification time, mold opening times, lubricating and air settings are important for obtaining robust parts. The design of high-pressure die casting molds (injection molding) is another factor that affects the quality of parts. If there is an error in the design of the mold, there will be accidents and stops in production. Mold surfaces must match each other. In the design, the conduits and channels through which the liquid metal can flow smoothly into the mold cavity must be opened. Proper venting ducts or pockets must be made to allow air to escape from the mold. Also if the match is used, the direction of the matches must be well adjusted. In addition, the quality of the mold steel is important in terms of mold life. Thermal fatigue, erosion, corrosion and abrasion are common mold damage mechanisms as aluminum injection molds are exposed to extreme thermal and mechanical stresses. There is also a problem of soldering in the pressure die casting method. All negative factors increase downtime in production, create dimensional problems and shorten mold usage period. These problems, which occur in production, increase the costs in the pressure die casting process. In addition, after the machine investment in aluminum injection, injection casting molds with the highest financial cost are expected to cast at least 100,000 shots. For this reason, increasing the working life of pressure die casting molds is one of the most studied subjects in recent years. There are two ways to increase the duration of use of casting molds. The first is to protect the mold with casting and cooling system. The second is to improve the surface properties of the mold steel. Thermomechanical surface treatments, hard coating applications are processes to prevent mold damage in injection molding process. Nitration, nitrocarburization or boron is the most common surface modification mediators. In addition, hard coating steels with PVD or CVD methods on mold steels is a way to extend the life of a mold that has become widespread in recent years. It is aimed to increase the thermal fatigue strength of the mold steel by gas or plasma nitriding process. Hard coatings provide benefits for adhesion and erosion resistance. TiAlN is the most commonly applied coating type because of its high oxidation resistance. AlCrN coating is applied in injection molds because it is harder than TiAlN coating and has better oxidation resistance. DLC (diamond-like carbon) coatings are generally preferred for improving the wear properties of steels. In this study, aluminum injection mold pins were reformed first. Then plasma nitration was applied to increase the surface hardness of the pins. TiAlN and AlCrN coatings were applied on the plasma nitriding by PVD method and DLC coating was applied by PECVD (Plasma-Enhanced Chemical Vapor Deposition) method to further improve the surface properties of the pins. Finally, double-coated samples were obtained by plasma-nitriding PVD with TiAlN and AlCrN coated pins by DLC coating with PECVD. Plasma nitrided (uncoated), TiAlN, AlCrN, DLC, TiAlN / DLC and AlCrN / DLC coated pins are used in aluminum injection casting 2000, 4000 and 6000 shots. The soldering strength, erosion resistance and crack formation of the pins used in serial production conditions were investigated after 2000, 4000 and 6000 shots operations. The masses of the pins before and after casting are weighed for soldering and erosion resistance. The crack length was measured with an optical microscope for the crack length. The hardness values of the coatings were compared with Vikers microhardness measurement. Coating images were examined both on optical microscope and on scanning electron microscope. Phase analysis of the coatings was performed by XRD and Raman spectroscopy. Generally, the number of shots, the amount of soldering, the amount of erosion, the total crack length and the average crack length were found to increase. Soldering and erosion resistance of single coatings are better than double coatings. The AlCrN coating has the best erosion resistance. It was observed that the amount of soldering, the amount of erosion, the total crack length and the average crack length increased. In general, the total length of cracks and the average crack length have increased with the amount of erosion. Generally, the total crack length and the average crack length have increased. The crack formation occurred in the AlCrN coating at the earliest. After 4000 prints cracks were formed in all coatings. It has been understood that the DLC coating has deteriorated after casting tests. At the end of the experiment, the relative damage values of the other samples were calculated with reference to the uncoated (nitrated) sample. According to these values, the TiAlN coating showed the best performance after 2000 shots, while the DLC coating performed best after 4000 and 6000 shots.