Toz metalurjisi ile üretilen takım çeliklerinin sıcak daldırma yöntemiyle alüminyum kaplama sonrası yüzey karakterizasyonu
Surface characterization of powder metallurgy tool steels after hot dip aluminizing
Yazar: ANIL ÇALIŞKAN
Danışman: PROF. DR. MURAT BAYDOĞAN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Malzeme Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Teknolojinin gelişmesi, üretim adetlerindeki artış, kimyasal bileşimlerine çeşitli alaşım elementleri eklenerek daha mukavemetli hale gelmesi ile çeliklerden de beklenen özellikler artmaktadır. Bu sebeple, imalat ve kalıpçılık sektöründe kullanılan modern takım çeliklerinin yerini toz metalurjisi ile üretilmiş takım çelikleri almaktadır.
Geleneksel çelik üretim yöntemi ile kıyaslandığında, toz metalurjisi ile üretilen çeliklerin daha homojen bir mikroyapıya sahip olduğu görülmektedir. Bu avantajın yanı sıra daha yüksek karbür yapıcı elementlerin varlığı da üretilen çeliğin mekanik özelliklerinde çok ciddi bir artışa neden olmaktadır. Isıl işlem sonrası yüksek sertleşebilme özelliği, özellikle sürtünmeli bölgelerde kullanılan geleneksel takım çeliklerine göre toz metalurjisi ile üretilen takım çeliklerinden yapılan parça ve kalıp ömürlerinin ciddi bir şekilde arttırılmasında da yardımcı bir rol oynamaktadır.
Toz metalurjisi ile üretilen ASP 2012 kalite soğuk iş takım çeliği, yapısında bulunan güçlü karbür yapıcı elementler ve üretim teknolojisi sayesinde homojen bir mikroyapıya sahiptir. Diğer toz metalurjisi ile üretilmiş çeliklere göre daha yüksek tokluğa sahip olan ASP 2012, özellikle darbe ve yük altında çalışan parçalarda, kesme ve sıvama kalıplarında kullanılmaktadır. ASP 2012 takım çeliğinin diğer modern takım çeliklerine göre bir diğer avantajı ise yüksek sertleşebilme kabiliyetidir. Isıl işlem sonrası sertliği 60 HRC'ye kadar artabilmektedir. Endüstride kullanılan nitrasyon, karbürleme ve borlama gibi diğer yüzey işlemlerine de uygundur.
Kimyasal bileşiminde bulunan yüksek karbon ve güçlü karbür yapıcı elementler sayesinde, yüksek aşınma direncine sahip bir diğer toz metalurjisi ile üretilmiş soğuk iş takım çeliği ise ASP 2053'tür. İnce taneli ve homojen bir mikroyapıya sahip olan ASP 2053, yüksek sertliklere ulaşabilmesi sayesinde, özellikle kesici takımlarda kullanılmaktadır. İmalat ve kalıpçılık sektöründe kullanılan bu çeliklerde parça ömründeki iyileşmenin yanı sıra uygulanan kaplama işlemlerinin yeterliliğinin geliştirilmesi ve ekonomik olarak malzemelerden alınan performansın da arttırılması hedeflenmektedir. Bu çalışmada ASP 2012 ve ASP 2053 takım çeliklerine uygulanan sıcak daldırma alüminyum kaplama işlemi sonrasında elde edilen yüzeyin yapısal karakterizasyonu yapılmış ve kaplama tabakasının sertliği ölçülmüştür.
Bu tez çalışmasında; toz metalurjisi ile üretilmiş ASP 2012 ve ASP 2053 takım çelikleri kullanılmıştır. Bu takım çelikleri, sıcak daldırma yöntemi ile saf alüminyum ve Al-ağ. %12Si alaşımı ile kaplanarak, iki farklı kimyasal bileşime sahip takım çeliğinin kaplama tabakasının karakterizasyonu yapılmıştır. Diğer alüminyum kaplama işlemleri, termal sprey yöntemi, giydirme yöntemi, elektroliz ile kaplama yöntemi, vakum yöntemi, buhar biriktirme yöntemi ve elektroforez yöntemidir. Bu çalışma kullanılan sıcak daldırma yönteminin seçilmesindeki en önemli kriter ise nispeten kısa işlem sürelerinde kalın ve altlık malzemeye iyi yapışan bir kaplama tabakası elde edilebilmesidir.
Sıcak daldırma yöntemi ile kaplama işlemi, sade karbonlu ve alaşımlı çelikler, titanyum alaşımları ve süperalaşımlara uygulanmakta ve bu konudaki akademik çalışmalar halen devam etmektedir. Kaplama işleminde kullanılan kaplama malzemesi, sıcaklık, süre ve altlık malzeme gibi parametreler değiştirilerek geliştirme çalışmaları yapılmaktadır. Endüstride kullanılan diğer kaplama işlemleri ile kıyaslandığında ise alüminyum kaplama işlemi yüksek sertlik ve sürekliliği yüksek bir kaplama bölgesi imkânı sunmaktadır. Özellikle borlama gibi yüksek sıcaklık ihtiyacı bulunan bir kaplamaya göre daha ekonomiktir.
Bu tez çalışmasında yapılan kaplama işlemleri, her iki malzeme için de 700 oC sıcaklıkta ergimiş saf alüminyum ya da Al-ağ. %12Si alaşımının bulunduğu grafit pota içerisinde gerçekleştirilmiştir. Kaplama işlemi 3 dakika süreyle yapılmıştır.
Bu çalışmada her iki çelik kalitesi için de çeliklerin östenitlenme sıcaklığının altında bir difüzyon tavlaması sıcaklığı belirlenmiştir. ASP 2012 kalite soğuk iş takım çeliği için östenitlenme sıcaklığı 1100 oC iken, ASP 2053 kalite soğuk iş takım çeliği için ise bu değer 1180 oC'dir. Kaplama sıcaklığının bu değerlerden düşük seçilmesinin nedeni, alüminid fazlarındaki Fe ve Al oranlarındaki değişimin belirlenmesidir. Kaplama işlem sonrasında 800 oC'de 1 saat difüzyon tavlama işlemi gerçekleştirilmiştir. Daha sonrasında, tavlama fırınından çıkarılan numuneler oda sıcaklığında soğumaya bırakılmıştır.
Kaplama ve difüzyon tavlaması işlemleri tamamlandıktan sonra XRD analizi ile her iki kalite soğuk iş takım çeliğinin yapısal karakterizasyonu yapılmıştır. Numunelerden alınan kesitler incelenerek difüzyon öncesi ve sonrası kaplama tabakasında gözlemlenen faz ve arayüzey değişimleri noktasal ve elementel haritalama EDS analizleriyle değerlendirilmiştir. Optik mikroskop altında kaplanmış̧ ve kaplama işleminden sonra difüzyon tavlaması yapılmış takım çeliklerinde farklı fazlarda yapılar gözlemlenmiş ve bu bölgelerin sertlik deneyleri yapılarak kaplama tabakasının farklı bölgelerinin sertlik değerleri elde edilmiştir.

Abstract:
The qualities anticipated from steels are improving along with technological advancements, production volume expansion, and the inclusion of diverse alloys in raw materials. For this reason, powder metallurgy (P/M) tool steels are replacing modern tool steels currently utilized in the manufacturing and molding industries. Because of the increased demand for better qualities and performance along with low- cost tooling per produced part, the demand for high-performance tool steels has increased dramatically. High alloy steels made conventionally show some limitations due to their susceptibility to alloy segregation during solidification. Their qualities and performances still further. In fact, non-uniform clusters of carbides continue to exist as traces of the as-cast microstructure regardless of how much forging or mill processing has taken place since casting. Utilizing powder metallurgy makes it possible to produce tool alloy grades with extremely high performance by facilitating HIP consolidation from tiny powder particles.
Traditionally, alloying steel in its liquid, molten state is done before putting it into ingot molds, where it progressively cools to become solid steel. The steel is subsequently finished via forging and rolling. Forging and rolling procedures are required to provide consistent characteristics because of the considerable segregation of alloying components and steel phases caused by the final ingot's sluggish cooling rates. Because there is more alloy to separate, this is especially true with highly alloyed steels. Sometimes to the extent that they cannot even be manufactured without problems in production. The problem is that increased alloy quantities cause the carbides to develop at higher temperatures.
The Osprey process, another steel production method, is based on the principle of spraying the droplets obtained from the atomization of the molten metal into a collector and depositing them on the collector. The advantages provided by Osprey tool steels are like the benefits of P/M products. The large volumes of gas sprayed by the Osprey method have a very high density. This high-density value enables high speed steels produced by the Osprey method to show uniform and fine carbide distribution in dimensions similar to P/M products.
Powder metallurgy is a process used to create very small ingots. Steel that is still liquid is dripped through a nozzle and "atomized" by liquid or gas sprays, which almost immediately turns the steel into powder. Tool steel is often produced using nitrogen. The powder's individual particles resemble small ingots. In comparison to traditional casting, the cooling rates are therefore substantially faster.
Following production, the powder is placed into a mild steel can and put through a process known as "hot isostatic pressing" (HIP), which involves heating the can to a high temperature (about forging temperature) and applying pressure to transform the individual powders into solid ingots. For damascus, the HIP method is comparable to forge welding. The ingot is subsequently treated by forging andor rolling it in the same manner as any other ingot.
Production of very high alloy tool steels was made possible by the invention of powder metallurgy. High alloy tool steels have the problem of forming very large carbides, which results in poor toughness.
The finished product has a homogenous internal structure and less porosity when powder metallurgy and conventional steel production processes are compared. PM can be used to create parts with controlled porosity. This is advantageous in applications where lubrication or permeability is needed, such as in self-lubricating bearings or filters. PM processes can achieve tight tolerances and produce parts with consistent quality. This is important in applications where uniformity and reliability are critical. Powder metallurgy can produce complex shapes and intricate geometries that may be difficult or costly to achieve through conventional manufacturing methods, such as casting or forging. Powder metallurgy can lead to improved material properties, such as increased strength, wear resistance, and dimensional stability, depending on the alloy composition and process parameters. The mechanical qualities of the steel produced are significantly improved by these benefits and the presence of high carbide-forming components. In comparison to conventional tool steels utilized, especially in frictional areas, it also helps to greatly extend the life of components and molds because to its excellent hardenability after heat treatment.
Strong carbide-forming components in its structure and manufacturing process provide ASP 2012 quality cold work tool steel, which is made using powder metallurgy, a homogeneous microstructure. In particular, ASP 2012 is utilized in parts that work under impact and stress, as well as in cutting and plastering molds since it is more durable than other powder-metallurgy-produced steels. The high hardening ability of ASP 2012 tool steel is another benefit over other contemporary tool steels. Its hardness can grow to 60 HRC after heat-treated. It is also appropriate for other industrial surface treatments like nitriding, carburizing, and boriding.
Additionally, ASP 2053 is a powder-metallurgy cold work tool steel with a high wear resistance due to its high carbon content and potent carbide-forming components. Due to its capacity to acquire high hardness, ASP 2053, which has a fine-grained and uniform microstructure, is employed particularly in cutting tools. These steels are used in manufacturing and molding, and in addition to increasing part life, it is intended to increase the suitability of the coating techniques used on them and the performance of the materials in an economically sensible way. As a result of the surface coating technique to be used on ASP 2012 and ASP 2053 tool steels, surface characterization was done in this study.
Tool steels made using powder metallurgy, ASP 2012 and ASP 2053, were used in this thesis. These tool steels were aluminized by hot dipping pure aluminum and Al- wt.12%Si. Aluminized powder metallurgy (PM) tool steels are a specialized type of tool steel that combines the advantages of both powder metallurgy and aluminizing processes. These steels are designed to provide exceptional wear resistance, heat resistance, and corrosion resistance, making them ideal for various cutting, machining, and tooling applications.
Other aluminum coating techniques include thermal spraying, cladding, electrolysis coating, vacuum coating, vapor deposition coating and electrophoresis coating. The most important criterion in choosing the hot dipping method used in this study is that a coating layer that is thick and adheres well to the substrate material can be obtained in relatively short processing times.
Spray coating is based on the principle of coating the metal with aluminum at a certain thickness by spraying aluminum onto the cleaned material surface via an air jet at appropriate pressure. Annealing can be applied after the process to ensure the strength between the main material surface and the spray-sprayed aluminum coating.
Electrolysis coating, in this method, the electrodes contain either aluminum chloride or aluminum soaked in ethyl bromide and benzene. The surface to be coated is first cleaned by dipping it in HCl solution. In this method, the coating speed is very slow. A coating thickness of 0.01 mm can be achieved in approximately 30 minutes.
The cladding method is based on the principle of coating the surfaces of metallic materials with aluminum by rolling them through mechanical energy and high pressure. It is a preferred method for coating boards. In the electrolytic aluminum coating method, the material to be coated is first cleaned from dirt and oil in HCl solution. In this process, the coating speed of aluminum on the substrate material to be coated is quite low.
Box cementation is a very costly and time-consuming coating method. It is recommended for materials with irregular geometries. It is generally applied to nickel- based alloys. The substrate surface to be coated is cleaned, then it is heated to the appropriate temperature with the aluminum mixture containing ferro-aluminum powders in an airtight still for up to 30 h.
Hot-dipped aluminizing involves immersing steel or iron components into a bath of molten aluminum or aluminum alloy at temperatures typically ranging from 870 oC to 900 oC. During the immersion, the aluminum or aluminum alloy adheres to the surface of the steel or iron through a metallurgical reaction, forming a protective coating. The resulting aluminum coating provides excellent corrosion resistance, making it highly effective in preventing rust and other forms of corrosion that can degrade the metal. The process ensures strong adhesion of the aluminum to the base metal, creating a durable and long-lasting protective layer. Hot-dipped aluminizing is commonly used in various industries, including automotive, construction, and manufacturing, to protect steel and iron components such as pipes, sheet metal, and structural members.
Studies are continuing on the coating process with the hot dipping method. Development studies are carried out by changing parameters such as coating material, temperature, time and substrate material used in the coating process. Compared to other coating processes used in the industry, the aluminum coating process offers a coating area with high hardness and high continuity. It is more economical, especially compared to a coating that requires high temperatures, such as boriding.
The parameters of the aluminum coating process by hot dipping method affect the thickness formed after the coating process, coating morphology, adhesion strength at the substrate material and coating interface, and the mechanical properties of the coating. As the coating temperature increases, the mutual diffusion between the substrate material and aluminum accelerates and affects the coating structure and morphology. While the coating time does not significantly affect the coating morphology, it significantly increases the coating thickness.
To increase the adhesion strength of the coating, the surface of the base material should be cleaned from oil and dirt before the process. The chemical composition of the aluminum used as the coating material affects the type and number of intermetallic phases formed.
The coating process used with molten pure aluminum and Al-wt 12% Si alloy at 700 oC for both materials. It was carried out in a graphite crucible and the coating process was done for 3 min.
In this study, a diffusion annealing temperature was determined for both steels below the austenitization temperature. While the austenitization temperature for ASP 2012 quality cold work tool steel is 1100 oC, this value is 1180 oC for ASP 2053 quality cold work tool steel. The reason why the coating temperature is chosen lower than these values is to determine the change in Fe and Al ratios in the aluminide phases. After the coating process, diffusion annealing was carried out at 800 oC for 1 h. Afterwards, the samples were removed from the surface and cooled to room temperature in air. With the diffusion annealing process, it is aimed to modify the aluminide layers which have been already obtained after the hot dipping process.
After the hot dip coating and diffusion annealing processes were completed, structural and morphological characterization of both cold work tool steels were performed by XRD analysis, optical microscopy, and SEM-EDS examinations, and the hardness of the coating layer was measured.
Both ASP 2012 and ASP 2053 cold work steels underwent XRD examination to determine the current phases that exist between the coating and the bulk material. In ASP 2053 and ASP 2012 quality powder metallurgy tool steel, phases rich in Al and Si, FeAl and Fe3Al are created from the outermost layer to the substrate after the diffusion annealing process, whereas FeAl2, FeAl, and Al-rich phases are only formed in HDA (Hot Dip Aluminizing) of ASP 2012 and ASP 2053 quality steel.