A new eco-friendly slurry has been developed for the chemical mechanical polishing process with a solution of malic acid, deionized water, and an oxidizing agent hydrogen peroxide (H2O2). The surface quality of Ti-6Al-4V workpieces with the proposed chemical mechanical polishing slurry with optimal parameters include oxidizers (H2O2), colloidal (SiO2) slurry, and deionized water by weight 8%, 45%, and 47% respectively, the pH concentration is adjusted 4 through the malic acid content present in the slurry. Experimental results obtained with the proposed chemical mechanical polishing method show a more improved surface quality than previous studies when applying for polishing Ti-6Al-4V alloy. The developed chemical mechanical polishing method's polishing results under optimal conditions obtain an ultra-fine surface quality with Ra = 0.696 nm over a measuring area of 53×70 μm 2 . X-ray photoelectron (XPS) and electrochemical measurements were used to study the chemical reaction mechanisms in the proposed chemical mechanical polishing process. The chemical mechanical polishing processes for the surface of the Ti-6Al-4V alloy workpiece with the H2O2 oxidizing agent showed high suitability with the reactants formed on the surface such as Ti, V, and Al oxide. With the proposed oxidant and the established chemical mechanical polishing slurry, the feasibility and surface quality of the super smooth Ti-6Al-4V workpiece formed after polishing were demonstrated. The established chemical mechanical polishing method shows high applicability in environmental protection and Ti-6Al-4V alloy ultra-precision machining industries. INTRODUCTIONAmong Ti alloys, Ti-6Al-4V alloy is the most common and also one of the most important engineering materials in Ti alloys [1-3]. Ti-6Al-4V alloy is widely used in many different industries such as medical equipment, sporting goods, petrochemical, aerospace, automotive, and many other industrial fields. Due to the relatively low density of Ti-6Al-4V alloy and high biocompatibility, chemical corrosion resistance and high strength [4,5]. With outstanding fatigue strength at high temperatures, Ti-6Al-4V alloy has been used in gas turbines in the aerospace industry with heat resistance up to 600°C [6,7]. With the parts subjected to temperature and cyclic rotational loads, the scratches caused by machining _____________

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Many machining processes would not be possible without the presence of cutting oils. There are many diffe­rent types of cutting oils on the market today, each with different properties. The difference of oils is manifested in many parameters such as viscosity, combustion temperature, recyclability, pollution tendency, stability, price, etc. Choosing the best oil is a difficult and tedious task for customers. In this work, we present the results of a study on the selection of cutting oil using multi­criteria decision­making (MCDM) methods. The selection of the best oil is made on the basis of ranking of seven different types. Two MCDM methods used in this study are Proximity Indexed Value (PIV) and Collaborative Unbiased Rank List Integration (CURLI). This two methods have been used to rank cutting oils. These are two methods with completely different characteristics. When using the PIV method, it is necessary to standardize the data and determine the weights for the criteria. Meanwhile, if using the CURLI method, these two tasks are not needed. In addition, three different weight methods were also used to calculate the weights for the criteria including EQUAL, Rank Order Centroid weight (ROC weight) and Rank Sum weight (RS weight). These three methods have been used to determine the weights for the criteria of cutting oil. The PIV method was used three times corresponding to three different weight methods. The results showed that out of the four ranking results (three using the PIV method and one using the CURLI method), the same best oil was unani­mously identified. It is recommended that the CURLI method should be used if weighting of criteria and data normalization are not desired