However the use of friction surfacing process for

However, the use of friction surfacing process for many applications has been limited due to the difficulty of monitoring and control of the process outputs, such as bond quality and coating dimensions [4]. Proper selection of process parameters is vital for obtaining the quality coatings using friction surfacing. THZ1 Hydrochloride of process parameters and torque-time characteristic are important for the quantum of heat generated at the contact surface and to maintain consumable at quasi steady status in entire process, which affect the quality of deposit [5].The three main friction surfacing parameters are rotational speed of mechtrode, substrate traverse speed and axial force on mechtrode by means of which the desired quality of the coating layer with improved bond strength and corrosion resistance can be achieved [6,7]. Empirical investigations are normally required to determine the optimum parameters that produce the required process response. High strength low alloy steel is widely used due to easy availability and good weldability. Corrosion resistance of low alloy steel can be improved by surface coating with stainless steel, high speed steel, tool steel and metal matrix composites [6–8]. A number of successful research studies on friction surfacing of similar and dissimilar combinations have been done especially in the areas of microstructural analysis of coating and mechanism during process [9,10]. However very few systematic studies have been performed on relationship between the various process parameters and resulting properties, especially bond strength and corrosion resistance. In the present study, AISI 304 was chosen considering its wide-spread industrial use as corrosion resistant clad material for high strength low alloy steels. This investigation is aimed at studying the microstructure, pitting corrosion resistance and bond integrity of friction surfaced austenitic stainless steel 304 coatings produced on high strength low alloy steel substrate in detail.
Materials and experiment The stainless steel AISI 304 (15 mm diameter and 250 mm length) and the low alloy steel plate (10 mm × 100 mm × 250 mm) are used as mechtrode and substrate, respectively. The chemical compositions of materials are shown in Table 1. The experiments are carried out using friction surfacing machine (50 kN capacity), specially designed and developed by Defence Metallurgical Research Laboratory, Hyderabad, India. Trial experiments are conducted to determine the working range of the factors, such as rotational speed of mechtrode (A), substrate traverse speed (B) and axial force on mechtrode (C). Feasible limits of the parameters are chosen in such a way that the coating should be free from any visible defects. In the present study, the temperature measurements were carried out close to the rubbing end of the rotating mechtrode using a caliberated infrared camera capable of measuring the temperatures up to 1500 °C. The setup is shown in Fig. 2. The camera was focused at the rotating mechtrode/substrate interface. Statistical design of experiment approach is used to minimize the number of trials required to optimize surfacing conditions. The three important parameters, i.e., rotational speed of mechtrode, traverse speed of substrate and axial load, were selected for the experimentation. Central composite design was chosen with three process parameters varying at five levels [11]. The generalized regression equation of experiment [12] is given aswhere Y is the response function, and b(i = 0, 1, 2, 3) is the unknown coefficient that is estimated by least square fitting of the model to the experimental results obtained at the design points. Table 2 indicates the selected factors and corresponding levels against which experimental design is prepared. Table 3 shows the 20 set of coded conditions used to form the design matrix and output value as pitting potential and bond strength. The friction surfaced coatings were subjected to ultrasonic testing (UT) by employing a specially developed calibration block in accordance with ASTM A578M. The good bond area was subjected to further investigation. Transverse section of friction surfaced coating was prepared using standard metallographic technique for microstructural examination. X-ray diffraction studies on friction surfaced coating were carried to identify various phases present by using Cu Kα radiation on a Philips X\'Pert Pro diffractometer. To evaluate the integrity of the friction surfaced coatings, the ram tensile test (Fig. 3) was carried out to find the tensile strength of the coating by employing specially designed ram tensile fixture. The ram tensile test specimens were prepared as per MIL-J-24445 (SH) standard.