Abstract
The “radius of contact” or the “real-rotational contact plane,” has been increasingly mentioned terminology in friction surfacing. However, the fundamental understanding of the flow dynamics behind this phenomenon is still very limited. The goal of this study was to understand the influence of spindle speed and consumable rod diameter on the flow dynamics and the radius of contact during friction surfacing of 304L stainless steel over a substrate of the same material. Friction surfacing was performed using consumable rods with diameters of 4.76 mm, 9.52 mm, and 12.7 mm while using spindle speeds from 1500 rpm to 20,000 rpm. The impact of spindle speed on deposition morphology, including the radius of contact, was studied. The radius of contact was calculated empirically and was found to be inversely proportional to the tangential velocity of the rod. The coupling between flow stresses and localized forces is hypothesized to be the key factor behind the variation of the radius of contact with processing conditions.
1 Introduction
1.1 Friction Surfacing of Austenitic Stainless Steels.
Friction surfacing is an emerging solid-state deposition process belonging to the family of “third-body region” based manufacturing processes such as friction stir welding, friction welding, etc. Figure 1(a) shows the schematic illustration of friction surfacing. During friction surfacing, a rotating consumable rod is pressed against a stationary substrate under an applied axial load. The friction contact leads to the generation of a visco-plastic boundary between the rod and the substrate that plastically deforms the rod. The plastically deformed consumable rod is traversed over a substrate to provide a dense uniform coating. Figure 1(b) shows the thermomechanical events occurring during friction surfacing. As the unconstrained rotating rod is traversed across the substrate, material at the frictional interface either becomes a flash or will roll onto the substrate [2–4].
![(a) Schematic illustration of friction surfacing and (b) thermomechanical events occurring during friction surfacing [1]](https://asmedc.silverchair-cdn.com/asmedc/content_public/journal/manufacturingscience/146/2/10.1115_1.4063653/1/m_manu_146_2_021005_f001.png?Expires=1739883608&Signature=g683bDq7etnhdQxSYiBp26dx1Wn1I~3dZdhuu4NPMlGFCy5VB8btZocDEuCRZqNGhtW98E44fuFdiKJvmjgEsTnrrwe9uVAnWDJwW7wDT9Y76IsIBdZcBsATfb8BHMEEqXoVY9W1~spI0RpACluvXd6nZo80Jk4WPkRbaSQZzix6vMtw-4zgf6OTxlafGGz7M0uYNywoCEj4nJwAFFoEEcD5iENt7WPaB0kc6j5wf9YhYDGysX-LFWtsRNBi32RI04MQca7P9br4opkoqf6YtmPv96bDWb4eBFZTkgUdQaDdt-jfzg15DC85mL6xOj1P~zVenABaNzJsQuFgQ8Ajmg__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA)
(a) Schematic illustration of friction surfacing and (b) thermomechanical events occurring during friction surfacing [1]
![(a) Schematic illustration of friction surfacing and (b) thermomechanical events occurring during friction surfacing [1]](https://asmedc.silverchair-cdn.com/asmedc/content_public/journal/manufacturingscience/146/2/10.1115_1.4063653/1/m_manu_146_2_021005_f001.png?Expires=1739883608&Signature=g683bDq7etnhdQxSYiBp26dx1Wn1I~3dZdhuu4NPMlGFCy5VB8btZocDEuCRZqNGhtW98E44fuFdiKJvmjgEsTnrrwe9uVAnWDJwW7wDT9Y76IsIBdZcBsATfb8BHMEEqXoVY9W1~spI0RpACluvXd6nZo80Jk4WPkRbaSQZzix6vMtw-4zgf6OTxlafGGz7M0uYNywoCEj4nJwAFFoEEcD5iENt7WPaB0kc6j5wf9YhYDGysX-LFWtsRNBi32RI04MQca7P9br4opkoqf6YtmPv96bDWb4eBFZTkgUdQaDdt-jfzg15DC85mL6xOj1P~zVenABaNzJsQuFgQ8Ajmg__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA)
(a) Schematic illustration of friction surfacing and (b) thermomechanical events occurring during friction surfacing [1]
Austenitic stainless steels are widely used across industries such as transport, marine, nuclear, etc., because of their high corrosion resistance. The scientific community has extensively researched friction surfacing using austenitic stainless steel consumable rods. In one of the earliest studies, Lambrineas and Jewsburg [5] performed friction surfacing of 304 and 316 stainless steel rods on mild steel substrates for aerial coverage applications. The effect of providing inclination to the consumable rod with respect to the substrate was reported in the study. Based on the corrosion performance, the potential use of friction surfacing for pressure vessels was proposed by Govardhan et al. [6]. In another study by Agiwal et al. [1], the efficacy of friction surfacing to repair ∼50 µm cracks up to depths of 200 µm was shown. Austenitic stainless steels possess dynamic nature for phase transformations, and thus a significant focus has been given to microstructural changes and material flow during the process. A “discrete laminar” layer formation was reported during the friction surfacing of 304 stainless steel rods over 5083 aluminum alloy [7]. Before attaining sufficient plasticity to flow on the substrate, the delaminated layers from the consumable rod rolled over the substrate, leading to a filament-like structure. The severe plastic deformation of the consumable rod during friction surfacing leads to dynamic recrystallization of the material and produces fine-grained coatings with superior mechanical and corrosion properties. An average grain size reduction from 40 µm to 5 µm was reported by Khalid Rafi et al. [8]. For a more detailed literature background on friction surfacing of austenitic stainless steels, please refer to the authors’ previous publications [1,9–11].
1.2 Effect of Process Parameters and Radius of Contact.
Summary of the effect of process parameters on the friction surfacing process
Process parameter | Effect of increasing the parameter | References |
---|---|---|
Spindle speed | Thinner coatings, improved bond strength, better corrosion resistance | [10–13] |
Traverse speed | Higher process force, improved bond strength, thinner and wider coatings | [10,11,14] |
Axial force/Axial speed | Improved bond strength, thinner and wider coatings | [10,11,15] |
Tilt anglea | Increased bonded width, thicker coatings | [5,10,11] |
Process parameter | Effect of increasing the parameter | References |
---|---|---|
Spindle speed | Thinner coatings, improved bond strength, better corrosion resistance | [10–13] |
Traverse speed | Higher process force, improved bond strength, thinner and wider coatings | [10,11,14] |
Axial force/Axial speed | Improved bond strength, thinner and wider coatings | [10,11,15] |
Tilt anglea | Increased bonded width, thicker coatings | [5,10,11] |
Increasing the positive tilt angle. Assuming that the tool tip is the pivot point, a positive angle has the tool spindle angle away from the workpiece normal, opposite of the traverse direction.
This work highlights the importance of the combination of spindle speed and consumable rod diameter on the process dynamics. To the authors’ best knowledge, this is the first study to investigate both the consumable rod diameter and spindle speed during friction surfacing. This study also employs spindle speeds up to 20,000 rpm, a novel regimen of working rotation speeds presented in previous publications by the group [9,21]. In literature, friction surfacing has been performed only up to 6000 rpm spindle speed. A fundamental understanding of plastic flow, flash formation, and the variation in the contact radius with spindle speed and rod diameter has been established in this study.
2 Experimental Setup
Friction surfacing experiments were performed using stainless steel 304L consumable rods (Wahsin Lihwa, USA) on 304L stainless steel substrates (North American Stainless, Minoka, IL). The composition of the consumable rods and the substrates is provided in Table 2. Consumable rods of three diameters were chosen: 4.76 mm, 9.52 mm, and 12.7 mm. The rod length was chosen to avoid buckling under initial contact force. The protruding length from the collet for the three rods was 15 mm, 30 mm, and 40 mm, respectively. The substrates were 3-mm-thick and 152 mm × 76 mm in dimensions. A stainless steel 304L backing plate of the same dimensions was used. Friction surfacing experiments were conducted on two computer numerical control (CNC) machine tools, a three-axis CNC milling machine (HAAS TM-1, Oxnard, CA; max. spindle speed: 4000 rpm) and a five-axis CNC mill-turn machine (NT-1000, Mori Seiki, Japan; max. spindle speed: 20,000 rpm). During the friction surfacing process across all tests, the consumable rod was positioned with an initial plunge (below the point of contact) of 1 mm at a constant plunge rate of 20 mm/min. After a dwell time of 0.25 s, the rod was traversed along the substrate with a lateral traverse feed rate (Vx) of 120 mm/min and a constant axial feed rate (Vz) of 80 mm/min. The consumable rod’s rotation speed varied across the three diameters, ranging from 1500 rpm to 20,000 rpm. Table 3 summarizes the processing parameters used in the experiments. The five-axis CNC mill-turn machine’s force and torque requirements constrained the selection of rod diameters for given speeds. Thus, for higher spindle speeds (>10,000 rpm), only the 4.76-mm-diameter rod was used. All friction surfacing experiments were conducted using position control.
Composition of the 304L stainless steel substrate and consumable rod
Element (wt%) | C | Mn | P | S | Si | N | Cr | Ni | Mo | Cu | Fe |
---|---|---|---|---|---|---|---|---|---|---|---|
Substrate | 0.015 | 1.74 | 0.031 | 0.001 | 0.273 | 0.094 | 18.01 | 8.061 | 0.373 | 0.467 | Balance |
Consumable rod | 0.02 | 1.68 | 0.033 | 0.028 | 0.33 | 0.082 | 18.23 | 8.09 | 0.2 | 0.56 | Balance |
ASTM A240 | 0.030–max | 2.00–max | 0.045–max | 0.030–max | 0.75–max | 0.10–max | 18.0–20.0 | 8.0–12.0 | Balance |
Element (wt%) | C | Mn | P | S | Si | N | Cr | Ni | Mo | Cu | Fe |
---|---|---|---|---|---|---|---|---|---|---|---|
Substrate | 0.015 | 1.74 | 0.031 | 0.001 | 0.273 | 0.094 | 18.01 | 8.061 | 0.373 | 0.467 | Balance |
Consumable rod | 0.02 | 1.68 | 0.033 | 0.028 | 0.33 | 0.082 | 18.23 | 8.09 | 0.2 | 0.56 | Balance |
ASTM A240 | 0.030–max | 2.00–max | 0.045–max | 0.030–max | 0.75–max | 0.10–max | 18.0–20.0 | 8.0–12.0 | Balance |
Friction surfacing parameters used in this study
Spindle speed, N | 1500–20,000 rpm (157–2095 rad/s) |
Consumable rod diameter, 2ro | 4.76, 9.52, and 12.7 mm |
Lateral traverse feed rate, Vx | 120 mm/min |
Axial feed rate, Vz | 80 mm/min |
Feed ratio, Vz/Vx | 0.67 |
Initial plunge | 1 mm |
Initial plunge rate | 20 mm/min |
Dwell at the end of the plunge | 0.25 s |
Spindle speed, N | 1500–20,000 rpm (157–2095 rad/s) |
Consumable rod diameter, 2ro | 4.76, 9.52, and 12.7 mm |
Lateral traverse feed rate, Vx | 120 mm/min |
Axial feed rate, Vz | 80 mm/min |
Feed ratio, Vz/Vx | 0.67 |
Initial plunge | 1 mm |
Initial plunge rate | 20 mm/min |
Dwell at the end of the plunge | 0.25 s |
To study the deposition morphology, both the consumable rod and friction-surfaced coatings were sectioned. The friction-surfaced coating was sectioned in the middle of the deposition length. This allows visualization of the process in the steady-state regimen since the initial part of the coating is in the preheating phase. The cross sections were ground successively with 240, 400, 600, 800, and 1200 grit SiC paper, followed by polishing with diamond slurry; 9 µm, 3 µm, 1 µm, 0.25 µm, and colloidal silica solution at 0.05 µm. The polished samples were imaged using a white light optical microscopy system (Alicona InfiniteFocus® G4, Austria).
3 Results and Discussion
3.1 Friction Surfacing With Different Spindle Speeds and Rod Diameters.
Figures 2(a)–2(c) show the consumable rods and coatings from friction surfacing experiments performed using the three different rod diameters. All three depositions show similarities in deposition appearance and the mushrooming flash on the consumable rods. However, the gradient of width from the start of the deposition into the steady-state region is bigger for depositions at higher spindle speeds (Fig. 2(a)). This is attributed to the reduced radius of contact with increased tangential velocity, which will be discussed later in this paper. Figures 2(d) and 2(e) also show a cross section of the coating and consumable rod from a 9.52-mm-diameter rod. These cross sections are typical of friction-surfaced coatings and are consistent across all experimental conditions tested in this study. For each coating cross section, the total width and thickness of the coating were measured. The typical flash formation around the rod was observed in cross sections of the consumable rods. The diameter of the cylindrical flash around the tool was also measured.

Images showing representative depositions and consumable rods after friction surfacing experiment with consumable rod diameter: (a) 4.76 mm (spindle speed: 20,000 rpm), (b) 9.52 mm (spindle speed: 4000 rpm), and (c) 12.7 mm (spindle speed: 20,000 rpm), all other conditions were kept constant. Cross sections of (d) coating and (e) consumable rod from specimen condition (b).

Images showing representative depositions and consumable rods after friction surfacing experiment with consumable rod diameter: (a) 4.76 mm (spindle speed: 20,000 rpm), (b) 9.52 mm (spindle speed: 4000 rpm), and (c) 12.7 mm (spindle speed: 20,000 rpm), all other conditions were kept constant. Cross sections of (d) coating and (e) consumable rod from specimen condition (b).
Figure 3 visualizes the effect of the various combinations of spindle speed and diameter on the deposition morphology metrics. The width of the coating cannot be compared directly because of the different consumable rod diameters used. Therefore, it has been divided by the rod’s diameter. In Fig. 3(a), the normalized width of the coating is similar across the different diameters for the same spindle speed. A reduction in the width of the coating was observed for the 4.76 mm diameter rod with increasing spindle speeds up to 20,000 rpm; however, the variation at lower speeds (<4000 rpm) was minimal across the three rods. This reduction can also be observed in Fig. 2(a), where there is a visible gradient in width from the starting rod diameter compared to the other two rods (Figs. 2(b) and 2(c)). The thickness of the coating shows a continuous reduction with increasing spindle speeds for all diameters (Fig. 3(b)). For a given spindle speed, the thickness is also reduced with increased in the rod diameter. For instance, at 4000 rpm, the thickness of the coating is 1.1 mm, 0.7 mm, and 0.5 mm for rod diameters 4.76 mm, 9.52 mm, and 12.7 mm respectively.

Variation of deposition morphology with the spindle speed and diameter of rod: (a) normalized coating width and (b) coating thickness
3.2 Radius of Contact.

Schematic illustration showing (a) the top-down view of the consumable rod and (b) the cross-sectional view of the consumable rod

Variation of (a) normalized contact radius and (b) cross-sectional area of the coating as a function of tangential velocity
The radius of contact reduces with increasing tangential velocities. At higher tangential velocities corresponding to higher rotation speeds or larger rod radii, the contact region drops below 40% of the initial contact of the entire rod. The reduction in the radius of contact corresponds to the reduction in the overall volume of deposited material, as can be seen in Fig. 5(b). The volume of material deposited is represented by the cross-sectional area of the coatings since all the depositions were cross-sectioned in the steady-state region, and the lateral traverse feed rate was held constant for all experiments. The cross-sectional area has been calculated as the product of the coating’s width and thickness.
The influence of tangential velocity on the calculated strain rates for all the experiments is plotted in Fig. 6. Strain rates and temperature play a critical role in the plastic flow during friction surfacing. The plastically deformed material is compressed between the rod and the substrate. Due to the axial compression, translational, inertial, and circumferential forces imparted by the rod and substrate on the deformed material, localized stresses are developed in the plastically deformed material. These localized stresses are directly impacted by the changing processing parameters. For instance, an increase in spindle speed would increase the circumferential forces; similarly, increased axial and traverse feed rates would increase the axial compression on the deformed material [12]. These localized stresses tend to move the deformed material; however, the inherent resistance to the plastic flow of the material (flow stress) counteracts these localized stresses. If the flow stresses are large enough to counteract the localized stresses, the deformed material stays between the rod and the substrate and, under the thermal boundary conditions, forms the coating. However, if the flow stresses aren’t large enough to counteract the localized stress, the material plastically flows out, typically as a flash [23]. The strain rates have also been shown to increase radially outwards during friction contact [24]. Since the strain rates are largest toward the outer periphery of the rod, we typically see the flash formation from the outer regions of the rod and deposition from the central regions or the contact region (Fig. 4).
These flow stresses strongly depend on the applied strain rates and process temperatures. For austenitic stainless steels at lower temperatures, flow stresses have a high strain rate sensitivity and increase with an increase in strain rates. A higher tangential velocity leads to a higher strain rate and, therefore, higher flow stresses in the plastically deformed material; however, a higher tangential velocity will also increase heat flux and temperature. The temperatures have been recorded near 80–90% of the solidus temperature during friction surfacing of 304L austenitic stainless steels [25]. The flow stresses reduce with temperature rise [26], and hence the increase in strain rates initially balances the increased temperature effects. Still, as the temperatures keep rising, the strain rate sensitivity of flow stresses reduces significantly and leads to a reduction in flow stresses. Since the strain rates increase radially outwards, the amount of material around the center of the rod that remains in contact with the substrate and subsequently gets deposited, reduces. Thus, the radius of contact reduces as we increase the tangential velocity.
To visualize the interplay between flow stress and localized stress and the influence of tangential velocity, two extreme conditions with respect to the tangential velocity have been compared and demonstrated, as shown in Fig. 7. The first condition represents low tangential velocity (0.74 m/s) with the process forces around 2000 N. The second condition represents extremely high tangential velocity (10 m/s) with similar process forces (1500 N). Since, for both these conditions, the axial forces are comparable, the tangential velocity plays a key role in both the localized and flow stresses. First, discussing localized stresses, a higher tangential velocity will increase the circumferential and inertial stresses for the second condition. Second, as mentioned previously, higher tangential velocities increase the heat flux and, therefore, temperature, which further reduces the strain rate sensitivity of the flow stresses and ultimately reduces the flow stress. The combined effect of an increased tangential velocity being higher localized stresses and reduced flowrates, and a significant amount of deformed material flowing out as the flash, resulting in a very thin and narrow coating (Fig. 7(b)). For the first condition, since the flow stresses dominate the localized stresses, most of the deformed material stays between the consumable rod and substrate and is deposited (Fig. 7(a)).

Demonstration of the interplay between flow stress and localized stress for two extreme conditions: (a) low tangential velocity and (b) high tangential velocity
4 Conclusion
Friction surfacing experiments were performed on 304L substrates using 304L austenitic stainless steel consumable rods. The spindle speed and consumable rod diameter were varied across the experiments, while other process parameters were constant. Using empirical and analytical equations, the radius of contact was calculated and compared against the input parameters. The radius of contact during friction surfacing was calculated as a function of the deposition morphology (width and thickness) and process speeds. The ratio of the radius of contact to the initial rod radius reduced as the tangential velocity of the rods was increased. The radius of contact is strongly influenced by the interplay between the flow stresses and localized stress resulting from the processing conditions. When the flow stresses dominate, more material is deposited on the coating, leading to a larger radius of contact. Moving forward, the impact of the radius of contact on the bonding characteristics during friction surfacing will be studied for a better understanding of the thermomechanical events occurring during friction surfacing.
Acknowledgment
The authors would like to acknowledge the support of this work by the Department of Energy (Grant No. DE-NE0008801), the Department of Mechanical Engineering, the Department of Material Science and Engineering at the University of Wisconsin-Madison, the Machine Tool Technology Research Foundation, and colleagues in the Multiscale Metal Manufacturing Processes Lab.
Conflict of Interest
There are no conflicts of interest.
Data Availability Statement
The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.