This paper presents a study of tool wear and geometry response when machinability tests were applied under milling operations on the Super Austenitic Stainless Steel alloy AL-6XN. Eight milling trials were executed under two cutting speeds, two feed rates, and two depths of cuts. Cutting edge profile measurements were performed to reveal response of cutting edge geometry to the cutting parameters and wear. A scanning electron microscope (SEM) was used to inspect the cutting edges. Results showed the presence of various types of wear such as adhesion wear and abrasion wear on the tool rake and flank faces. Adhesion wear represents the formation of the built-up edge, crater wear, and chipping, whereas abrasion wear represents flank wear. The commonly formed wear was crater wear. Therefore, the optimum tool life among the executed cutting trails was identified according to minimum length and depth of the crater wear. The profile measurements showed the formation of new geometries for the worn cutting edges due to adhesion and abrasion wear and the cutting parameters. The formation of the built-up edge was observed on the rake face of the cutting tool. The microstructure of the built-up edge was investigated using SEM. The built-up edge was found to have the austenite shear lamellar structure which is identical to the formed shear lamellae of the produced chip.
Super Austenitic Stainless Steel (SASS) AL-6XN alloy is a special type of Austenite Stainless Steel (ASS) distinguishable by its high alloying contents. High corrosion resistance of the alloy is gained by the presence of chromium, nickel, molybdenum, and nitrogen alloying elements. The alloying elements enhance the resistance of the alloy to stress corrosion cracking, crevices, and pitting corrosion. In addition, AL-6XN has high ductility, toughness, and strength, especially at elevated temperatures, compared to nominal austenitic grades. AL-6XN alloy is applied in various applications such as in offshore structures, chemical waste processing, food containers, transformer cases, and pumps parts [
A machinability study has been conducted by the authors of this work on the AL-6XN SASS alloy [
The austenitic stainless steel alloy type X5 CrMnN 18 18 has been machined using cemented carbide tools [
In the reviewed literature, wear investigations on machining various materials and alloys, including common ASS grades, have been conducted; however, a research gap exists in regard to the gradual progress of machining alloys progressing from ASS to SASS. To the best of our knowledge, machinability assessment due to tool wear analysis has not been studied on AL-6XN SASS alloy in machining field. Therefore, this paper’s aim is to continue the authors’ investigations on machinability of AL-6XN SASS alloy using tool wear analysis. This investigation consists of two parts. The first includes an observation analysis of tool wear using SEM according to the applied cutting parameters. The second part studies the geometry response of the cutting tool during the machining process. It is important in the machining field to establish the edge profile of the cutting tool, as this profile influences chip formation, performance, cutting tool life, and the thermal and mechanical loads created on the edges of the cutting tool during the machining process [
A spectrometry test was used to reveal the chemical composition of AL-6XN SASS alloy. Table
AL-6XN SASS chemical composition.
Element | Weight (%) |
---|---|
C | 0.02586 |
Si | 0.3586 |
Mn | 0.3696 |
P | 0.0439 |
S | 0.00163 |
Cr | 21 |
Mo | 6.066 |
Ni | 24 |
The designation of the cutting trials was selected based on the authors’ recently conducted work [
Arrangement of the cutting trials.
Trial | Cutting speed (m/min) | Feed/tooth (mm/tooth) | Cutting depth (mm) | Coolant |
---|---|---|---|---|
1 | 100 | 0.1 | 2 | On |
2 | 100 | 0.1 | 3 | |
3 | 100 | 0.15 | 2 | |
4 | 100 | 0.15 | 3 | |
5 | 150 | 0.1 | 2 | |
6 | 150 | 0.1 | 3 | |
7 | 150 | 0.15 | 2 | |
8 | 150 | 0.15 | 3 |
A 5-axis SPINNER U620 CNC machining centre was used to execute the cutting trials. The specifications of the CNC machine were 15 kW spindle power and table diameter of 650 mm. An ISCAR HELIDO 490-09 insert of cutting grade IC830 was used during the machining test. The cutting insert designation standard is H490 ANCX 090416PDR and has dimensions and geometry as provided by the manufacturer’s data as shown in Figure
Dimensions and geometries of (a) milling cutter (cutting tool). (b) AL-6XN block.
Experimental setup.
A Zeiss Supra 55VP SEM was used to inspect the edges of the cutting inserts (Figure
SEM system. (a) Monitors and joysticks used to apply the adequate settings. (b) SEM vacuum chamber.
An Alicona InfiniteFocus (AIF) optical profilometer was used to scan the profile of the cutting edge. More details about the technical specification and scanning process using AIF are available online [
The edges of the cutting inserts were examined under the SEM detector to reveal the presence of the BUE. The BUE formation was identified on the rake faces of cutting inserts 5 and 7, respectively, as shown in Figures
SEM images of the cutting insert of trial 5 revealing wear and BUE formation.
SEM images of the cutting insert of trial 7 revealing wear and BUE formation.
When the BUE was examined under the SEM detector, the BUE was found to consist of austenite shear lamellae. Figure
(a) Inspection of the microstructure of the formed BUE in trial 7. (b) Inspection of the formed chip of trial 7 to make a comparison with the microstructure of the BUE.
Figure
SEM images of the cutting insert of trial 6 to reveal wear types.
SEM image of the chip cross section which reveals the presence of carbides and the adhered particles causing FW on the insert’s flank faces.
In Figure
Generally, FW was detected on the cutting edges of the eight cutting trials but with low values. However, when a high cutting speed of 150 m/min was applied, the FW of cutting trials 5–8 was smaller compared to the FW in the cutting trials 1–4, where a low speed of 100 m/min was used. When high feed of 0.15 mm/tooth was used in trials 3 and 7, the depth of FW increased. The effects of the depth of cut on FW were noticed and studied in trials 2, 3, and 7. When the depth of cut value was changed from 3 mm in trial 2 to 2 mm in trial 3, FW increased, which was the same effect when the depth of cut of 2 mm was used in trial 7. The FW on the flank faces of the cutting inserts was probably induced due to the presence of the formed carbides in the alloy microstructure during the cutting process, as shown in Figure
It can be seen from Figures
Estimation of the length and the depth of CW formed on the cutting edges of the eight cutting trials.
The cutting inserts were subjected to profile measurement analysis using IF-MeasureSuite 5.1 software to determine the relationship between tool geometry and wear and with cutting parameters. The changes in the cutting tool geometry, locations of the wear, and BUE were shown through this analysis. The profile of the new cutting edge was measured first to indicate the positions of the tool tip and rake and flank faces; then the measurements were applied on the eight worn cutting edges to view the new shaped profiles as presented in Figure
Cutting tool path profile estimation for the new and worn cutting edges with respect to the applied cutting conditions and the generated FW, CW, chipping, and BUE.
The effects of the FW and CW were found on the flank and rake faces, respectively, of the eight cutting inserts. Chipping wear significantly affected the geometries of the cutting inserts used in trials 3, 4, and 8 by lowering cutting edges. Sharp and long edges were formed on the inserts’ rake faces in trials 5 and 7 due to the presence of the BUE.
At the low cutting speed and low feed rate in trials 1 and 2, the cutting edges were worn and their locations were shifted to the right of the reference edges (Figure
It can be summarised from the profile measurements of the cutting tool edges that the newly formed profiles affected the obtained results from the authors’ preliminary machining study [
In this work, eight cutting trials were conducted using a milling machine on AL-6XN SASS alloy to study wear and geometry response of cutting tools to the applied cutting parameters. The main outcomes are as follows. Abrasive and adhesive wear types were identified on the cutting tool edges using SEM. FW and CW represented abrasive wear, whereas the adhered BUE represented adhesive wear. The located carbides and the adhered small pieces of the cutting edges on the backside of the formed chip motivated and increased the spread of FW and FW, respectively, when the chip slipped on the tool during the cutting process. Chipping wear was recognised on the cutting wedge of the cutting tool. This wear was created due to the surface subtraction phenomenon found in machining when a BUE is evident. The BUE is an unstable adhered layer that can be separated easily from the hosting surfaces and causes pitting and microcracks which are the chipping wear motivation. The common wear in this study was CW. Therefore, the dimensions of CW (length and depth) were used to find the optimum life of the eight cutting tools. The length and the depth of CW were calculated. Maximum length and depth of 2624 Trials 5 and 7, where the high cutting speed, depth of cut, and low feed rate were applied, showed a significant BUE formation on the rake faces of the cutting inserts. The BUE microstructure was inspected and found to be similar to the microstructure of the formed chip due to material plastic flow and chip slip mechanism during machining. Profile measurements of the cutting tool edges were executed to reveal the geometry response of the cutting edges to the applied cutting parameters as well as to the generated wear types, BUE, and roughness values. At low cutting speed in trials 1–4, the cutting edges were shifted to the right and their heights were changed. At high cutting speeds in trials 5–8, the cutting edges were severely deformed and new sharp, shifted edges were generated. The tip radii of the cutting inserts were changed according to wear type. Large radii were identified when the cutting wedge was lowered in trials 1, 2, and 8. Two large and four small rounded edges were formed due to chipping wear in trials 3 and 4, respectively. BUE formation resulted in edges of sharp angles in trials 5 and 7, whereas CW on the cutting edge of trial 6 formed a long and flat (chamfered) edge. For workshop practice, these concluding results can serve as reference data to predict tool life and to machine SASS using cutting parameters which best avoid aggressive tool wear and frequent BUE formation.
The authors declare that there are no competing interests regarding the publication of this paper.