Skip to content
6CCVD Research

Investigation of the Effect of End Mill-Geometry on Roughness and Surface Strain-Hardening of Aluminum Alloy AA6082

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2020-07-10
JournalMaterials
AuthorsP. G. Filippov, Michael Kaufeld, Martin Ebner, Ursula Koch
InstitutionsMunich University of Applied Sciences, Rohde & Schwarz (Germany)
Citations4
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This study investigates the influence of micro-end mill geometry and feed rate on the surface roughness and subsurface strain-hardening of AA6082 aluminum alloy, focusing on requirements for high-precision manufacturing.

  • Dominant Factor Identified: Tool cutting edge geometry (radius $r$ and roughness) is the primary factor determining surface integrity, while the feed per tooth ($f_z$) in the tested range (3-14 ”m) showed no significant influence.
  • Roughness Achievement: The Monocrystalline Diamond (MD) tool, possessing an ultra-sharp edge ($r$ = 17 nm), achieved superior horizontal surface roughness ($R_q$ = 26 nm). This is approximately 4.5 times smoother than surfaces milled by the Solid Carbide (SC) tool ($R_q$ = 119 nm, $r$ = 671 nm).
  • Strain-Hardening Control: The sharper MD tool minimized the ploughing effect, resulting in a thin strain-hardened zone (< 40 nm deep) with a maximum hardness increase of 125% relative to the bulk material.
  • Ploughing Effect: The SC tool (larger $r$) induced significant strain-hardening (up to 160% increase over bulk) and a deeper hardened zone (< 200 nm), confirming the anticipated micro-milling size-effect due to the undeformed chip thickness being closer to the minimum chip thickness ($h_{min}$).
  • Methodology: Depth-dependent hardness was measured using the Enhanced Stiffness Procedure (ESP) nanoindentation technique, and the resulting data was analyzed using the Korsunsky film-substrate model.
  • Application Relevance: These findings are crucial for the reliable fabrication of high-frequency components, such as terahertz waveguides, where low surface roughness is essential for minimizing transmission loss.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Workpiece MaterialAA6082-T651 (AlSi1MgMn)N/AMedium strength aluminum alloy
Bulk Hardness (AA6082)1195 ± 46MPaMeasured HIT at max indentation depth
Tool 1 TypeSolid Carbide (SC) End-millN/ASingle-tooth
Tool 2 TypeMonocrystalline Diamond (MD) End-millN/ASingle-tooth
SC Tool Cutting Edge Radius ($r$)671nmDetermined via SEM
MD Tool Cutting Edge Radius ($r$)17nmDetermined via SEM
SC Tool Edge Roughness ($R_q$)2355 ± 1551nmAverage cutting edge roughness
MD Tool Edge Roughness ($R_q$)90 ± 48nmAverage cutting edge roughness
Cutter Diameter ($D$)500”mNominal tool size
Rotational Speed ($n$)18,000min-1Milling parameter
Cutting Velocity ($v_c$)28.3m/minMilling parameter
Feed Per Tooth ($f_z$) Range3, 8, 14”mVaried parameter (undeformed chip thickness)
SC Machined Surface $R_q$ (Horizontal)119nmAverage roughness perpendicular to milling
MD Machined Surface $R_q$ (Horizontal)26nmAverage roughness perpendicular to milling
SC Max Hardness IncreaseUp to 160%Relative to bulk HIT
MD Max Hardness IncreaseUp to 125%Relative to bulk HIT
SC Strain-Hardened Zone Depth< 200nmEstimated from 8%-Onset Depth
MD Strain-Hardened Zone Depth< 40nmEstimated from 8%-Onset Depth
Nanoindenter Tip Radius153nmBerkovich indenter
Nanoindentation Max Load500mNESP procedure parameter

Key Methodologies

Section titled “Key Methodologies”

The study employed a combination of micro-milling, advanced microscopy, and depth-sensing indentation to characterize surface integrity.

  1. Reference Sample Preparation: AA6082 and 99.999% pure aluminum (Al5N) specimens were cold mounted and electropolished to create non-strain-hardened reference surfaces for bulk hardness determination.
  2. Tool Geometry Characterization: New SC and MD single-tooth end-mills were analyzed using optical microscopy and Scanning Electron Microscopy (SEM) to determine the wedge angle ($\beta$), cutting edge radius ($r$), and cutting edge roughness ($R_q$, $R_t$).
  3. Micro-Milling Process: Samples were milled on a KERN Pyramid Nano CNC center.
    • Fixed parameters: $v_c$ = 28.3 m/min, $n$ = 18,000 min-1, $a_e$ = 15 ”m, $a_p$ = 500 ”m.
    • Varied parameter: Single feed per tooth ($f_z$) at 3 ”m, 8 ”m, and 14 ”m.
  4. Surface Roughness Analysis: Confocal microscopy (Leitz Ergoplan, 20× lens) was used to image the peripherally milled surfaces.
    • One-dimensional roughness parameters ($R_q$, $R_t$) were extracted from profiles measured perpendicular (horizontal roughness, influenced by tool edge) and parallel (vertical roughness, influenced by $f_z$) to the milling direction.
  5. Depth-Dependent Hardness Testing: Instrumented indentation was performed using a Picodentor HM500 with a Berkovich indenter.
    • The Enhanced Stiffness Procedure (ESP) was utilized, applying the maximum test load (500 mN) in 20 partial load/relief cycles at a single location to obtain HIT values across varying depths (0.2 ”m to 9 ”m).
  6. Strain-Hardening Modeling: The averaged hardness-depth curves were fitted using the Korsunsky film-substrate model (based on a volume law of mixtures) to quantify the film hardness ($H_f$) and the thickness ($t$) of the strain-hardened layer.

Commercial Applications

Section titled “Commercial Applications”

The ability to precisely control surface roughness and minimize subsurface strain-hardening in aluminum alloys is critical for high-performance micro-components, particularly in the following fields:

  • Terahertz (THz) and High-Frequency RF: Manufacturing of high-precision waveguides, filters, and transmission line modules. Ultra-low roughness (e.g., $R_q$ = 26 nm achieved by the MD tool) is mandatory to reduce signal reflection and transmission loss at high frequencies.
  • Measurement Technology: Production of modules for advanced measurement equipment requiring extremely tight dimensional tolerances and stable mechanical properties near the surface.
  • Micro-Electro-Mechanical Systems (MEMS): Fabrication of micro-scaled metallic structures where controlled surface integrity (minimal work hardening) is necessary to ensure predictable long-term mechanical behavior and reliability.
  • Precision Tooling and Molds: Creating micro-molds and dies where the surface finish must be near-perfect to avoid replication defects, requiring tools that minimize both roughness and subsurface plastic deformation.
View Original Abstract

Micro-milling is a promising technology for micro-manufacturing of high-tech components. A deep understanding of the micro-milling process is necessary since a simple downscaling from conventional milling is impossible. In this study, the effect of the mill geometry and feed per tooth on roughness and indentation hardness of micro-machined AA6082 surfaces is analyzed. A solid carbide (SC) single-tooth end-mill (cutting edge radius 670 nm) is compared to a monocrystalline diamond (MD) end-mill (cutting edge radius 17 nm). Feed per tooth was varied by 3 ÎŒm, 8 ÎŒm and 14 ÎŒm. The machined surface roughness was analyzed microscopically, while surface strain-hardening was determined using an indentation procedure with multiple partial unload cycles. No significant feed per tooth influence on surface roughness or mechanical properties was observed within the chosen range. Tools’ cutting edge roughness is demonstrated to be the main factor influencing the surface roughness. The SC-tool machined surfaces had an average Rq = 119 nm, while the MD-tool machined surfaces reached Rq = 26 nm. Surface strain-hardening is influenced mainly by the cutting edge radius (size-effect). For surfaces produced with the SC-tool, depth of the strain-hardened zone is higher than 200 nm and the hardness increases up to 160% compared to bulk. MD-tool produced a thinner strain-hardened zone of max. 60 nm while the hardness increased up to 125% at the surface. These findings are especially important for the high-precision manufacturing of measurement technology modules for the terahertz range.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2006 - Effects of surface roughness on reflection spectra obtained by terahertz time-domain spectroscopy [Crossref]
  2. 2014 - Fabrication of multilayered SU8 structure for terahertz waveguide with ultralow transmission loss [Crossref]
  3. 2019 - Influence of cutting parameters on surface roughness and strain hardening during milling NiTi shape memory alloy [Crossref]
  4. 1996 - Materials Processing Technology
  5. 2007 - An experimental investigation of micro-machinability of copper 101 using tungsten carbide micro-endmills [Crossref]
  6. 2017 - Surface quality in micro milling: Influences of spindle and cutting parameters [Crossref]
  7. 2017 - Experimental study of micro-milling mechanism and surface quality of a nickel-based single crystal superalloy [Crossref]
  8. 2009 - Size effects of micro drilling in steel [Crossref]
  9. 2008 - Modelling and analysis of micro scale milling considering size effect, micro cutter edge radius and minimum chip thickness [Crossref]

Reads Similar to This

A Wire Bow Model of Diamond Wire Sawing with Asymmetric Arc Hypothesis Texture for Diamond Wire Sawn Multicrystalline Silicon Wafers by a Vapor Etching Method The 4-inch free-standing diamond film prepared by MPCVD at 10kW