U.S. ARMY RESEARCH, DEVELOPMENT AND ENGINEERING COMMAND

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1 U.S. ARMY RESEARCH, DEVELOPMENT AND ENGINEERING COMMAND Solid State Joining of Rolled Homogeneous Armor to Aluminum Martin McDonnell Experimenter Mechanical Engineer DISTRIBUTION A. Approved for public release; distribution unlimited. OPSEC #: OPSEC2120, OPSEC1870 Materials Division - Joining and Additive Manufacturing (JAM) Team 19 FEB 2019

2 ACKNOWLEDGEMENTS This work was partially funded through the Advanced Vehicle Powertrain and Technology Alliance (AVPTA) Thanks to PNNL Staffs- Md. Reza-E-Rabby and Kenneth Ross - FSW Nicole R. Overman - Scanning Electron Microscopy Matthew J. Olszta STEM Timothy Roosendaal- Mechanical Testing 2

3 BACKGROUND: FSW OF DISSIMILAR MATERIALS Dissimilar Alloys One of the hallmarks of FSW well developed for certain combinations Dissimilar Thickness Generally used for TWBs mass-production ready Dissimilar material systems Available in literature for around one decade Problematic results Generally not considered production viable 3

4 BACKGROUND: FSW SCRIBE TOOLING FSW Scribe Technology Developed at PNNL to overcome the challenge of joining dissimilar metals with vastly differing melting temperatures Combines two well understood technologies into a single process: FSW tool and a milling cutter are combined into a single tool Allows the cutter to contact the steel without overheating the aluminum Creates a geometrically beneficial shape in the steel that increases bond strength Shows great promise in many aluminum to steel applications 4

5 BACKGROUND: FSW SCRIBE RESULTS IN RHA STEEL Used to create joints between ½ AL 6061 and RHA Steel FSS experienced a number of challenges including: Premature scribe failure Scribe clogging Low strength Brittle failure Uncontrolled hook formation Not repeatable Use of multiple scribes results in RHA buildup between scribes and inability of scribes to cut material 5

6 FRICTION STIR SCRIBE (FSS) Key observations from FSS: IMC formed only at locations where scribe contacted RHA IMC thickness was too thick (>2µm) and uncontrollable Hook geometry acted as a stress concentration Mechanical performance unpredictable Determined that FSS was not a viable process due to some significant limitations 6

7 FRICTION STIR DOVETAILING (FSD) Transitioned to FSD in order to address challenges encountered with FSS: Interlocking repeatability controlled through pre-machined dovetail groove IMC thickness controlled through force and temperature control Eliminates early failure and clogging Higher strength and less brittle than FSS FSD became primary approach when preliminary data showed dramatic strength, elongation, and process control advantages over FSS 7

8 BACKGROUND: MECHANICAL INTERLOCK Traditional Friction Stir Welding Friction Stir Dovetailing 8

9 BACKGROUND: MECHANICAL INTERLOCK+ METALLURGICAL BONDING The joint strength is improved by the combined effect of dovetail interlock and metallurgical bonding 9

10 BACKGROUND: MECHANICAL INTERLOCK + METALLURGICAL BONDING FEA Effective plastic strain contours from the LS DYNA structural analyses. (a) Without IMC formation and (b) with IMC layer formation along the Al-RHA dovetail interface. 10

11 Reported at TMS BACKGROUND: MECHANICAL INTERLOCK + METALLURGICAL BONDING EXPERIMENTAL RESULTS Corners break or deform Base metal fails in HAZ No metallurgical bonding With metallurgical bonding 11

12 FRICTION STIR DOVETAILING (FSD) Friction stir dovetailing: FSP created mechanical interlock strengthen by metallurgical bonding within the mechanical interlock How is metallurgical bonding achieved within the dovetail? Can this be done without overheating the aluminum? How is formation of detrimental intermetallics suppressed? Is the process repeatable? What is the tool life? 12

13 FRICTION STIR DOVETAILING (FSD) WC Insert Enables formation of intermetallic with very low heat input Long tool life Temperature is controlled Limits intermetallic growth Prevents overheating of aluminum Contact Between WC and RCA is controlled Deflection compensated position control 13

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15 EXPERIMENTAL SETUP Joining Trials Lap Joint Configuration Interlock Geometry: Type mm A Dovetail: Mechanical Interlocking B Flat Interface: Metallurgical Bonding C Dovetail: Interlocking + Bonding D Trench: Interlocking + Bonding Shoulder Temperature is 470 C for all welds Plunge Depth of WC into RHA 15

16 EXPERIMENTAL SETUP 1 A) Dovetail: Mechanical Interlocking B) Flat Interface: Metallurgical Bonding C) Dovetail: Interlocking + Bonding D) Trench: Interlocking + Bonding 16

17 RESULTS: SEM OF INTERFACE 17

18 STEM/TEM OF INTERFACE STEM results obtained from the Al-RHA interface show a banded layer of refined, Si-rich, second phase dispersoids along the interface extending ~1.5 μm into the Al layer Formation of a crystalline, Si-rich intermetallic layer averaging nm in thickness is also observed at the Al-RHA interface. 18

19 PULL TESTS 19

20 FRICTION STIR DOVETAILING IMPROVEMENTS Demonstrated strength and elongation benefits of FSD compared to other Al-Steel friction stir joining techniques 20

21 FRICTION STIR DOVETAIL ACCOMPLISHMENTS Journal Articles Md. Reza-E-Rabby, K. Ross, N. Overman, M. Olszta, M. McDonnell, S. Whalen, Joining Thick Section Aluminum to Steel with Suppressed FeAl Intermetallic Formation via Friction Stir Dovetailing, Scripta Materialia, 148, 63-67, Md. Reza-E-Rabby, Kenneth Ross, Martin McDonnell, Scott A. Whalen, Numerical Simulation and Experimental Validation of Joint Performance in Al-Steel Lap Welds Formed Using Friction Stir Dovetailing, In preparation, to be submitted in Journal of Materials Processing Technology. Md. Reza-E-Rabby, Kenneth Ross, Martin McDonnell, Scott A. Whalen, Joining of 7099 to RHA via hybrid FSD and Cold Spray Techniques, In preparation. Patent Applications S. Whalen, K. Ross, Md. Reza-E-Rabby, Y. Hovanski, System and Process for Joining Dissimilar Materials and Solid-State Interlocking Joint with Intermetallic Interface Formed Thereby, US Patent Application 15/694,565 filed on 9/1/2017. K. Ross, S. Whalen, Md. Reza-E-Rabby, Friction Stir Interlocking, US Patent Application 15/794,687 filed on 10/26/2017. S. Whalen, Md. Reza-E-Rabby, K. Ross, M. McDonnell, Joining AA7XXX Series Aluminum to Steel using AA6XXX Friction Stir Dovetail Interlayer, Provisional Patent Application, Sept Conference Papers and Presentations 12th International Symposium on Friction Stir Welding, June 26-28, SAE WCX World Congress Experience, Apr , Defense Manufacturing Conference (DMC) 2017, 2018 Friction Stir Welding and Processing IX TMS 2017, 7th Annual Global Automotive Lightweight Materials Summit, Aug , 2017 TARDEC Ground Vehicle Survivability Symposium (GVSS), Nov , 2016, Global Automotive Lightweight Materials Series: Lightweight Vehicle Manufacturing, Feb 24-25, 2016 PNNL Graduate Research Symposium, 2016, 2017 Media Coverage Press release accompanying Scripta Materialia Paper KNDU-TV spot Featured articles in Design News and Tribology & Lubrication 21

22 CONCLUSIONS FSD is a promising new technique for joining thick section Al-steel SEM and TEM results of the Al-RHA interface have confirmed the presence of a sub-micron interlayer formation, likely FeAl3 The ability to inhibit the growth of IMC layers within mechanically interlocking dovetail grooves has been demonstrated The novel tooling and temperature control approach developed herein may be used to tailor the IMC thickness in FSD or more classical FSW Al-steel joints Joining other dissimilar materials that are otherwise unable to be welded by conventional means, such as Al-Cu, Cu-Steel, Mg-Steel, Al-Ti or metal matrix composites, may also be possible with the FSD technique. 22

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