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Title: MODELING OF HIGH SPEED FRICTION STIR SPOT WELDING USING A LAGRANGIAN FINITE ELEMENT APPROACH

Abstract

Friction stir spot welding (FSSW) has been shown to be capable of joining steels of very high strength, while also being very flexible in terms of controlling the heat of welding and the resulting microstructure of the joint. This makes FSSW a potential alternative to resistance spot welding (RSW) if tool life is sufficiently high, and if machine spindle loads are sufficiently low so that the process can be implemented on an industrial robot. Robots for spot welding can typically sustain vertical loads of about 8kN, but FSSW at tool speeds of less than 3000 rpm cause loads that are too high, in the range of 11-14 kN. Therefore, in the current work tool speeds of 3000 rpm and higher were employed, in order to generate heat more quickly and to reduce welding loads to acceptable levels. The FSSW process was modeled using a finite element approach with the Forge® software package. An updated Lagrangian scheme with explicit time integration was employed to model the flow of the sheet material, subjected to boundary conditions of a rotating tool and a fixed backing plate [3]. The modeling approach can be described as two-dimensional, axisymmetric, but with an aspect of three dimensionsmore » in terms of thermal boundary conditions. Material flow was calculated from a velocity field which was two dimensional, but heat generated by friction was computed using a virtual rotational velocity component from the tool surface. An isotropic, viscoplastic Norton-Hoff law was used to model the evolution of material flow stress as a function of strain, strain rate, and temperature. The model predicted welding temperatures and the movement of the joint interface with reasonable accuracy for the welding of a dual phase 980 steel.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1214912
Report Number(s):
PNNL-SA-98845
VT0505000
DOE Contract Number:
AC05-76RL01830
Resource Type:
Conference
Resource Relation:
Conference: Computational Plasticity XII: Proceedings of the XII International Conference on Computational Plasticity - Fundamentals and Applications (COMPLAS XII), September 2-5, 2013, Barcelona, Spain
Country of Publication:
United States
Language:
English
Subject:
Friction Stir Spot Welding; Lagrangian Finite Element Method; Advanced High strength steel

Citation Formats

Miles, Michael, Karki, U., Woodward, C., and Hovanski, Yuri. MODELING OF HIGH SPEED FRICTION STIR SPOT WELDING USING A LAGRANGIAN FINITE ELEMENT APPROACH. United States: N. p., 2013. Web.
Miles, Michael, Karki, U., Woodward, C., & Hovanski, Yuri. MODELING OF HIGH SPEED FRICTION STIR SPOT WELDING USING A LAGRANGIAN FINITE ELEMENT APPROACH. United States.
Miles, Michael, Karki, U., Woodward, C., and Hovanski, Yuri. Tue . "MODELING OF HIGH SPEED FRICTION STIR SPOT WELDING USING A LAGRANGIAN FINITE ELEMENT APPROACH". United States. doi:.
@article{osti_1214912,
title = {MODELING OF HIGH SPEED FRICTION STIR SPOT WELDING USING A LAGRANGIAN FINITE ELEMENT APPROACH},
author = {Miles, Michael and Karki, U. and Woodward, C. and Hovanski, Yuri},
abstractNote = {Friction stir spot welding (FSSW) has been shown to be capable of joining steels of very high strength, while also being very flexible in terms of controlling the heat of welding and the resulting microstructure of the joint. This makes FSSW a potential alternative to resistance spot welding (RSW) if tool life is sufficiently high, and if machine spindle loads are sufficiently low so that the process can be implemented on an industrial robot. Robots for spot welding can typically sustain vertical loads of about 8kN, but FSSW at tool speeds of less than 3000 rpm cause loads that are too high, in the range of 11-14 kN. Therefore, in the current work tool speeds of 3000 rpm and higher were employed, in order to generate heat more quickly and to reduce welding loads to acceptable levels. The FSSW process was modeled using a finite element approach with the Forge® software package. An updated Lagrangian scheme with explicit time integration was employed to model the flow of the sheet material, subjected to boundary conditions of a rotating tool and a fixed backing plate [3]. The modeling approach can be described as two-dimensional, axisymmetric, but with an aspect of three dimensions in terms of thermal boundary conditions. Material flow was calculated from a velocity field which was two dimensional, but heat generated by friction was computed using a virtual rotational velocity component from the tool surface. An isotropic, viscoplastic Norton-Hoff law was used to model the evolution of material flow stress as a function of strain, strain rate, and temperature. The model predicted welding temperatures and the movement of the joint interface with reasonable accuracy for the welding of a dual phase 980 steel.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Tue Sep 03 00:00:00 EDT 2013},
month = {Tue Sep 03 00:00:00 EDT 2013}
}

Conference:
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  • Superplastically-formed structural panels are growing in their applications in aerospace, aircraft, automotive, and other industries. Generally, monolithic sheets are employed, limiting the size and complexity of the final part. However, more complex and larger final geometries are possible if individual sheet materials can be joined together through an appropriate joining technology, then SPF formed to final shape. The primary challenge in this type of SPF fabrication has been making a joint between the sheets that will survive the SPF forming event and display the correct amount of elongation in the joint relative to the base materials being formed. Friction Stirmore » Welding is an ideal joining technology for SPF applications because the forming response of the weld metal at SPF conditions is adjustable by selecting different weld process parameters during initial joining. This allows the SPF deformation in the weld metal to be “tuned” to the deformation of the parent sheet to prevent early failure from occurring in either the weld metal or the parent sheet due to mismatched SPF flow stresses. Industrial application of the concept of matching flow stresses is currently being pursued on a program at the Pacific Northwest National Laboratory on room temperature formed friction stir welded tailor welded blanks for heavy truck applications. Flow stress matching and process parameter “tuning” is also important in the fabrication of SPF multisheet structural panels. These panels are fabricated by joining three sheets together with alternating welds top and bottom, so that each weld penetrates only two of the three sheets. This sheet pack is then sealed with a weld seam around the outside and hot gas is introduced between the sheets through a welded tube. Under SPF conditions the sheet pack inflates to produce an internally supported structure. In this paper we presents results on an investigation into using FSW and Refill Friction Stir Spot Welding to fabricated 5083 aluminum multisheet packs that can be SPF formed into 3-D structural or integrally stiffened panels. Several configurations of 3-sheet egg crate and truss structures were friction stir welded and hot gas SPF formed in a parallel-platen SPF press. Data on weld conditions for optimum SPF forming as well as pre- and post- forming microstructures will be presented. It is found that FSW process conditions are a key feature of a successful SPF forming operation and the nugget microstructures and other features of the weld zone can be optimized to produce a wide range of weld region elongations. Friction Stir Welding may prove to be the enabler that allows aluminum to be considered in multisheet and integrally stiffened SPF Aluminum structures.« less
  • Friction stir spot welding was used to join two advanced high-strength steels using polycrystalline cubic boron nitride tooling. Numerous tool designs were employed to study the influence of tool geometry on weld joints produced in both DP780 and a hot-stamp boron steel. Tool designs included conventional, concave shouldered pin tools with several pin configurations; a number of shoulderless designs; and a convex, scrolled shoulder tool. Weld quality was assessed based on lap shear strength, microstructure, microhardness, and bonded area. Mechanical properties were functionally related to bonded area and joint microstructure, demonstrating the necessity to characterize processing windows based on toolmore » geometry.« less
  • Friction stir spot welding was used to join two advanced high-strength steels using polycrystalline cubic boron nitride tooling. Numerous tool designs were employed to study the influence of tool geometry on weld joints produced in both DP780 and a hot-stamp boron steel. Tool designs included conventional, concave shouldered pin tools with several pin configurations; a number of shoulderless designs; and a convex, scrolled shoulder tool. Weld quality was assessed based on lap shear strength, microstructure, microhardness, and bonded area. Mechanical properties were functionally related to bonded area and joint microstructure, demonstrating the necessity to characterize processing windows based on toolmore » geometry.« less
  • Friction stir spot welding techniques were developed to successfully join several advanced high strength steels. Two distinct tool materials were evaluated to determine the effect of tool materials on the process parameters and joint properties. Welds were characterized primarily via lap shear, microhardness, and optical microscopy. Friction stir spot welds were compared to the resistance spot welds in similar strength alloys by using the AWS standard for resistance spot welding high strength steels. As further comparison, a primitive cost comparison between the two joining processes was developed, which included an evaluation of the future cost prospects of friction stir spotmore » welding in advanced high strength steels.« less