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Challenges in Resistance Welding of Ultra High Strength Steels
KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Welding Technology.
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Increasing the use of Ultra High Strength Steels (UHSS) in vehicle bodystructures is important for reducing weight and cutting CO2 emissions. This thesis investigates challenges in resistance welding that can be a barrier to implementing UHSS as a replacement for low strength steels in vehicle structures. Empirical research has been performed to offer new approaches for improved joint strength and to increase knowledge on cracking mechanisms in resistance projection welding and resistance spot welding of UHSS. By optimising the current build-up phase and peak current during the first milliseconds of weld time, it was shown that the strength could be improved by up to two-fold for projection welded joints. An approach to improve the ductility and strength of resistance spotwelds in UHSS using reduced cooling time was unsuccessful. The reduced cooling rate after weld metal solidification did not fully create the desired softened microstructure. The study on the surface cracking mechanism in resistance spot welded dual-phase UHSS showed that cracking is linked to the galvanization method. It is proposed that formation of aluminium oxide layers on the electrode tips increases the surface temperature and thereby increases the probability for liquid metal embrittlement and surface cracking.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , xvii, 59 p.
Series
TRITA-IIP, ISSN 1650-1888 ; 15:02
Keyword [en]
Resistance welding, spot welding, surface cracks, nut welding, UHSS, projection welding
National Category
Manufacturing, Surface and Joining Technology
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-167985ISBN: 978-91-7595-577-3 (print)OAI: oai:DiVA.org:kth-167985DiVA: diva2:813651
Presentation
2015-06-03, M311, Brinellvägen 68, KTH, Stockholm, 10:00 (Swedish)
Opponent
Supervisors
Funder
XPRES - Initiative for excellence in production research
Note

QC 20150526

Available from: 2015-05-26 Created: 2015-05-25 Last updated: 2015-05-26Bibliographically approved
List of papers
1. Resistance Nut Welding: Improving the Weldability and Joint Properties of Ultra High Strength Steels
Open this publication in new window or tab >>Resistance Nut Welding: Improving the Weldability and Joint Properties of Ultra High Strength Steels
2007 (English)In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 51, no 3-4, 28-36 p.Article in journal (Refereed) Published
Abstract [en]

This study covers resistance projection welding of three different M6 and M8 weld nut types to high- and ultra high strength thin sheet steels ranging up to 1 500 MPa tensile strength. Static tensile testing, torsion testing and fracture evaluation as well as metallographical examinations were used to evaluate the suitability of the selected nut types for applications in high and ultra high strength steels. Different resistance welding equipment were compared and both alternating current and direct current were evaluated. A new etchant was used for examining the fusion zon ein the nut joints, which indicated that no significant melting of the interfaces had occurred. This was evident for all the weld nut types welded to the galvanized thin sheet steels. The electrical characteristic of the power source plays an important role for the weld result. A rapid current build-up during the first period (AC) of weld time increased the initial heating of the weld nut projections that improved the joint strength. Significant differences in pull-out strength were recorded in joints welded with different AC power sources. Current curve modification for MFDC welding machines is a promising way of increasing the joint strength. A welding cycle with an initial high peak current pulse followed by a current plateau improved the pull-out strength by more than 100 %

Place, publisher, year, edition, pages
Springer-Verlag: , 2007
National Category
Materials Engineering
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-167983 (URN)10.1007/BF03266557 (DOI)
Note

QC 20150526

Available from: 2015-05-25 Created: 2015-05-25 Last updated: 2017-12-04Bibliographically approved
2. Influence of Reduced Cooling Time on the Properties of Resistance Spot Welds
Open this publication in new window or tab >>Influence of Reduced Cooling Time on the Properties of Resistance Spot Welds
2008 (English)In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 52, no 3-4, 43-53 p.Article in journal (Refereed) Published
Abstract [en]

Reducing the cooling time in a resistance spot welding schedule offers two possible advantages. The first is decreased weld cycle time, which is of great importance in particular to car body assembly lines performing hundreds of millions of spot welds each year. Decreasing each resistance spot weld cycle time by a mere 0.1 s leads to substantial cost savings. The second advantage is the possibility to reduce the hardness in the weld metal and thereby improve fracture behaviour. Reducing the cooling time leads to reduced cooling rate of the weld metal and potentially a softer material. Two and three-sheet joint combinations of martensitic, dual phase, transformation induced plasticity (TRIP), complex phase and hardened boron steels were investigated in this study. The joints were evaluated by shear-, peel and cross-tension testing as well as metallographical examinations and hardness measurements. With knowledge of the materials weldability, and evaluation of the behaviour when welding with reduced cooling time, it was possible to develop optimised weld schedules and thereby increase productivity. It is recommended that the programming of the hold time in the power sources is modified in order to enable full flexibility in setting of the hold time /cooling time. This study showed that the cooling time could be reduced significantly without endangering the joint integrity.

Place, publisher, year, edition, pages
Springer-Verlag: , 2008
National Category
Manufacturing, Surface and Joining Technology
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-167984 (URN)10.1007/BF03266631 (DOI)
Note

QC 20150526

Available from: 2015-05-25 Created: 2015-05-25 Last updated: 2017-12-04Bibliographically approved
3. Surface breaking cracks in resistance spot welds ofdual-phase steels with electro and hot dip zinc coating
Open this publication in new window or tab >>Surface breaking cracks in resistance spot welds ofdual-phase steels with electro and hot dip zinc coating
2013 (English)In: Science and technology of welding and joining, ISSN 1362-1718, E-ISSN 1743-2936, Vol. 18, no 1, 25-31 p.Article in journal (Refereed) Published
Abstract [en]

In normal production of resistance spot welded galvanised structures, it is difficult to completely avoid surface breaking cracks. Known key factors to cause cracking are zinc coating, electrode wear during subsequent welding and insufficient electrode cooling. In this report, an embrittlement mechanism was investigated that could be coupled to the galvanisation method for dual phase steels. With identical bulk material and weld parameters, the first 50 spot welds were crack free with electrogalvanised coating, while only 10 out of 50 were crack free with hot dip galvanised coating. Energy dispersive X-ray spectroscopy analysis of the worn electrode surfaces used for welding of the hot dip galvanised coating revealed areas of aluminium oxide. Since aluminium oxide is a very strong isolator, the electrical resistance will increase, which in turn is suggested to increase the surface temperature of the spot weld and thereby increase the probability for liquid metal embrittlement and surface cracks.

Keyword
Resistance spot welding, Liquid metal embrittlement, Surface cracking, Advanced high strength steel
National Category
Engineering and Technology
Research subject
SRA - Production
Identifiers
urn:nbn:se:kth:diva-105376 (URN)10.1179/1362171812Y.0000000068 (DOI)000312215500004 ()2-s2.0-84871349725 (Scopus ID)
Funder
XPRES - Initiative for excellence in production research
Note

QC 20130220

Available from: 2012-11-20 Created: 2012-11-20 Last updated: 2017-12-07Bibliographically approved

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