INDUSTRIAL MATERIALS

Development of a Friction Spot Joining Tool for980 MPa Tensile Strength Steels Joining
Hiroka MIYAZAKI*, Yoshiharu UTSUMI and Keiichi TSUDA---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Car manufacturers have been increasing the use of ultra-high tensile strength steels with strengths exceeding 980 MPa. This
is to improve collision safety and reduce vehicle weight, thereby reducing CO2 emissions. Due to their high carbon content,these steels cannot achieve sufficient joining strength when joined using the current resistance spot welding technology. To
achieve high joining strength in high-carbon content steels, we focused on the friction spot joining (FSJ) method, a solidphase joining method that employs the frictional heat of the joining tool to soften and ultimately join the sheets through aplastic flow process. FSJ has already been commercialized for the joining of aluminum sheets, but its poor tool life has
prevented its application to steel sheet joining. In order to make the FSJ method practically applicable to steel joining, wedeveloped a cemented carbide tool base material with good resistance to breakage, plastic deformation and thermal
cracking, a PVD coating that provides hardness and oxidation resistance, as well as a tool geometry. We also determined
appropriate joining conditions to reduce tool wear. By integrating all these elements, we have successfully developed an FSJ
tool with superior tool life (7,000 spots) for ultra-high tensile strength steel joining.

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Keywords: friction spot joining (FSJ), friction stir welding (FSW), ultra-high tensile strength steel sheets, resistance spot
welding (RSW)

1. Introduction

2. Friction Spot Joining Technology

Given the growing focus on global warming inrecent years, reducing the weight of vehicles to cut CO2
emissions while at the same time improving collisionsafety has been a critical issue for auto manufacturers,
and this has accelerated the adoption of ultra-hightensile strength steel sheets with a tensile strength of980 MPa or higher. For joining these automobile steel
sheets, resistance spot welding (RSW)*1 and laser beam
welding*2 are currently the mainstream techniques.
However, to increase the tensile strength of the steel
sheets a higher carbon content is required. The
currently used methods have the drawback of being
difficult in achieving sufficient joining strength inprecisely these high carbon content steels. The reason
is that an embrittlement phase (martensite phase)
forms in the weld during fusion joining, something that
hardens the material, resulting in breakage.(1) As higher
tensile strength steel sheets with higher carbon content
are required in the future, it will therefore become
increasingly difficult to join them using RSW.
In contrast, as it is a non-fusion joining technique,friction spot joining (FSJ) has recently been attracting
attention as a method for joining two high-carbon steel
materials or sheets of two different materials. However,since the tools used for joining steel sheets by FSJ havea limited tool life, the method has not yet been put into
practice. With the goal of realizing FSJ tools for ultrahigh tensile strength steel sheets, by using the technology we have accumulated in manufacturing
cemented carbide*3 cutting tools, which are our mainstream products, and by cooperating with Kawasaki
Heavy Industries, Ltd., a FSJ machine manufacturer, wehave developed a tool with long tool life for joining 980
MPa-class ultra-high tensile strength steel sheets.

FSJ is a new solid phase joining technique developed in 1991 by The Welding Institute (TWI), a researchorganization in the UK.(2) It is a non-fusion joining technique that relies on frictional heat generated by letting atool, with a protrusion called a probe, rotating at highspeeds and then pressing the tool against the sheets tobe joined. This will soften the work material and join the
sheets through a plastic flow process. The joining
method is called friction stir welding (FSW) if
performed by linearly moving the tool while pressing it
against the materials to be joined, or FSJ if the materials
are joined in spots. FSJ is an improved and more developed version of FSW, which was realized in 2004
through joint development by Kawasaki Heavy
Industries, Ltd. and Mazda Motor Corporation.(3) Photo 1
shows external views of the FSJ robot system and thejoining tool that is attached to the end of the joining
gun. Figure 1 is a schematic representation of the FSJ
process, and Photo 2 is an exterior view of the two 980
MPa-class ultra-high tensile strength steel sheets joinedusing FSJ.
While FSJ uses a robot system similar to that usedin RSW, it does not, however, require auxiliary equipment such as the water cooling system and the resistance welding machine that accompanies RSW, thus
reducing facility and running costs. The main energyconsuming parts in FSJ are only two motors that drive
the joining tool. Thus, FSJ is an energy-efficient technique compared with RSW. Moreover, in contrast to
RSW that generates both fumes and expulsion, it ispossible to maintain a clean working environment
during joining using FSJ joining.
The characteristics of RSW, laser beam welding,and FSJ are summarized in Table 1. As FSJ is a non-

SEI TECHNICAL REVIEW · NUMBER 81 · OCTOBER 2015 · 67

fusion joining (solid phase joining) technique, it ispossible to join two high-carbon steel materials or two
entirely different materials. However, the method has
not been put into practice due to its problems of short
tool life. Joining aluminum has already been realizedusing tools made of tool steel*4 as the tool temperatureduring joining is low, only 400 to 500°C. In the case of
joining steel, however, the tool temperature reaches
around 1,000°C, and this results in a much shorter tool
life. It is against this background that we started developing a tool with long tool life using cemented carbide.

Joining Gun
Joining Tool

ShoulderFSJ Robot

Probe

3. FSJ Joint Strength
Photo 1. External views of the FSJ robot system and joining tool

Pressure

Rotation

Rotation

Rotation

Materials to be joined(superimposed)

Completion
Stirring (plastic flow) area

Fig. 1. Schematic representation of the FSJ process

Front SideIn aluminum FSJ, the probe of the tool is threaded
to promote plastic flow by stirring. However, in the caseof joining ultra-high tensile strength steel, the stirring
effect from the thread is smaller than when joining
aluminum, and the threaded probe is easily chipped.
Therefore, we optimized the diameter and length of theprobe instead of threading it, and found conditions
under which a high joint strength (tensile shear
strength) that exceeds the JIS-A-class average can beobtained. A simple circular truncated conical probe isless likely to wear away and be chipped and also has
the added advantage of reducing machining costs(Photo 3).

Back Side

Photo 2. External views of an FSJ joint (980 MPa-classultra-high tensile strength steel sheets)

Photo 3. Threaded probe tool (left) and circular truncatedconical probe tool (right)

Table 1. A comparison of different joining methods
Resistance Spot
Welding (RSW)

Laser beam
Welding

Friction Spot
Joining(FSJ)

Joining
methodFusion joining

Fusion joining

Sold phasejoining

Joiningdifferentmaterials

Not possible

Not possible

Possible

Joining
medium andhigh carbon
steel

Not possible

Not possible

PossiblePower
consumption

MediumMediumLow

Workingenvironment

Not good(Fume andexpulsion)

Not good(Spatter and
fume)

Good

Joining cost

Low

High
(Equipment
cost)

High
(Tool cost)

Using steel sheets with various strengths, theTensile Shear Strength (TSS) and Cross Tensile Strength(CTS) of the FSJ and RSW (4) joints were compared toeach other. The results are displayed in Figure 2. While
the TSS of RSW increases gradually for steel sheetswith lower strengths, this increase levels off above 980
MPa. For FSJ, however, the TSS continues to increase
even for 1,500 MPa strength steels. The CTS of both
RSW and FSJ show similar behaviors, although FSJ
displays higher values than RSW. As stated above, RSW,
which is a fusion joining method, generate cracks due
to embrittlement of the joint that often become theorigin of a rupture, and we believe this to be the reason
for the higher CTS values of FSJ. Consequently, onemust conclude that FSJ is superior to RSW in terms ofboth TSS and CTS.

68 · Development of a Friction Spot Joining Tool for 980 MPa Tensile Strength Steels Joining

TSS（kN）

20

FSJ

15

RSW

10

(1) Heat Cracking(2) Plastic Deformation(3) Chipping(4) Wear(5) Oxidation

New Tool

5
0
0

500

1000

1500

2000

Base Material Strength (MPa)(a) Tensile Shear Strength (TSS)CTS（kN）

20
15
10

Photo 4. Tool damage modes

FSJ

5

RSW

0
0

500

1000

1500

2000

Base Material Strength (MPa)demonstrated the difficulty in realizing a tool with asufficiently long tool life.

(b) Cross Tensile Strength (CTS)
Fig. 2. Tensile shear strength and cross tensile strength of
FSJ and RSW

Considering the trend towards higher tensile
strength and greater carbon content, FSJ is a promisingtechnology that can be expected to replace RSW in the
future.

4. Tool Damage Evaluation
There are numerous grades of cemented carbides
used for different applications. During the course of thiswork, we manufactured tools using several typicalcemented carbides grades generally used for cutting
tools to evaluate the potential of the existing grades. For
the evaluation, two 1.2-milimeter-thick sheets of 980
MPa-class ultra-high steel were laid on top of each otherand joined using FSJ. The resulting damage to the tool
can be classified into (1) to (5) as shown in Photo 4.
The experiments revealed that it is necessary to achieve
high heat crack resistance, high plastic deformation
resistance, high chipping resistance, high wear resistance, and high oxidation resistance in order to realize atool with a long tool life. However, the tools manufactured using the existing cemented carbide grades
displayed the damage modes shown in Photo 4 (1) to(5) after mere tens to hundreds of tool passes. This

5. Development of a FSJ Tool with
Superior Tool LifeAs FSJ requires frictional heat to be generated
between the tool and the materials to be joined, for efficient joining the base material of the tool should have
low thermal conductivity to prevent heat dissipation.
However, if the thermal conductivity is too low, the heat
cycles during continuous spot joining can cause heatcracking at the tool shoulder. Therefore, it was necessary to optimize the thermal conductivity of thecemented carbide base material.
As the FSJ tool is exposed to heavy loads, it mustbe hard enough to resist plastic deformation. However,in cemented carbides there is generally a trade-off
between the hardness and toughness, thus excessive
hardness can result in chipping. It was therefore necessary to develop a base material with a good balance
between hardness and toughness appropriate for FSJ.
In addition, since the probe, which is the mostimportant part for maintaining joint strength, during
joining will be rotating at high speeds and high temperatures in the materials to be joined, it is subject toaggressive wear, resulting in degradation of the joining
strength. Also, as the tool shoulder is repeatedly
exposed to high temperatures in the atmosphere, theoxidized base material wears out, resulting in burrs at
the joint and leading to a degradation of the joint
quality (Photo 5).

SEI TECHNICAL REVIEW · NUMBER 81 · OCTOBER 2015 · 69

tests under Conditions 1 and 2. For the same joining
time, the simulations show that the rise in tool temperature is smaller under Condition 2 compared to Condition
1. By selecting Condition 2, it was thus possible to lower
the tool temperature during joining, minimize the probe
wear and the oxidative wear of the shoulder.
Through the development work described above,
the tool life, which was originally a mere several hundred
spots, was improved to maintain a joint strength above
the JIS-A-class minimum even after approximately
7,000 spots (Figure 4). Considering the fact that RSW

With burrs (bad joining quality)[K]

Without burrs (good joining quality)
Photo 5. Exterior photos of the joined materials

To fulfill all necessary requirements, we developed
a base material that has heat conductivity, hardness,and toughness appropriate for FSJ by optimizing thecomposition of the material as well as the WC particle
diameter, thereby succeeding in simultaneously
attaining high thermal crack resistance, high plastic
deformation resistance, and high chipping resistance.
Wear resistance was improved by ceramic coating thetool surface using physical vapor deposition (PVD).*5
When joining steel, the tool temperature reaches
approximately 1,000°C, and thus a coating able to withstand oxidation at this temperature or above wasneeded. By adding a new element to the standard TiAlN
coating widely used for cutting tools, we have developed a PVD coating for FSJ with both high hardness
and an oxidation onset temperature of at least 1,100°C(Table 2).

Joining Condition 1[K]

Joining Condition 2
Fig. 3. Result of the simulation analysis

Film hardness (GPa)Oxidation onset
temperature (°C)

TiAlN

32

850

Developed coating

36

1129

To reduce the load on the tool, we also exploredthe optimum conditions for lowering tool temperatureby simulation using Abaqus finite element analysis software. Young’s modulus, Poisson’s ratio, heat conductivity, specific heat, specific gravity, coefficient ofthermal expansion, yield strength, and coefficient offriction were used as the physical properties of the steel
sheets and tool base material. Figure 3 illustrates theresults of simulating the tool temperatures in joining

Tensile Shear Strength (kN)

Table 2. Film hardness and oxidation onset temperature of
TiAlN and the developed coating

14
12
10
8

JIS-A-Class Minimum

6
4
2
0
0

2000

4000

6000

8000

Number of Spots
Fig. 4. Result of tool life evaluation in the joining of980 MPa-class ultra-high tensile steel

70 · Development of a Friction Spot Joining Tool for 980 MPa Tensile Strength Steels Joiningrequires dressing after approximately 200 to 300 spots
and replacement of the electrode after 2,000 to 4,000
spots, we believe that the new FSJ tool is superior to
RSW in both maintainability and cost performance.

6. Conclusion
FSJ is a ground-breaking new non-fusion joining
technique able to join ultra-high tensile steel as well as
different materials that cannot be joined using RSW. A
suitable FSJ tool for 980 MPa-class ultra-high tensile
steel was developed through improving the base material of the cemented carbide tool, enabling the simultaneous achievement of high thermal crack resistance,high plastic deformation resistance, and high chipping
resistance. Furthermore, the development of the PVD
coating, improvement of the tool shape, and optimization of the joining conditions raised the wear resistanceof the tool, resulting in a tool with long tool life thatmaintains a joint strength of the JIS-A-class minimum
up to approximately 7,000 spots. As increasingly higher
tensile strength steel sheets are used for automobiles
and the carbon content increases to a level that makes
the use of RSW impossible, we believe that FSJ will
have great potential. We will, therefore, continue todevelop tools that responds to the need for increasingly
high tensile strength automobile steel sheets.

• ‌Abaqus is a trademark or registered trademark of Abaqus Inc.

References(1)	H. Oikawa, G. Murayama, T. Sakiyama, Y. Takahashi,
T. Ishikawa, “Resistance Spot Weldability of High Strength
Steel (HSS) Sheets for Automobiles,” NIPPON STEEL &SUMITOMO METAL Technical Papers and Technical Reports
385 (2006)(2)	Y. Murakami, “Development and application of Friction Stir
Welding, FSW,” NMC News, 12 (2004)(3)	Kawasaki News, 135, Aug. 2004, pp. 18-19(4)	H. Fujimoto, T. Okada “Resistance Weldability of Hot Press
Steel Sheets, High tensile Strength Steel Sheets,” Welding
Technique 3 (2011), p 48

Contributors (The lead author is indicated by an asterisk (*).)

H. MIYAZAKI*• ‌Advanced Materials R&D Laboratories,
Sumitomo Electric Industries, Ltd.

Y. UTSUMI• ‌Assistant General Manager,
Advanced Materials R&D Laboratories,
Sumitomo Electric Industries, Ltd.

K. TSUDA• ‌Group Manager, Advanced Materials R&D
Laboratories, Sumitomo Electric Industries,
Ltd.

＊ 1	
ResistanceTechnical Terms

spot welding: A technique for joining
two metal sheets by pressing them together andpassing a current through them in order to fuse
the sheets using ohmic heat.
＊ 2	
Laser beam welding: A technique for joining two
metal sheets by locally fusing them using the heatof a laser beam and then waiting for the joint tosolidify.
＊ 3	Cemented carbide: A hard material mainly composedof tungsten carbide (WC) that forms a hard phaseand cobalt (Co) that forms a binding phase, widelyused for cutting tools.
＊ 4	
Tool steel: A kind of steel mainly used for metal
processing tools.
＊ 5	
Physical vapor deposition (PVD): A method forforming a film on the surface of a base material byvaporizing a metal at high temperature and relying
on a physical reaction to form the film.

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