Original Article

Proc IMechE Part B:J Engineering Manufacture1–8� IMechE 2017Reprints and permissions:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/0954405417720954journals.sagepub.com/home/pib

Joining by forming of additivemanufactured ‘mortise-and-tenon’joints

Diogo FM Silva1, Ivo MF Bragancxa2, Carlos MA Silva1, Luis M Alves1 andPaulo AF Martins1

AbstractThis article is aimed at extending the ‘mortise-and-tenon’ joining concept commonly utilized in corner or tee joints tolap joints in which one sheet is partially placed over another without any change in their shape. The approach makes useof wire arc additive manufacturing to fabricate the tenons and allows various shapes and thicknesses to be made from awide range of metallic materials. Upset compression of the tenons is utilized to mechanically lock the two sheets beingjoined. Experimental and finite element simulation works performed with monolithic (aluminium–aluminium) and hybrid(aluminium–polymer) ‘unit cells’ consisting of a single lap joint are utilized to investigate the deformation mechanics andthe feasibility of the new proposed joining process. Tensile-shear loading tests were carried out to determine the maxi-mum force that the new proposed joints are capable to withstand without failure. Pull-out forces of approximately 8 and6 kN for the monolithic and hybrid joints allow concluding on the potential of additive manufactured ‘mortise-and-tenon’lap joints to connect sheets made from similar and dissimilar materials.

KeywordsForming, additive manufacturing, lap joints, sheets, experimentation, finite element method

Date received: 18 January 2017; accepted: 25 June 2017

Introduction

Lap joints formed by partially placing one sheet overanother without any change in their shape are widelyutilized to assemble products from individual parts.These joints are generally manufactured by welding,adhesive bonding and mechanical fastening using oneof the specific processes shown in Figure 1.

Welding,1 for example, comprises fusion welding(e.g. arc and resistance welding, Figure 1(a)), brazingand soldering (Figure 1(b)) and solid state welding (e.g.friction stir welding, Figure 1(c)). Adhesive bonding2

comprises plain adhesive bonding (Figure 1(d)) andweld-bonding. Mechanical fastening comprises the uti-lization of threaded fasteners and rivets3 (Figure 1(e))and the so-called joining by forming4 that includes clin-ching and self-piercing riveting (Figure 1(f)), amongother processes.

Each lap joint shown Figure 1 has its own character-istics, advantages and disadvantages, and choosingbetween them is dependent upon the materials, geo-metric features, equipment needed and design considera-tions regarding performance, safety and environment.

Now, bearing in mind the aim and objective of join-ing two metal sheets (or a metal and a polymer sheet)partially placed over one another along complex paths,without any change in shape, it follows that alternativesolutions to conventional welding, adhesive bondingand mechanical fastening need to be developed. This isbecause the weldability limitations arising from sheetmaterials with very different chemical, mechanical andthermal properties and the changes in dimensionscaused by excessive distortion and warping during theheating–cooling cycles may compromise the overall fea-sibility and quality of the joints. In case of adhesivebonding, major limitations may also arise from surface

1IDMEC, Instituto Superior Tecnico, Universidade de Lisboa, Lisboa,

Portugal2Instituto Superior de Engenharia de Lisboa, Instituto Politecnico de

Lisboa, Lisboa, Portugal

Corresponding author:

Paulo AF Martins, IDMEC, Instituto Superior Tecnico, Universidade de

Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal.

Email: pmartins@ist.utl.pt

preparation, time to cure, long-term durability andenvironmental requirements. The utilization of mechan-ical fastened joints is generally constrained by the

geometry and variety of the available threaded fastenersor rivets and by the overall size and aesthetics of theresulting joints.

Figure 2 shows a schematic assembly between twosheets made from dissimilar materials in which conven-tional solutions based on welding seams, adhesiveseams and threaded fasteners or rivets (Figure 2(a)) arereplaced by the new proposed joining process devel-oped by the authors (Figure 2(b)). The process utilizeslap joints built upon the ‘mortise-and-tenon’ conceptand makes use of additive manufactured tenons thatare plastically deformed to mechanically lock the twosheets to be joined.

The additive manufactured tenons are deposited onthe surface of the metal sheet by wire arc additive man-ufacturing (WAAM).5,6 The utilization of WAAMinstead of electron beam additive manufacturing7 orlaser sintering additive manufacturing8 is advantageousbecause it uses standard robotic metal inert gas/metalactive gas (MIG/MAG), tungsten inert gas (TIG) orplasma welding systems commonly available in metalworking companies. WAAM can also be considered alow-cost process for fabricating tenons due to its highdeposition rates and high efficiency9 resulting from theutilization of welding wire as feedstock instead of usingpowder bed of fed-based systems.

The combination of the above-mentioned advan-tages with the flexibility and precision of metal deposi-tion provided by computer numerical control (CNC)10

allows fabricating tenons of various shapes and thick-nesses, repeatedly and economically, along straight orcurved (continuous or discontinuous) paths (Figure2(b)). This is important for applications that comprisethe joining of regularly and irregularly shaped partsfrom sheets partially placed over one another.

Figure 2. Schematic joining of two sheets partially placed over one another along complex paths by means of (a) conventionalprocesses based on welding (left), adhesive bonding and threaded fasteners or rivets (right) and by means of (b) the new proposedprocess built upon the ‘mortise-and-tenon’ joining concept that makes use of additive manufactured tenons.

Figure 1. Schematic illustration of various processes utilized toconnect two sheets partially placed over one another withoutany change in their shape: (a) arc and resistance welding, (b)brazing (and soldering), (c) friction stir welding, (d) adhesivebonding, (e) threaded fasteners (and rivets) and (f) clinching andself-piercing riveting.

2 Proc IMechE Part B: J Engineering Manufacture 00(0)

In practical terms, and in contrast to fasteners, weld-ing studs and elements, additive manufactured tenonscan be tailored to a particular application, not only asto shape and thickness but also by the use of welding-compatible materials with the metal sheets to be joined.

Under these circumstances, the aim and objectivesof this article are to present a simple, flexible, low-costjoining process to connect two sheets partially placedover one another, without any change in their shape.The new proposed process is a variant of a previousdevelopment of Bragancxa et al.11 to connect two sheetsperpendicular to one another by means of ‘mortise-and-tenon’ joints cut out from the two sheets. However, andin contrast to the previously developed process that islimited to corner and tee joints, the new proposed join-ing process is capable of applying for the first time everthe ‘mortise-and-tenon’ concept to the production oflap joints. This is accomplished by combining WAAMand upset compression to fabricate and plasticallydeform the tenons utilized to mechanically lock the twosheets (Figure 2(b)). Moreover, this innovative utiliza-tion of the ‘mortise-and-tenon’ concept to produce lapjoints is the reason why the article is built upon lapjoints, despite its applicability to setup corner and teejoints.

The first part of the article is focused on the weldingparameters of the deposited material of the tenons and onthe determination of the stress–strain curves for all thematerials utilized in the investigation. The second part ofthe article starts by characterizing the onset of plasticinstability of the additive manufactured tenons subjectedto upset compression between two flat parallel platensand continues with the analysis of the overall feasibility ofthe new proposed joining process by means of finite ele-ment simulation and experimentation. Two different typesof lap joints are utilized: (1) a monolithic aluminium–aluminium joint and (2) a hybrid aluminium–polycarbonate joint. The third and last part of the articleincludes experimental data from tensile-shear loading teststhat were carried out to determine the maximum forcethat the new proposed lap joints are capable to withstandbefore detachment of the two sheets.

Experimentation

The development of the new proposed joining processwas supported by experimentation with two different

types of lap joints: (1) monolithic metal joints and (2)hybrid metal–polymer joints that make use of alumi-nium AA5754-H111 and polycarbonate sheets with 5-mm thickness. The tenons were fabricated by WAAMusing aluminium AA5356 wire with 1-mm diameter.

Welding parameters

Figure 3 shows a typical deposited metal sample builtthrough the deposition of multi-bead layers with a beadwidth w=12 mm and a distance z=4 mm betweenthe centres of the layers from which cylinder test speci-mens were fabricated by machining.

A KUKA six-axis robotic system using a FroniusCMT VR 7000 cold metal transfer welding machinewas utilized for metal deposition. The machine worksin controlled dip transfer mode with a lower heat inputthan that of conventional machines operating in diptransfer mode and is capable of delivering beads withexcellent quality without spatter. Computer interfacesare utilized to control the movement of the weldingtorch and the welding parameters.

The welding torch was programmed with a linearreciprocating motion and its inclination angle was setto 90�. The wire electrode was aluminium AA5356 with1-mm diameter, and the shielding gas was 99.9% ofargon. Table 1 provides the major WAAM parametersthat were utilized to fabricate the cylinder test speci-mens and the tenons.

Mechanical characterization of the materials

The stress–strain curve of the deposited aluminiumalloy AA5356 was obtained by means of standard com-pression tests. The cylinder test specimens with 10-mmdiameter and 10-mm height were machined out from

Figure 3. Schematic diagram and deposited aluminium alloy AA5356 from which cylinder test specimens were fabricated by machining.

Table 1. The WAAM parameters utilized in the investigation.

Current(A)

Voltage(V)

Wire feedrate(m/min)

Weldingspeed(mm/min)

Stick-outlength(mm)

Gas flowrate(L/min)

120 17.3 10.4 240 15 19

WAAM: wire arc additive manufacturing.

Silva et al. 3

the sample shown in Figure 3 in the ‘as-deposited’condition.

The stress–strain curve of the aluminium AA5754-H111 sheets was determined by means of stack com-pression tests. The stress–strain curve of the polycarbo-nate sheets was determined by means of tensile andstack compression tests because polycarbonate is apressure-sensitive polymer. The cylinder test specimenswere assembled by piling up three circular discs with 10-mm diameter machined out of the supplied aluminiumand polycarbonate sheets by a hole saw. The tensile testspecimens were machined out of the supplied sheets inaccordance to the ASTM D638 standards. The testswere performed at room temperature on a hydraulictesting machine (Instron SATEC� 1200kN) with across-head speed equal to 10mm/min, and the resultingstress–strain curves are shown in Figure 4.

New proposed joining process

The investigation on the new proposed joining processwas carried out in ‘unit cells’ consisting of aluminiumAA5356 tenons produced by WAAM and mortises(rectangular cavities) that were cut out in the alumi-nium AA5754-H111 or polycarbonate sheets. The

tenons are longer than they are wider and are machinedto their final desired shapes after being deposited. Noheat treatment is performed on the tenons. The mor-tises have straight walls and go all the way through thesheets. The upset compression of the free length of thetenons along the z-axis ensures the mechanical lockbetween the two sheets to be joined.

As a result of this, the experimental work plan onthe new proposed joining process was split into two dif-ferent parts. The first part was exclusively focused onthe upset compression of the additive manufacturedtenons and was aimed at determining the operatingparameters under which compression can be performedwithout signs of plastic instability and failure by out-of-plane buckling. This is because the tail of the tenonmust plastically deform like a rivet in order to producea sound flat shaped head during mechanical locking ofthe two sheets.

The upset compression tests were carried out atroom temperature, in displacement control, under avertical velocity equal to 10mm/min. The only operat-ing parameter that was analysed was the free length lfbecause the width w0 and thickness t0 of the tenonswere kept unchanged. This explains the values of thefree length-to-width ratio lf=w0 and of the thickness-to-width ratio t0=w0 in the first part of the experimentalwork plan that are included in Figure 8.

In what regards the second part of the experimentalwork plan, the main goal was to demonstrate the effec-tiveness of additive manufacture to extend the ‘mortise-and-tenon’ concept to the production of lap joints. Thetests were performed on the previously mentionedmonolithic (aluminium AA5754-H111–aluminiumAA5754-H111) and hybrid (aluminium AA5754-H111–polycarbonate) joints and made use of aluminiumAA5356 tenons fabricated by WAAM over the alumi-nium AA5754-H111 sheet. The work plan is summar-ized in Figure 5 and uses appropriate values of the

Figure 5. Summary of the experimental work plan to investigate the effectiveness of the monolithic and hybrid joints produced bymeans of the new proposed joining process.

Figure 4. Stress–strain curves of the deposited aluminium AA5356and of the aluminium AA5754-H111 and polycarbonate sheets.

4 Proc IMechE Part B: J Engineering Manufacture 00(0)

length-to-width ratio lf=w0 of the tenons retrieved fromthe first part of the investigation.

Finite element modelling

The upset compression of the additive manufacturedtenons and the production of the new lap joint builtupon a variant of the conventional ‘mortise-and-tenon’joint11 were numerically simulated with the in-housefinite element computer program I-form. The programis based on the irreducible finite element formulationand is being developed by the authors and validatedagainst experimentation since the end of the 1980s

P =

ð

V

�s_�edV+1

2K

ð

V

_e2vdV�ð

ST

TiuidS

+

ð

Sf

ðjurj

0

tfdur

0B@

1CAdS+

1

2PXNc

c=1

gcn� �2

+1

2PXNc

c=1

gct� �2

ð1Þ

In the above functional, the symbol �s denotes the effec-tive stress according to von Mises and Caddell et al.12

plasticity criteria for metals and polymers, respectively;_�e is the effective strain rate; _eV is the volumetric strainrate; K is a large positive constant imposing the incom-pressibility of volume V; Ti and ui are the surface trac-tions and velocities on surface ST, respectively; tf andur are the friction shear stress and the relative velocityon the contact interface Sf between the rigid (tool) anddeformable objects (tenon and sheets), respectively.

The contact between deformable objects (tenons andsheets) is defined by Nc contact pairs that are identifiedby a two-pass node-to-element algorithm in which theelements are extracted from the surfaces of the three-dimensional (3D) finite elements utilized in the discretiza-tion of the volume (refer to the last two terms in equa-tion (1)). The symbols gcn and gct denote the normal andtangential gap velocities of the contact pairs, respectively,which are penalized by a large number P in order toavoid penetration and to ensure full sticking between thecounter facing surfaces of the deformable objects. In case

of frictional sliding, the penalization of the tangentialgap velocity is absent and shear stresses are applied tothe contacting surfaces through the third term of equa-tion (1) which involves surface tractions Ti. Detailedinformation on the contact with friction between 3Ddeformable objects is provided in the Nielsen et al.13

The finite element models that were utilized in thenumerical simulation of the upset compression of theadditive manufactured tenons and in the connection ofsheets by means of the new proposed joining processmade use of 3D hexahedral elements. The tenons andthe sheets were modelled as deformable objects whereasthe tool was modelled as a rigid object and its geometrywas discretized by means of contact friction spatial tri-angular elements. Figure 6 illustrates the upset com-pression of two different tenons at the beginning andend of deformation.

The overall central processing unit (CPU) time for atypical analysis shown in Figure 6 was approximatelyequal to 3 h on a computer equipped with one Intel i7-5930K CPU (3.5GHz) processor.

Results and discussion

Upset compression of the additive manufacturedtenons

Figure 6 shows two different numerically predictedmodes of deformation that develop during the upsetcompression of additive manufactured tenons. Whenthe slenderness ratio lf=w041:5, the plastic deformationis symmetric and the tenons deform like a rivet and giverise to a flattened top surface head (Figure 6(b)) that issuitable for application in the ‘mortise-and-tenon’-based lap joints. The cross section of the flattened topsurface head of the tenons is approximately rectangularinstead of circular like in conventional riveting, due tothe geometric constraint on material flow imposed bythe corners and edges of the initial rectangular crosssection of the additive manufactured tenons.

In case the slenderness ratio lf=w0 . 1:5, there isplastic instability and failure by out-of-plane buckling(Figure 6(c)). Therefore, the slender tenons cannot beused in ‘mortise-and-tenon’-based lap joints.

Figure 6. Finite element modelling of the upset compression of an additive manufactured tenon (Figure 8): (a) schematicrepresentation of the model, (b) geometry of a tenon with a length-to-width ratio lf =w0 = 1:5 before and after upset compressionand (c) geometry of a tenon with a length-to-width ratio lf =w0 = 2 before and after upset compression.

Silva et al. 5

Figure 7 shows the experimental and finite elementpredicted evolution of the force with displacement forthe upset compression of the additive manufacturedtenons listed in Figure 8. As seen, the force–displacement evolution discloses two different trends.The tenons with a ratio lf=w0 =1 experience the highestforce and force growth rate as a result of strain harden-ing and of the increase in the contact area between thetail of the tenon and the compression platen (upper flattool). The tenons with lf=w0 =2 behave differently dueto the occurrence of plastic instability. In fact, after afirst rise of the force similar to that observed in thetenons undergoing symmetric plastic deformation, theforce growth rate becomes smaller. This is attributed todeformation being localized at the plastic hinge and to aminor increase in the contact area between the tail ofthe tenon and the compression platen.

Lap joints based on the ‘mortise-and-tenon’ concept

Figure 9(a)–(c) shows the new proposed lap joints basedon the ‘mortise-and-tenon’ concept with additive manu-factured tenons for connecting two sheets partiallyplaced over one another, without any change in their

shape. Figure 9(a) shows the additive manufacture tenonfabricated on top of the aluminium AA5754-H111 sheet,whereas Figure 9(b) and (c) shows the monolithic (alumi-nium AA5754–H111) and the hybrid (aluminiumAA5754-H111–polycarbonate) joints produced by thenew proposed process, respectively. The slenderness ratiolf=w0 of the tenon was chosen to be equal to lf=w0 =0:5in order to ensure symmetric deformation during upsetcompression (refer to section ‘Upset compression of theadditive manufactured tenons’) and to guarantee a small‘bump’ of upset material on the joint.

Figure 9(d) and (e) shows the corresponding finiteelement models at the beginning and end of the joiningprocess. The models are limited to the vicinity of theplastically deforming region in order to reduce the over-all computational requirements but the correlationbetween the experimental and numerical predicted evo-lution of the force with displacement shown in Figure9(f) is very good. In fact, both experimental results andfinite element predictions disclose the required formingforce to be smaller in case of the hybrid lap joints prob-ably due to some expansion of the polycarbonate mor-tise during the upset compression of the aluminiumtenon. Still, a sound mechanical lock is formed betweenthe two sheets as will be shown in the following sectionfocused on the tensile-shear loading tests.

Tensile-shear loading tests

Tensile-shear loading tests were utilized to determinethe maximum force that the new proposed joints arecapable to withstand without failure. The test is sche-matically shown in Figure 10(a) together with themonolithic and hybrid joints after being tested.

As seen, the separation of the two sheets is accom-plished in two different ways. In case of the monolithicaluminium joint, the sheets are separated by shearingoff the tenon along its interface (intermediate zone)with the lower aluminium sheet. The maximum experi-mental shear force Fmax is equal to 8.3 kN, and the mor-phology of the cross-sectional area As sheared off fromthe tenon reveals a small smooth (burnished) regionand a large rough fractured region caused by crack pro-pagation that is typical of shearing.

Figure 7. Experimental and finite element predicted evolutionof the force with displacement for various additivemanufactured tenons with different length-to-width ratios lf =w0

(Figure 8) during upset compression. The insert shows theexperimental test specimens.

Figure 8. Summary of the experimental work plan for investigating the upset compression of the additive manufactured tenons.

6 Proc IMechE Part B: J Engineering Manufacture 00(0)

In order to compare the performance of the new pro-posed joint with that of conventional riveting, the fol-lowing expression retrieved from the mechanical designof rivets14 is utilized to calculate the maximum admissi-ble shear force Fadm that a rivet can withstand withoutfailure

Fadm= taAs ð2Þ

In the above equation, ta is the allowable shear stressof the rivet material, which is assumed to be equal tosUTS=

ffiffiffi3p

and uniformly distributed over the cross-sectional area As of the rivet to be sheared off. So, byconsidering the ultimate tensile strength of the depos-ited material AA5356 equal to sUTS =241:7 MPa, themaximum admissible shear force of a rivet with a cross-sectional area As =50 mm2 identical to that of thetenon is equal to

Fadm=sUTSffiffiffi

3p As =0:5773 241:73 50=6973 N ð3Þ

This value is 16% smaller than that obtained in thetensile-shear loading tests performed with the new pro-posed joint (refer to the dashed line in Figure 10(b)). Apossible reason for the difference between Fadm andFmax is attributed to the fact that the material shearedoff is from the intermediate region.

In case of the hybrid (polycarbonate–aluminium)joint, the tensile-shear force to separate the two sheetsis smaller and does not drop after reaching its peakvalue (approximately 6 kN). In fact, it remains practi-cally unchanged because the joint is not destroyed byshearing off the tenon, but by plastically deforming thepolycarbonate mortise with the aluminium tenon (referto Figure 10(a)).

The results obtained with monolithic and hybridjoints prove the feasibility of the new proposed processto extend the concept of ‘mortise-and-tenon’ joints withadditive manufactured tenons to the production of lapjoints.

Conclusion

The ‘mortise-and-tenon’ fixture commonly utilized incorner or tee joints is successfully extended to lapjoints in which one sheet is partially placed overanother by the use of tenons fabricated by WAAM.The proposed approach allows fabricating tenonswith various shapes and thicknesses in a wide rangeof metallic materials for applications in simple orcomplex joining paths.

The feasibility of the new joining process was inves-tigated by means of ‘unit cells’ consisting of a single lap

Figure 9. Joining two sheets partially placed over one anotherby the new proposed process: (a) additive manufactured tenonmade from aluminium AA5083 and of two mortises cut out inaluminium AA5754-H111 and polycarbonate sheets, (b) lap jointformed between two aluminium AA5754-H111 sheet, (c) lapjoint formed between a polycarbonate and an aluminiumAA5754-H111 sheet, (d) finite element model corresponding to(b) before and after joining, (e) finite element modelcorresponding to (c) before and after joining and (f)experimental and finite element predicted force-displacementevolution for both test cases.

Figure 10. Tensile-shear loading tests of the new proposedjoint: (a) schematic representation of the tensile-shear loadingtest and joints after being tested and (b) experimental evolutionof the force with displacement for the monolithic and hybridjoints.

Silva et al. 7

joint of similar (aluminium) or dissimilar (aluminium–polycarbonate) materials. The process window wasestablished as a function of the free length-to-width(slenderness) ratio of the additive manufactured tenonslf=w041:5. Higher values of this ratio lead to plasticinstability and failure by out-of-plane bucking and,therefore, are unsuitable for joining purposes.

Destructive tensile (pull-out) tests were utilized tocharacterize the overall performance of the new pro-posed lap joints. Two different detachment modes wereobserved. In case of monolithic aluminium joints, thesheets were separated by shearing off the tenon alongthe intermediate region with the lower aluminium sheetafter being subjected to tensile-shear loads in the rangeof 8.3 kN. In case of hybrid (aluminium–polycarbo-nate) joints, the maximum withstandable force drops toapproximately 6 kN and separation is accomplished byplastic deformation of the polycarbonate mortise whilethe tenon is drawn by means of the applied tensile-shearloading.

Results allow concluding that the proposed exten-sion of the ‘mortise-and-tenon’ concept to lap joints haspotential to connect sheets made from dissimilar mate-rials with very different chemical, mechanical and ther-mal properties.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interestwith respect to the research, authorship and/or publica-tion of this article.

Funding

The author(s) disclosed receipt of the following financialsupport for the research, authorship, and/or publicationof this article: This work was supported by Fundacxaopara a Ciencia e a Tecnologia of Portugal and IDMECunder grants LAETA – UID/EMS/50022/2013 andPDTC/EMS-TEC/0626/2014. I.M.F. Braganca thanksInstituto Politecnico de Lisboa for support (IPL-IDI&CA 2016 – CompSBJ).

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