Impulse Pressure-Assisted Diffusion Bonding (IPADB): Review and Outlook

metals

Review Impulse Pressure-Assisted Diffusion Bonding (IPADB): Review and Outlook

Abdulaziz AlHazaa 1,2,* , Nils Haneklaus 2,* and Zeyad Almutairi 3,4,5

1 Research Chair for Tribology, Surface, and Interface Sciences (TSIS), Physics and Astronomy Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia 2 King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia 3 Mechanical Engineering Department, College of Engineering, King Saud University, Riyadh 11451, Saudi Arabia; [email protected] 4 Sustainable Energy Technologies Center (SET), King Saud University, Riyadh 11451, Saudi Arabia 5 King Abdullah City for Atomic and Renewable Energy (K.A.CARE), Energy Research and Innovation Center at Riyadh, Riyadh 11451, Saudi Arabia * Correspondence: [email protected] (A.A.); [email protected] (N.H.)

Abstract: Diffusion bonding is a solid-state welding technique used to join similar and dissimilar materials. Relatively long processing times, usually in the order of several hours as well as ﬁne polished surfaces make it challenging to integrate diffusion bonding in other production processes and mitigate widespread use of the technology. Several studies indicate that varying pressure during diffusion bonding in contrast to the traditionally applied constant load may reduce overall processing- and bonding times. Such processes are referred to as impulse pressure-assisted diffusion bonding (IPADB) and they are, for the ﬁrst time, reviewed in this work using the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) methodology. Results of the review indicate that varying pressure can indeed reduce bonding times in diffusion bonding and reduce the requirements for pre-bond surface preparation. Additional research is required and should go beyond small and Citation: AlHazaa, A.; Haneklaus, simple sample geometries to concentrate on making IPADB accessible to industrial applications. N.; Almutairi, Z. Impulse Pressure-Assisted Diffusion Bonding Keywords: diffusion bonding; transient liquid phase bonding; impulse pressure; dissimilar joints (IPADB): Review and Outlook. Metals 2021, 11, 323. https://doi.org/ 10.3390/met11020323 1. Introduction Academic Editor: António Pereira Diffusion bonding, also known as diffusion welding, is a solid-state welding technique by which two surfaces are joined under high-temperature and mechanical pressure in a Received: 14 January 2021 vacuum or nonoxidizing environment [1]. Diffusion bonding is a high-tech process that Accepted: 9 February 2021 Published: 12 February 2021 was originally developed in the Soviet Union (and shortly after the United States) in the 1960s for lightweight aerospace parts. Today diffusion bonding is used in various

Publisher’s Note: MDPI stays neutral industries such as aerospace, petrochemical, and energy applications. Diffusion bonding with regard to jurisdictional claims in can be used to join dissimilar parts and create holohedral structures, such as the cooling published maps and institutional affil- channels in plate-type heat exchangers. Compared with competing joining technologies iations. such as brazing and gluing, diffusion bonding is relatively expensive. Cooke and Atieh [2] identified the relatively slow welding times (diffusion bonding of metals usually requires process times between 30 min and several hours) as the major key challenge that needs to be overcome to better integrate diffusion bonding in other production processes. Three steps are commonly differentiated in diffusion bonding as indicated in Figure1. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. The base materials are brought in intimate contact at the mating surfaces that will show This article is an open access article some asperities. These asperities will need to be overcome to create a sound bond. In a distributed under the terms and first step, pressure is used to deform the asperities and thus increase the area of contact conditions of the Creative Commons between the two mating surfaces. The quality of the surface finish is an important factor Attribution (CC BY) license (https:// in diffusion bonding as larger asperities need to be overcome if surfaces could not be creativecommons.org/licenses/by/ properly polished in preparation of the diffusion bonding process [3,4]. The applied force 4.0/). will further break the protective oxide layer of the mating surfaces and create interfacial

Metals 2021, 11, 323. https://doi.org/10.3390/met11020323 https://www.mdpi.com/journal/metals Metals 2021, 11, 323 2 of 9

Metals 2021, 11, 323 2 of 9 in diffusion bonding as larger asperities need to be overcome if surfaces could not be properly polished in preparation of the diffusion bonding process [3,4]. The applied force will further break the protective oxide layer of the mating surfaces and create interfacial boundaries.boundaries. AtAt thethe secondsecond stage,stage, graingrain boundariesboundaries furtherfurther migratemigrate andand porespores along along the the bondbond line line will will be be reduced. reduced. In In the the last last step, step, stage stage 3, 3, porespores will will be be largelylargely eliminated eliminated through through volumevolume diffusion,diffusion, andand thethe bondbond willwill be be homogenized. homogenized. IfIf donedone correctly,correctly, aa bondbond ofof the the highesthighest quality quality is is formed formed that that is is largely largely indistinguishable indistinguishable from from the the base base material material itself itself and and showsshows a a strength strength that that is is nearly nearly as as high high as as that that of of the the base base material. material.

Initial contact of the base (1) Deformation and formation materials asperities of interfacial boundaries

(2) Migration of grain boundaries (3) Pore elimination through and elimination of pores volume diffusion

Figure 1. Overview of the different stages during diffusion bonding. Figure 1. Overview of the different stages during diffusion bonding. Bonding temperature, bonding time, and bonding pressure are the most important parametersBonding characterizing temperature, the bonding diffusion time, bonding and bonding process. Besidespressure these are the three most main important parameters,parameters atmosphere characterizing (usually qualitythe diffusion of the bond vacuuming process. or protective Besides gas these atmosphere), three main surface param- treatment/roughness,eters, atmosphere (usually and interlayers quality of (ifthe applied) vacuum are or reported.protective gas atmosphere), surface treatment/roughness,To reduce bonding and time interlayers of the diffusion (if applied) bonding are reported. process, Cooke and Atieh [2] sug- gestedTo to reduce use nanostructured bonding time interlayers. of the diffusion Using bonding thin interlayers process, that Cooke melt and during Atieh diffusion [2] sug- bondinggested to is use not nanostructured a new process butinterlayers. referred Using to as transient thin interlayers liquid phase that melt (TLP) during bonding diffusion that can dramatically reduce bonding times. The thin interlayer, which has a lower melting bonding is not a new process but referred to as transient liquid phase (TLP) bonding that point than the base materials and melts below the bonding temperature, is placed between can dramatically reduce bonding times. The thin interlayer, which has a lower melting the base materials that are to be joined. The interlayer element (or constituent if an alloy point than the base materials and melts below the bonding temperature, is placed between interlayer is used) diffuses into the base materials, causing isothermal solidification at the the base materials that are to be joined. The interlayer element (or constituent if an alloy bonding temperature. As a result, a bond that has a higher melting point than the initial interlayer is used) diffuses into the base materials, causing isothermal solidification at the TLP bonding temperature is formed [5–8]. Using nanostructured interlayers that have a bonding temperature. As a result, a bond that has a higher melting point than the initial high surface energy as a result of their large surface to volume ratio is a clever way to TLP bonding temperature is formed [5–8]. Using nanostructured interlayers that have a reduce the bonding time (and often even the bonding temperature) in diffusion bonding or high surface energy as a result of their large surface to volume ratio is a clever way to related processes such as vacuum brazing [9–11]. reduce the bonding time (and often even the bonding temperature) in diffusion bonding Applying interlayers on complicated geometries, such as the 10–200 platelets of a or related processes such as vacuum brazing [9–11]. diffusion-bonded printed circuit heat exchanger (PCHE) plate package [12] without block- ing theApplying micro-channels interlayers of the on final complicated product cangeomet be technicallyries, such challengingas the 10–200 and platelets expensive. of a Processesdiffusion-bonded that decrease printed the circuit bonding heat timeexchange withoutr (PCHE) additional plate package pretreatment [12] without or usage block- of (exoticing the and, micro-channels as a result, expensive) of the finalnanomaterials product can be would technically thus bechallenging most desirable. and expensive. A good exampleProcesses is that a process decrease proposed the bonding by Shirzadi time withou and Wallacht additional [13– 15pretreatment] who showed or usage that a of sim- (ex- pleotic temperature and, as a result, gradient expensive) perpendicular nanomateri to theals bondwould plane thus has be most a benevolent desirable. effect A good on the ex- finalample bond is a quality. process Other proposed recent by promising Shirzadi and innovations Wallach in[13–15] diffusion who bonding showed maythat includea simple in-linetemperature surface gradient treatment perpendicular and diffusion to the bonding bond plane [16] as has well a benevolent as spark plasma effect on diffusion the final bondingbond quality. [17–19 Other]. recent promising innovations in diffusion bonding may include in- line Itsurface is not treatment uncommon and to diffusion vary temperature bonding and[16] pressureas well as during spark plasma the diffusion diffusion bonding bond- process.ing [17–19]. Song et al. [20], for instance, used a two-stage diffusion bonding process for joining 316 LIt stainless is not uncommon steel with ato Ti vary alloy. temperature In the process, and samplespressure are during first heatedthe diffusion to 750 bonding◦C and keptprocess. there Song under et 30al. MPa[20], for load instance, for 20 min used before a two-stage moving diffusion to the second bonding diffusion process bonding for join- stageing 316 where L stainless samples steel are with diffusion a Ti alloy. bonded In the at 900 process,◦C under samples 5 MPa are pressurefirst heated for to another 750 °C 30–120 min.

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Metals 2021, 11, 323 3 of 9 and kept there under 30 MPa load for 20 min before moving to the second diffusion bonding stage where samples are diffusion bonded at 900 °C under 5 MPa pressure for another 30–120 min. InsteadInstead of of varying varying temperature andand pressurepressure “only” “only” two two times times over over the process,the process, rapid rapidchanges changes of more of more than than two two times times can can be performed.be performed. Sheng Sheng et al.et al. [21 [21]] rapidly rapidly cycled cycled the thetemperature temperature during during diffusion diffusion bonding bonding of Tiof (TA17)Ti (TA17) and and 304 stainless304 stainless steel. steel. Besides Besides rapid rapidtemperature temperature changes changes that that will will have have little little effect effect on larger on larger parts parts in a in vacuum a vacuum furnace furnace with withinduction induction heating, heating, rapid rapid pressure pressure variations variations are are anotheranother promising process process to to reduce reduce processingprocessing times times in in diffusion diffusion bonding. bonding. Rapidly Rapidly varying varying the the applied applied load load during during diffusion diffusion bondingbonding is isa aprocess process referred referred to to as as impulse impulse pressure-assisted pressure-assisted diffusion bondingbonding (IPADB)(IPADB) or orimpulse impulse pressuring pressuring diffusion diffusion bonding bonding (IPDB). (IPDB). In In this this work, work, we we refer refer to itto as it IPADB as IPADB since sinceit does it does not havenot have to be to performed be performed over over the whole the whole bonding bonding time time and theand description the description that it that“assists” it “assists” the ongoing the ongoing diffusion diffusion process process by more by more efﬁciently efficiently breaking breaking the protective the protective oxide oxidelayer layer seems seems most accurate.most accurate. Figure Figure2 schematically 2 schematically shows the shows difference the difference between traditionalbetween traditionaldiffusion diffusion bonding bonding with constant with constant bonding bonding pressure pressure (Figure 2(Figurea) and 2a) IPADB and withIPADB varying with varyingbonding bonding pressure pressure (Figure (Figure2b). 2b).

FigureFigure 2. 2.SchematicSchematic comparison comparison of bonding of bonding temperatur temperaturee and -pressure and -pressure during during the traditional the traditional dif- fusiondiffusion bonding bonding process process (a) and (a) impulse and impulse pressure pressure diffusion diffusion bonding bonding (IPADB) (IPADB) (b). (b).

ThisThis work work provides, provides, for for the the first ﬁrst time, time, a crit a criticalical review review on on reported reported experimental experimental work work usingusing IPADB IPADB with with the the aim aim to to quantify quantify to to which which degree degree bonding bonding times times can can be be reduced reduced and and thethe joint joint quality quality can can be be improved improved by by applying applying this this technique. technique.

2.2. Materials Materials and and Methods Methods WeWe used used the the PRISMA PRISMA (Preferred (Preferred Reporting Reporting Items Items for for Systematic Systematic Reviews Reviews and and Meta- Meta- Analysis)Analysis) methodology methodology for aa systematic systematic review review and and searched searched for publicationsfor publications using Googleusing GoogleScholar Scholar and Espacenet and Espacenet of the of European the European Patent Patent Ofﬁce Office (EPO). (EPO). In total, In total, we screened we screened more morethan than 300 records300 records looking looking for pressurefor pressure variation variation during during diffusion diffusion bonding bonding of which of which only only17 qualiﬁed17 qualified to be to included be included in this in reviewthis review as these as these studies studies can clearly can clearly be attributed be attributed to IPADB. to IPADB. 3. Results and Discussion

Varying the pressure to break the protective oxide layer on materials that are otherwise challenging to bond appears as a straight forward approach to improve diffusion bonding in a way that the bonding time can be reduced or joints can be produced at higher quality using bonding times similar to traditional diffusion bonding without pressure variation. The idea to use systematic pressure pulses or IPADB may be accredited to the E.O. Paton

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3. Results and Discussion Varying the pressure to break the protective oxide layer on materials that are otherwise challenging to bond appears as a straight forward approach to improve diffusion

Metals 2021, 11, 323 bonding in a way that the bonding time can be reduced or joints can be produced at higher4 of 9 quality using bonding times similar to traditional diffusion bonding without pressure variation. The idea to use systematic pressure pulses or IPADB may be accredited to the E.O. Paton Electric Welding Institute of the National Academy of Sciences in Ukraine, located in ElectricKiev [22]. Welding Institute of the National Academy of Sciences in Ukraine, located in KievSeveral [22]. publications report positive effects of varying pressure. Norajitra et al. [23] report thatSeveral the publicationsuse of dynamic report pressure positive is be effectsnevolent of varyingfor diffusion pressure. bonding Norajitra of refractory et al. [23 ] metalsreport in that a way the that use ofthe dynamic bonding pressure time can is benevolentbe reduced. for Xiong diffusion et al. bonding[24] used of high refractory fre- quencymetals vibrations in a way that(140–170 the bonding Hz) to timereduce can pa berameters reduced. during Xiong etbonding al. [24] usedof aluminum high frequency and Ti.vibrations In case of (140–170aluminum Hz) (Alloy to reduce LD2) where parameters vibrations during at bondinga frequency of aluminumof 167 Hz were and Ti.ap- In plied,case bonding of aluminum temperature, (Alloy LD2)bonding where time, vibrations and bonding at a frequency pressure could of 167 be Hz reduced were applied, from 520bonding to 480 °C, temperature, 210 to 60 min, bonding and 3 time,to 2.5 andMPa, bonding respectively, pressure while could the bebond reduced strength from meas- 520 to ◦ ured480 byC, tensile 210 to testing 60 min, was and improved 3 to 2.5 MPa,from 72 respectively, to 106 MPa. while In case the of bond Ti (TC4) strength where measured vibra- tionsby tensileat a frequency testing wasof 140 improved Hz were fromapplied, 72 tobonding 106 MPa. temperature In case of and Ti (TC4) bonding where time vibrations could at a frequency of 140 Hz were applied, bonding temperature and bonding time could be be reduced from 1280 to 1000◦ °C and 90 to 45 min, respectively, while bond strength meas- uredreduced by tensile from testing 1280 to increased 1000 C and from 90 102 to 45 to min, 144 respectively,MPa. The bonding while bondpressure strength was measuredslightly increasedby tensile from testing 2 to 2.5 increased MPa. from 102 to 144 MPa. The bonding pressure was slightly increased from 2 to 2.5 MPa. Research groups from the Chongqing University in China (College of Materials Sci- Research groups from the Chongqing University in China (College of Materials Science ence and Engineering) and the Indian Institute of Technology (IIT) Roorkee (Department and Engineering) and the Indian Institute of Technology (IIT) Roorkee (Department of of Mechanical and Industrial Engineering) conducted systematic analysis of IPADB that Mechanical and Industrial Engineering) conducted systematic analysis of IPADB that are are most relevant for this review. most relevant for this review.

3.1.3.1. Chongqing Chongqing Group Group QinQin et etal. al. [25] [25 reported] reported first ﬁrst IPADB IPADB experi experimentsments ofof TiTi (TA17)(TA17) and and 304 304 stainless stainless steel steel that thatindicate indicate advantages advantages of of IPADB IPADB over over traditional traditional diffusion diffusion bonding bonding with with constant constant pressure. pressure.IPADB IPADB of theof the same same material material combination combination was was subsequently, subsequently, systematically systematically investigated investi- gatedby Yuan by Yuan et al. et [ 26al.]. [26]. First, First, the engineeringthe engineering optimum optimum bonding bonding temperature temperature was was determined deter- minedto be to 825 be ◦825C (see °C (see Figure Figu3a).re 3a). Subsequently, Subsequently, the the maximum maximum impulse impulse pressure pressure (Figure (Figure3b), 3b),the the number number of of pulses pulses (Figure (Figure3c), 3c), and and the the pulse pulse frequency frequency (Figure (Figure3d) 3d) was was optimized optimized as as indicatedindicated inin FigureFigure3 3a–d.a–d.

Figure 3. Optimization of IPADB parameters: (a) bonding temperature, (b) maximum impulse Figure 3. Optimization of IPADB parameters: (a) bonding temperature, (b) maximum impulse pressure, (c) number of pulses and (d) pulse frequency for bonding of Ti (TA17) and 304 stainless pressure, (c) number of pulses and (d) pulse frequency for bonding of Ti (TA17) and 304 stainless steelsteel after after Yuan Yuan et etal. al.[26]. [26 ]. Following these initial experiments, researchers from the Chongqing University deployed IPADB for joining of TiC cermet to 304 stainless steel using a Ti/Cu/Nb (30/20/30 µm)

interlayer [27] and a Ti/Nb (20/30 µm) interlayer [28]. Best results were achieved at a bonding temperature of 885 and 890 ◦C with bonding times of 5 and 10 min, respectively. In both studies, pressure variations of 2–10 MPa at 0.5 Hz under a 1 × 10−3 Pa vacuum were used. The studies compared the results at different bonding times (2,3,5,7, and 8 min and 4,6,8,10, Metals 2021, 11, 323 5 of 9

and 12 min) while leaving all other parameters constant. A direct comparison between IPADB and diffusion bonding with constant load was not provided. Li and Sheng [28] do, however, point out that due to IPADB, bonding times required for traditional diffusion bonding of similar materials could be significantly reduced from 45–120 min to the 10 min required in the presented study. The same research group also conducted experiments on IPADB of commercially pure Ti to 304 stainless steel using a Ni (200 µm thickness) interlayer in vacuum at a temperature of 850 ◦C and pressure ranging from 8 to 20 MPa [29]. The reached ultimate tensile strength (UTS) of 358 MPa is comparable with results achieved by traditional diffusion bonding and TLP bonding of commercially pure Ti and 304 stainless steel [30]. However, signiﬁcant reduction in bonding times of 60–180 s was achieved. The needed bonding time for IPADB were 60 times shorter than those (60–180 min) of traditional diffusion bonding of these materials. The experiments were based on previous IPADB joining experiments of Ti- 4Al-2V and 304 stainless steel with different Ni-interlayers in the form of nanopowder (d = 20 µm), plating (10, 15, and 20 µm thickness), and foil (13 µm thickness) that showed the best reported bond quality [31]. Different pressure pulsation: 8–30, 8–30, 8–50, and 8–80 MPa were used at a frequency of 0.5 Hz. A direct comparison between samples produced using static pressure and IPADB was again not provided. The effective bonding time was only 180 s. Slightly lower ultimate tensile strength of 346 MPa could be realized by Deng et al. [32] during IPADB of commercially pure Ti to 304 stainless steel using a Cu (50 µm thickness) interlayer in vacuum at a temperature of again 850 ◦C and pressure ranging from again 8–20 MPa. In line with previous experiments, the effective bonding time was again only 180 s. The research group from the Chongqing University further conducted IPADB experiments between pure Cu and 304 L stainless steel with and without an Ni (12.5/50 µm) interlayer examining different bonding times (5, 10, and 20 min) and bonding temperatures (825, 850, and 875 ◦C) in a vacuum of 5 × 10−2 Pa and pressure variation of 5–20 MPa at 0.5 Hz [33]. Best results were obtained with a 12.5 µm Ni interlayer at 850 ◦C and a bonding time of 20 min. The authors remark that compared with other studies [30] using constant pressure, microvoids could be signiﬁcantly reduced using varying pressure. The group also used IPADB for joining of pure Cu (100 µm thickness) and 304 L stainless steel at 850 ◦C, at a bonding time of 20 min and impulse pressure of 5–20 MPa at a frequency of 0.5 Hz. Subsequently, AZ31 magnesium was bonded on the Cu at 520 ◦C as well as 495 ◦C and 2 MPa constant pressure [34].

3.2. Roorkee Group Another set of systematic experiments on IPADB has been conducted at the IIT Roor- kee. Besides reducing the processing time during diffusion bonding, another major mo- tivation for using IPADB was the potential ability to diffusion bond relatively to rough surfaces. First experiments, therefore, focused on comparison IPADB of ferritic stainless steel samples with different surface treatments [35]. Surface grinding with roughness of 2.29 µm was compared with rough polishing (0.72 µm roughness) and ﬁne polishing (0.02 µm roughness). Pulse pressures of 20, 30, and 40 MPa were used at pulsation times of 180 s (6 pulses à 30 s), 360 s (6 pulses à 60 s), and 540 s (6 pulses à 90 s), respectively. The bonding temperature was set to 850 ◦C, and the overall bonding time was 30 min. Pulsa- tion was performed at the beginning of the diffusion bonding process when the bonding temperature was reached. Although IPADB seems to do better with rough surfaces than traditional diffusion bonding, the best results were still reached with the ﬁne polished (0.02 µm) surface. Highest pressures of 40 MPa and pulsation time of 540 s proved to produce the best diffusion bonds in these experiments. Similarly, IPADB experiments were conducted for a ferritic low carbon steel using an Ag foil (40 µm thickness) interlayer [36]. For these experiments, the temperature was slightly increased to 875 ◦C, while the pulse pressure was reduced to 10 MPa. In total, only Metals 2021, 11, 323 6 of 9

ture was reached. Although IPADB seems to do better with rough surfaces than traditional diffusion bonding, the best results were still reached with the fine polished (0.02 μm) surface. Highest pressures of 40 MPa and pulsation time of 540 s proved to produce the best diffusion bonds in these experiments. Metals 2021, 11, 323 6 of 9 Similarly, IPADB experiments were conducted for a ferritic low carbon steel using an Ag foil (40 μm thickness) interlayer [36]. For these experiments, the temperature was slightly increased to 875 °C, while the pulse pressure was reduced to 10 MPa. In total, only 10 pressurepressure pulsespulses were were found found to to produce produce the the best best bond bond quality quality using using again again a bonding a bonding time timeof 30 of min. 30 min. The same ferriticferritic low low carbon carbon steel steel (surface (surface roughness: roughness: 0.85 0.85µm) μ wasm) was used used for additional for addi- tionalIPADB IPADB experiments experiments [37] that [37] specifically that specifically looked looked at the at voidsthe voids after after diffusion diffusion bonding bonding as asit wasit was postulated postulated earlier earlier that that IPADB IPADB can can reduce reduce those those voids. voids. ItIt isis postulatedpostulated that IPADB breaks the the oxide oxide layer layer and and fills fills the thevoids voids at the at interface the interface resulting resulting from asperities from asperities by plastic by flowplastic of flow the ofmetal the metalin the in overlapped the overlapped area. area. Figure Figure 4 shows4 shows the the results results of of Sharma andand Dwivedi [37] [37] who compared different maximum pressures (Pmax = 20, 20, 30, and 40 MPa) and different number of pulses (0, 6, and 18) with regards to the number and size of voids on the diffusion-bonded interfaces. Representative Representative lengths lengths of of 400 400 μµm of the diffusion bonds were analyzed for this purpose. The to totaltal number of voids clearly reduced with the applied maximum pressurepressure fromfrom 130 130 (Pmax (Pmax = = 20 20 MPa) MPa) to to 86 86 (Pmax (Pmax = 30= 30 MPa) MPa) and and lastly lastly 62 62(Pmax (Pmax = 40 = MPa).40 MPa). The The same same holds holds true true for thefor numberthe number of pulses of pulses where where voids voids are reduced are re- ducedafter increased after increased pulsation: pulsation: 116 (0 pulses)116 (0 pulses) to 87 (6 to pulses) 87 (6 pulses) and 75 and (18 pulses).75 (18 pulses).

Figure 4. InfluenceInﬂuence of maximum pressure (Pmax = 20, 30, and 40 MPa) and number of pulses (0, 6, and 18) on the number and size of voids along the bonding interface of ferritic stainl stainlessess steel samples produced by IPADB after data from Sharma and Dwivedi [[37].37].

Sharma and Dwivedi [37] [37] conclude in this work that the use of IPADB (18 pulses at Pmax = 40 40 MPa) was able to improve the bond quality by nearly 44% (measured using shear tests) tests) compared compared to to traditional traditional diffusion diffusion bonding bonding performed performed using using constant constant pressure. pressure. The successfully developed IPADB IPADB process at IIT Roorkee was used again for the bonding of a creep creep strength enhanced ferritic steel steel (CSEF (CSEF P92) P92) with with higher higher surface surface rough- rough- ◦ ness (0.52 μµm) [38]. [38]. IPADB IPADB was performed at a bonding temperature ofof 10001000 °CC and a total overall bonding time of 60 min. Pressure Pressure pu pulsationlsation was performed for 10 min with pulses reaching 20 MPa and a duration of 10 s. Another series of systematic IPADB experimentsexperiments was carried out for bonding of 304 ◦ stainless steel steel at at different different temperatures temperatures (850 (850,, 900, 900, and and 950 950°C), atC), maximum at maximum bonding bonding pressurepressure of 15 of MPa 15 MPaand minimum and minimum bonding bonding pressure pressure of 7.5 ofMPa 7.5 for MPa 30 for min 30 [39]. min Tensile [39]. Tensile shear testing,shear testing, optical optical microscope, microscope, and andfield-emissi ﬁeld-emissionon scanning scanning electron electron microscope microscope (FESEM) (FESEM) ◦ were used to investigate the produced samples. Samples produced at 950 °CC showed the best results. ComparedCompared withwith other other studies studies that that diffusion diffusion bonded bonded 304 304 stainless stainless steel steel samples sam- plesusing using diffusion diffusion bonding bonding [30], [30], the accomplished the accomplished 86 MPa 86 MPa shear shear strength strength is at theis at lower the lower end. end.3.3. Discussion 3.3. DiscussionIt is well known that increased pressure during diffusion bonding will increase the bond quality [40]. Diffusion bonding usually presents a balancing act between the required minimum pressure and the allowable maximum pressure/deformation of the workpiece. Li et al. [41] demonstrate that if deformation is not of concern and high bonding pressure can be applied virtually, defect free bonds can be produced at short bonding times (roughly half the time of comparable diffusion bonding with little induced deformation) that reach strength equal to those of the base material. The high bonding pressure induces a signif- icant macroscopic deformation that breaks the oxide layer, promotes void closure, and reﬁnes the grain structure at the bond, which again promotes diffusion. Deformation of samples/workpieces is rarely reported in diffusion bonding studies, and the reviewed Metals 2021, 11, 323 7 of 9

literature is no exception. In one of the reviewed reports, Han et al. [42] mention a com- pressibility of 16%, which is well above the <2% desired in traditional diffusion bonding. To better understand whether improved bond quality was reached as a results of pressure variations or simply increased pressure, deformation during the process should be reported. Larger deformation of more than 2% for the very simple sample geometries used by both research groups in this review, pose the question to which degree IPADB can be applied for joining of larger industrial parts. Large industrial diffusion bonding furnaces usually have more than one stamp that applies pressure and it will need to be investigated if IPADB could actually be implemented properly and bonding over much larger surfaces can be achieved at uniform high quality. Besides the actual bonding time during diffusion bonding, considerable time is used for controlled heat-up and cool-down of samples (particularly large samples). Controlled heat-up and cool-down, since it is often performed in vacuum and workpieces cannot be stressed too much, can easily be longer than the actual bonding time. Even if IPADB holds the promise of shortening bonding times, it needs to be seen to which degree this has a relevant impact on the overall processing time.

4. Conclusions and Outlook Laboratory-scale experiments reviewed here proved that IPADB is a promising im- provement to traditional diffusion bonding that may reduce bonding times and could enable processing of parts with reduced surface treatment if compared to traditional diffusion bonding. Additional systematic experiments, particularly, experiments with larger parts, larger bonding areas, and an increase number of parts as they are found in sand- wich structures for heat exchanger plate packages, are required to better understand to which degree IPADB could be used industrially. In following IPADB experiments, the sample/workpiece deformation should be reported to better distinguish the impact of the pressure variation from simple high-pressure application. Besides, quantitative testing such as tensile testing or tensile shear testing should be performed to generate data that can be compared with other experimental work. In this context, common materials such as stainless steels should be preferred over more exotic materials, as this allows for a better comparison of the applied process.

Author Contributions: All authors discussed the content, analyzed the data, reviewed, edited the manuscript, and have agreed to the published version of the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Data Availability Statement: The data presented in this study is openly available. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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