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航空工業(yè)焊接的新趨勢
麻省理工學院 帕特里西奧樓門德斯
摘要:
焊接在航空業(yè)正經歷著令人振奮的發(fā)展。廣泛應用計算機和改善設備和設計新材料塑造的方式焊接，是實施過程和產品正在設計的主要要方法。有一種普遍的趨勢，減少鉚釘在在飛機結構組件的使用有一種普遍的趨勢. 擴散焊和激光，電子束焊接是在加入材料情況下使用的。軍用飛機的電子束焊接在加入鈦合金下使用，并且有擴大趨勢.大型商業(yè)飛機的激光束焊接，慢慢取代鉚釘在大部份機身上使用, 航天工業(yè)也顯示:航空業(yè)界承諾一些新的進程的發(fā)展.其中包括：攪拌摩擦焊接和變極性等離子弧焊，這已經被應用火箭的關鍵部件上, 目前, 鑄件在飛機有日益增加的趨勢,這樣開辟了新的機遇和挑戰(zhàn). 而一些進程，包括擴散焊接鋁合金和線性摩擦加入葉片盤。似乎并沒有得到廣泛應用. 本文側重于焊接的基礎，就其影響的焊接航空組件，以及對趨勢的行業(yè)的預計，因此是一項基本的水平。維修焊接，無損檢測，釬焊是本文章討論的范圍。
導言:
焊接的過程，就是人類一個古老的處理金屬的過程.其在歷史上的大多數的時間,它一直被視為一個粗淺的藝術或施工技術. 19世紀推動現代焊接新的發(fā)展而且發(fā)展趨勢比以往任何時候都要快. 不同的焊接工藝可以由不同強度的熱源融合.也揭示了許多重要的趨勢當中, 滲透率衡量的比例，深度,寬度（四/瓦特）焊縫截面的急劇增加與熱源強度有關。這允許較高的焊接速度,使得焊接過程更有效率. 一個更有效率的過程中在焊接過程中需要較少的熱輸入，從而形成一個強大的焊縫 . 較小的熱源,移動速度更快,也意味著停留時間在任何特定的點的時間大大減少. 如果停留時間太短，在過程中，無法手動控制，必須加以自動化, 最低的時間，仍然是可以手動控制的對應的電弧焊接（約0.3秒）. 因此，他們只能用自動控制熱源更激烈的的焊接. 焊接工藝在更加集中熱源的地方創(chuàng)建一個較小的熱影響區(qū)（ HAZ組織）和形成較低后焊縫扭曲力. 它可以推斷：所帶來的好處是: 更加集中熱源, 資本設備的費用大約與強度熱源成正比的.
航空業(yè)的特點是：低單位生產，單位成本較高，在營運條件極其嚴重情況下,焊接就十分重要了, 這些特征對較昂貴的和更集中熱源如等離子弧，激光束和電子束焊接作為焊接進程的選擇,是焊接的關鍵部件的重要選擇
焊接過程在航空業(yè)中的使用.
摩擦焊接）
在這個過程中，通過機械變形加入金屬。既然沒有熔化,就不存在基礎的材料熔化-凝固現象的相關缺陷. 這個過程中可以加入鋁起落架組件,組成了一個比較簡單的橫截面. 性摩擦（微動）焊接被認為是由通用電氣公司和普惠公司發(fā)明的一種替代，為制造和修理高溫合金盤為噴氣發(fā)動機. 雖然沒有透露這些進程，他們也會演變成商業(yè)應用.
攪拌摩擦（攪拌摩擦焊）
契維語在1991年發(fā)明了這一焊接方法, 這是一個堅實的焊接進程，通過機械變形加入金屬, 在這個過程中,圓柱等工具與異型探頭旋轉，慢慢地陷入了聯(lián)合線之間的兩塊資產負債表或板材，這樣就對接在一起，這個過程可以焊接以前報告鋁合金飛機結構在使用. 實力焊縫與弧相比是弧30 ％ 或50 ％ . 在一些小熱影響區(qū)的地方，殘余應力，微觀結構也得到了改變. 波音公司出了1500萬美元的投資在使用攪拌摩擦焊焊接助推器為三角洲的范圍的國家運載火箭，這是美國的第一生產攪拌摩擦焊的地方. 1998年8月, 在德爾塔II,首次發(fā)射一個使用攪拌摩火箭. 這一進程目前正在考慮加入鋁berilium合金，如2005年，為中央智囊團的航天飛機，就得到了應用。鈦合金也有其他航空用途. 作為攪拌摩擦焊的一種,為了更好地使用，它可以取代等離子弧焊（足）和電子束焊接（電子束焊接）,在一些具體的應用中用鋁和用鈦是有分別。
閃光焊
為是一種在熔化過程中, 應用一對焊接接頭焊接在短期的內弧和壓力而作用的。這是能夠產生強大的焊縫的基礎材料. 這個過程可以焊接鋁和表面耐高溫合金使用沒有特別準備或屏蔽氣體。它可以焊接各種復雜的截面, 這是用在航空業(yè)加入環(huán)噴氣發(fā)動機,主要是出于耐高溫合金和擠壓鋁構件的作用而考慮的。
氣體金屬弧焊
這個過程中，其中一個是世界上進心最熱門的焊接工藝，因為它的靈活性和低成本是呆以廣泛使用在航空業(yè). 缺點是大尺寸的熱源(處理流程，與如電子束焊接，運作）的焊縫有著不太好的力學性能. 這個過程是在主要的焊接工藝用于建造該燃料和氧化劑坦克火箭（ 2219為第一階段）, 目前應用在自動焊接的葉片愛國者導彈上. 這些葉片由一個框架17-4 pH值超級不銹鋼組成，其中金屬薄板的相同的組成是welded8, 此應用程序的好處，從成本降低，而可靠性增強。
鎢極氬弧焊（氬弧焊）
氬弧焊可以使用比的GMAW更激烈的熱源. 因此，它可以產生較小的焊縫，從而降低的成本。對于大多數結構,在應用這一過程中，不能與其他焊接方法如電子束焊接，激光焊接或等離子弧相焊接相單混用。氬弧焊是一起使用的GMAW與焊接在2014年和2219的鋁合金在燃料和氧化劑坦克在土星v rocket7使用. 梅塞施米特b?lkow blohm在德國目前使用的GMAW為噴嘴延長鎳，阿麗亞娜發(fā)射vehicles9上也使用過這種焊接. 大部分的焊接主要表現在商用飛機及對管道及油管使用焊接. 這個過程也可以用在換熱器的核心上，噴氣發(fā)動機百葉窗和排氣外殼上，不銹鋼和inconel1無論是在商業(yè)和軍事都得到了使用. 不銹鋼葉片在多倫多也用在堵塞焊縫在愛國者導彈也得到了使用. 允許應用到航空焊接結構的組成部分包括弧長控制和救濟的應力用散熱器在焊接上, 這項技術，是由洛克希德馬丁公司在土衛(wèi)六四運載火箭上使用的. 它是一種通過測量電弧電壓來測量所期望的滲透率.這種技術在中國 北京航空制造技術研究所也得到了應用. 它已用于噴氣發(fā)動機案件中的耐熱合金和火箭燃料箱的鋁合金。在這方面的技術，散熱器步道背后的焊接電弧就是這樣一種方式，他們的熱領域的互動，大大降低了氬弧焊的過程產生的殘余應力和扭曲力. 企圖以取代鉚由多倫多的焊接邁出皮膚板尚未成力，但由于嚴重歪曲了一些問題.
等離子弧焊
使用constricted弧之間的nonconsumable電極和熔池（轉移弧）或之間的電極和制約噴嘴（ nontransferred?。? 如果熱強度不夠高，這個過程是不可以運作，類似一個小孔模式，有人認為，激光或電子束焊接，雖然與規(guī)模較小但滲透率最高. 這個過程是用于焊接的先進的固體火箭發(fā)動機,使用材料是惠普- 9 - 4 - 30鋼.
其中一個最新的變化，就是霍巴特兄弟將這個過程如變極性等離子弧焊焊接（ vppa ）商品化. 這種變化在航空航天工業(yè)焊接較厚路段鋁合金，特別是為外部燃料箱的航天飛機得到了使用.這個過程中熔化的是在小孔模式中進行的.不好的一部分，是循環(huán)提供了一個陰極清洗鋁工件，而好的部分，提供了理想的滲透和熔融金屬流. 測試結果表明，最佳占空比為這個過程中涉及的負序電流15-20 MS和一個積極的2-5的電流中，一個積極作用是:當前的30-80 1高于負序電流. 集中供熱原因是為了明顯減少角扭曲力.
激光焊接
這個過程, 優(yōu)勢是電子束焊接技術可以提供最集中的熱源焊接, 更高的精度,焊縫質量和規(guī)模較小的扭曲. 這個過程是用于焊接噴氣發(fā)動機部件,其由耐熱合金制成，如hastelloy，激光加工燃燒在普惠公司噴氣發(fā)動機jt9d ， pw4000 ， pw2037和F - 100 - ○ – 22019得到了運用.
激光焊接將很快取代鉚在空中客車318飛機中使用. 顯著的優(yōu)點是可以預期并取得的取代鉚接接頭的不足. 鉚，估計消費占制造業(yè)的40 ％左右 .
電子束焊接
如上所述，高強度的電子束產生焊縫與熱影響區(qū)小，這個過程中的優(yōu)勢，電子束對熔融金屬的對接已沒有問題. 不過，它需要在真空中運作. 這一特點在使用這一進程中，特別適合焊接鈦合金而不能焊接在一個開放的氣體中的部件. 鈦合金被廣泛用于軍用飛機，因為它重量輕，強度高，性能在高溫下也較好. 應用電子束焊接，以焊接鈦部件的軍用飛機一直在不斷擴大. 塔員額和機翼部件在Ti 6Al - 4V和f15戰(zhàn)斗機也得到了廣泛的應用. 機翼盒舉行可變幾何的翅膀，在旋風式戰(zhàn)斗機，如f14 “雄貓”得到了使用. 在控制系統(tǒng)中,以及在以及在實施電腦自動化中有著顯著的差異. 這項新技術，使連續(xù)一通焊縫超過曲線和曲面，并通過不同厚度來進行. 波音公司的F - 22的關鍵結構部件現在用鈦電子束焊接這種方式來焊接的. F - 22是第一次飛機在60年的特點焊接機進行的. 前的前身用了鉚接鋁.將他們焊接在一起. 最近的應用的鈦鑄件在F - 22戰(zhàn)斗機的焊接出現了問題，因此延遲開始生產時間至少五個月. 俄羅斯能源火箭應用電子束焊接建造該氧氣和油缸. 由于龐大真空，是造成當地密封與鐵電產生影響.
擴散焊
這是一個固態(tài)焊接,在焊接過程中在焊縫所在處產生應用的壓力，在高溫下，該件沒有宏觀變形或相對變化. 航空業(yè)是主要用戶是dfw, 這個過程已證明,超塑成形（ SPF ）則鈦合金特別有用相結合. 在這種情況下，復雜的幾何形狀可以得到在短短得到應用. 在某些情況下替代鉚接鋁構件,從而使成本降價. 傳統(tǒng)的制作由500緊固件構成的16個部分,并一起進行. 有人建議，以取代設計，整體加筋所產生的SPF / dfw會得到很好的作用. 應用的SPF / dfw可以減少了原來的鉚接的鋁材構件，來自76個詳細的零件和1000緊固件，以鈦金屬版只有14個細節(jié)和90緊固件與總成本可節(jié)省30 ％左右. 成功的SPF / dfw鈦刺激了大量的研究與目標，完成了類似的過程與鋁焊接過程. dfw鈦和鋁鈦根本區(qū)別是鈦可以解散其氧化物而鋁不可以，因此, 剩余氧化氮在界面形成鋁聯(lián)合,極大地降低了力量的焊接在焊接中的擴散. 這個問題已妨礙了SPF級/ dfw鋁的普遍采用.
結論
驅動的成本和重量的積累，技術進步使得更換鉚釘和緊固件與焊縫得到緊密的結合. 在商用飛機中,一些鉚接鋁構件由SPF級/ dfw鈦的替代品（ SPF級/ dfw鋁仍處于試驗階段）形成了一種趨勢. 在不久的將來，空中客車飛機（ a318和a3xx ）功能將機身出現激光焊接，以在飛機上的形成. 展望進一步,邁向未來, 這是有可能將攪拌摩擦焊用于對飛機結構組件的焊接, 它可以可靠地加入合金系列等材料.
變極性等離子弧焊焊接（ vppa ） ，原本是設計為空間應用可能深入飛機工業(yè)入中的厚度較厚的鋁。實施計算機控制使用電子束焊接鈦合金的應用程序在過去是不可行的，制造業(yè)等焊接第一次為噴氣式戰(zhàn)斗機機身中使用，電子束焊接鈦在未來的軍用飛機的運用將增加，這種預期是合理的，在飛機使用鑄件正在增加，這必將帶來新的挑戰(zhàn)。
Proceedings of the conference “New Trends for the Manufacturing in the AeronauticIndustry”, Hegan/Inasmet, San Sebastián, Spain, May 24-25, 2000, pp. 21-38.
NEW TRENDS IN WELDING IN THE AERONAUTIC INDUSTRY
Patricio F. Mendez
Massachusetts Institute of Technology
9
Cambridge, MA 02139, USA
Abstract
Welding in the aeronautic industry is experiencing exciting developments. The widespread application of computers and the improved knowledge and design of new materials are shaping the way welding is implemented and process and product are being designed. There is a general trend to reduce the use of rivets in structural components in airplanes. Diffusion welding and laser, and electron beam welding are used to join the materials in these cases. In military airplanes electron beam welding is continually gaining ground in the joining of titanium alloys. In large commercial planes laser beam welds are posed to replace rivets in large parts of the fuselage. Some new processes developed for the space industry also show promise for the aeronautic industry, among them: friction stir welding and variable polarity plasma arc welding, which are already being used for critical applications in rockets. A current trend of increasing the use of castings in newer airplanes opens up new opportunities and challenges. Some processes that do not seem to have gained widespread application include the diffusion welding of aluminum alloys and the linear friction joining of blades for blisks. This paper focuses on the welding fundamentals, on its implications for welding of aeronautical components, and on the trends in the industry that can be expected from progress at a fundamental level. Repairs by welding, NDT, and brazing are left outside the scope of this paper.
Introduction
Welding is a process almost as old as the processing of metals by humans. For most of its history it has been regarded as an obscure art or a crude construction technique. New discoveries and the availability of electric energy in the nineteenth century pushed the development of modern welding with an ever-accelerating rate (Figure 1).
The different welding processes can be ordered by the intensity of the heat source used for fusion (Figure 2). This ordering reveals many important trends among them. The penetration measured as the ratio of depth to width (d/w) of the weld cross section increases dramatically with the intensity of the heat source. This makes the welding process more efficient and allows for higher welding speeds. A more efficient process requires less heat input for the same joint, resulting in a stronger weld, as indicated in Figure 3. A smaller heat source moving at a faster speed also implies a much reduced dwell time at any particular point. If the dwell time is too short, the process cannot be manually controlled and must be automated, as shown in Figure 4. The minimum dwell time that can still be controlled manually corresponds to arc welding (approximately 0.3 seconds). Heat sources more intense than arcs have shorter dwell times; therefore, they can only be used automatically. Welding processes with a more concentrated heat source create a smaller heat affected zone (HAZ) and lower post-weld distortions, as indicated in Figure 5 to Figure 7. The benefits brought by a more concentrated heat source come at a price: the capital cost of the equipment is roughly proportional to the intensity of the heat source as it can be deduced from Figure 8.
The nature of welding in the aeronautical industry is characterized by low unit production, high unit cost, extreme reliability, and severe operating conditions1. These characteristics point towards the more expensive and more concentrated heat sources such as plasma arc, laser beam and electron beam welding as the processes of choice for welding of critical components.
Welding Processes used in the Aeronautic Industry
Friction Welding (FRW)
In this process, the joining of the metals is achieved through mechanical deformation. Since there is no melting, defects associated with melting-solidification phenomena are not present and unions as strong as the base material can be made. This process can join components with a relatively simple cross section. It is used for the joining of aluminum landing gear components. Linear friction (fretting) welding was considered by General Electric and Pratt & Whitney as an alternative for the manufacture and repair of high temperature alloy blisks for jet engines2. Although little was disclosed about these processes, they do not seem to have evolved into commercial applications.
Friction Stir (FSW)
TWI invented this process in 1991. It is a solid-state process that joins metals through mechanical deformation. In this process a cylindrical, shouldered tool with a profiled probe is rotated and slowly plunged into the joint line between two pieces of sheet or plate material, which are butted together, as shown in Figure 9. This process can weld previously reported unweldable aluminum alloys such as the 2xxx and 7xxx series used in aircraft structures. The strength of the weld is 30% 50% than with arc welding3. The fatigue life is comparable to that of riveted panels. The improvement derived from the absence of holes is compensated by the presence of a small HAZ, residual stresses, and microstructural modifications in the welding zone4.
Boeing made a $15 million investment in the use of FSW to weld the booster core tanks for the Delta range of space launch vehicles, which was the first production FSW in the USA5. The first launch of a FSW tank in Delta II rocket happened in August 19993. This process is currently being considered for the joining of aluminum–berilium alloys such as 2195 for the central tank of the Space shuttle, and also titanium alloys for other aeronautical uses. As FSW becomes better established, it can replace plasma arc welding (PAW) and electron beam welding (EBW) in some specific applications in aluminum and titanium respectively.
Flash Welding (FW)
FW is a melting and joining process in which a butt joint is welded by the flashing action of a short arc and by the application of pressure. It is capable of producing welds as strong as the base material. This process can weld aluminum and temperature resistant alloys without especial surface preparation or shielding gas. It can join sections with complicated cross sections, and it is used in the aeronautical industry to join rings for jet engines made out of temperature resistant alloys and extruded aluminum components for the landing gear6.
Gas Metal Arc Welding (GMAW)
This process, one of the most popular welding processes in the world because its flexibility and low cost is not used extensively in the aeronautic industry. The drawback for its is that the large size of the heat source (compared with processes such as EBW, LW, PAW) causes the welds to have poor mechanical properties. This process was the main welding process used for the construction of the fuel and oxidizer tanks for the Saturn V rocket (2219 aluminum alloy for the first stage)7. One of the current applications of GMAW is in the automatic welding of the vanes of the Patriot missile. These vanes consists of an investment cast frame of 17-4 PH stainless steel over which sheet metal of the same composition is welded8. This application benefits from the low cost of GMAW, while extreme reliability is not as important as in manned airplanes.
Gas Tungsten Arc Welding (GTAW)
GTAW can use a more intense heat source than GMAW, therefore it can produce welds with less distortion at a similar cost. For most structural critical applications this process cannot compete with other welding methods such as electron beam welding, laser beam welding or plasma arc welding. GTAW was used together with GMAW to weld the 2014 and 2219 aluminum alloy in the fuel and oxidizer tanks in the Saturn V rocket7. Messerschmitt B?lkow Blohm in Germany currently uses GMAW for the nozzle extensions of Inconel 600 in the Ariane launch vehicles9. Most of the welds performed on commercial aircraft are done on ducting and tubing using GTAW10. This process is also used in heat exchanger cores, louvers and exhaust housings for jet engines, both commercial and military in stainless steel and Inconel11. GTA plug welds are also used in the stainless steel vanes of the Patriot missile8. Creative innovations that permit the application of welding to aeronautical structural components include arc-length control (ALC)12 and relief of stress by using a heat sink during welding (LSND)13. This technology, shown schematically in Figure 10, was developed by Lockheed Martin for the Titan IV launch vehicle. It permits one to detect the desired penetration by measuring the arc voltage, as shown in Figure 11. The Low Stress No-Distortion (LSND) technique has been developed at the Beijing Aeronautical Manufacturing Technology Research Institute in China. It has been applied to jet engine cases of heat resistant alloys and rocket fuel tanks of aluminum alloys. In this technique, a heat sink trails behind the welding arc in such a way that their thermal fields interact, significantly reducing the residual stresses and distortions created by the GTAW process. Attempts to replace riveting by GTA welding of stringers to the skin plate have not been sucessful yet due to serious distortion problems14.
Plasma Arc Welding (PAW)
PAW uses a constricted arc between a nonconsumable electrode and the weld pool (transferred arc) or between the electrode and the constricting nozzle (nontransferred arc). If the heat intensity of the plasma is high enough, this process can operate in a keyhole mode, similar to that of laser or electron beam welding, although with smaller maximum penetration. A schematic of PAW is shown in Figure 12. This process is used for the welding of the Advanced Solid Rocket Motor (ASRM) for the Space Shuttle15. The ASRM is made of HP-9-4-30 steel by Lockheed.
One of the latest variations of this process is variable-polarity plasma arc welding (VPPA) commercialized by Hobart Brothers. This variation was developed by the aerospace industry for welding thicker sections of alloy aluminum, specifically for the external fuel tank of the space shuttle16. In this process the melting is in the keyhole mode. The negative part of the cycle provides a cathodic cleaning of the aluminum workpiece, while the positive portion provides the desired penetration and molten metal flow. Tests showed that the best duty cycle for this process involves a negative current of 15-20 ms and a positive one of 2-5 ms, with a positive current 30-80 A higher than the negative current17, 18. The concentrated heat of VPPA causes significantly less angular distortions than GTAW, as shown in Figure 13.
Laser Beam Welding (LBW)
This process, together with electron beam welding can deliver the most concentrated heat sources for welding, with the advantages of higher accuracy and weld quality and smaller distortions. This process is used for welding and drilling of jet engine components made of heat resistant alloys such as Hastelloy X. Laser-processed combustors are used in the Pratt & Whitney jet engines JT9D, PW4000, PW2037 and F-100-PW-22019
Laser beam welding will soon replace riveting in the joining of stringers to the skin plate in the Airbus 318 and 3XX aircraft20. A schematic comparing a riveted and a welded stringer is shown in Figure 14. Significant savings are expected to be made by replacing riveted joints by LBW. Riveting is estimated to consume 40% of the total manufacturing man-hours of the aircraft structure4.
Electron Beam Welding (EBW)
As mentioned above, the high intensity of the electron beam generates welds with small HAZ and little distortion as shown in Figure 5 and Figure 6. This process presents the advantage over LBW that it has no problems with beam reflection on the molten metal; however, it needs to operate in a vacuum. This characteristic makes this process especially suitable for the welding of titanium alloys that cannot be welded in an open atmosphere. Titanium alloys are widely used in military aircraft because of its light weight, high strength, and performance at elevated temperatures. The application of EBW to the welding of titanium components for military aircraft has been expanding constantly. Pylon posts and wing components in Ti 6Al-4V for the F15 fighter have been EB welded by McDonnell Douglas since the mid 70’s21. The wing boxes that hold the variable geometry wings in the fighters Tornado, and F14 “Tomcat”, are also Ti 6Al-4V EB welded (Figure 15 and Figure 16). Progress in control systems and in the implementation of computers for automation made a significant difference in the EBW of titanium alloys for military aircraft. This new technology enables continuous one-pass welds over curved lines and surfaces, and through varying thicknesses. Critical titanium structural components are being EB welded this way for the Eurofighter (attachment of the wings and fin to the fuselage22) and Boeing’s F-22 (aft fuselage10). The F-22 is the first airplane in 60 years to feature a welded fuselage. Prior fuselages were made of riveted aluminum. The recent application of titanium castings in the F-22 presented welding problems that delayed the start of production by at least five months23.
A remarkable application of EBW is in the construction of the oxygen and fuel tanks of the Russian Energia rocket (Figure 17). Due to the large size of the tanks, the vacuum is created locally, and sealed with ferroelectric liquids24.
Diffusion Welding (DFW)
It is a solid-state welding process that produces a weld by the application of pressure at elevated temperature with no macroscopic deformation or relative motion of the pieces. The aeronautic industry is the major user of DFW25. This process has proven particularly useful when combined with the superplastic forming (SPF) of titanium alloys. In this case, complicated geometries can be obtained in just one manufacturing step as shown in Figure 18. The quality and low cost of the joint enables in some cases the substitution of riveted aluminum components with SPF/DFW titanium replacements. Figure 19 shows a possible improvement for the door panel of an aircraft fuselage. The conventional fabrication consisted of 16 parts held together by 500 fasteners. It was proposed to replace that design by a 2-sheet assembly, integrally stiffened produced by SPF/DFW. Figure 20 shows an exit hatch for the British Aerospace Bae 125/800. The application of SPF/DFW reduces the original riveted aluminum design from 76 detail parts and 1000 fasteners to a titanium version with only 14 details and 90 fasteners with a total cost savings of 30%. Figure 21 shows a wing access panel for the Airbus A310 and A320 in which switching from riveted aluminum to SPF/DFW titanium achieved a weight saving in excess of 40%. The success of SPF/DFW with titanium stimulated much research with the goal of accomplishing a similar process with aluminum. The fundamental difference between DFW of titanium and aluminum is that titanium can dissolve its oxides, and aluminum cannot. Therefore, the residual oxide at the interface of an aluminum joint dramatically reduces the strength of the diffusion weld. This problem has prevented the SPF/DFW of aluminum from being generally adopted.
Conclusions
Driven by cost