He took part in various excavations in Turkey and Syria, and began work on the TellKamid al lawz in Lebanon in 1963 together with Arnulf Kuschke, professor for Old Testament studies at the University of Mainz. He directed this excavation from 1966 until 1981. In 1982, he initiated the excavations near the village of Drama in the Yambol Province of southeast Bulgaria together with Jan Lichardus and Alexander Fol, heading the efforts until his retirement. His research covers a broad spectrum in both periods and geography. He authored important works on topics which included the Bronze Age in central Europe and the archaeology and history of Germanic peoples. In 1962 together with Georg Kossack and philologistHans Kuhn he published a study on the Nordwestblock. In 1970, Hachmann published his study Die Goten und Skandinavien, which investigated the origins of the Goths and their presumed original homeland in Scandinavia. In this study Hachmann concluded that it could neither be proven nor disproven that the Goths originated in Scandinavia. He nevertheless noted that the trail of human migration will not always be reflected in the archaeological record. Hachmann proposed that the Goths originated in the Przeworsk culture, but a later study by Herwig Wolfram has found this assumption to be incorrect. Hachmann continued his work after becoming an emeritus professor at the university. In 1991, he published his studies on the origin and creation of the Gundestrup cauldron. The excavation results from Kamid al lawz were published in more than 20 volumes under his direction in the series Saarbrücker Beiträge zur Altertumskunde. Those whose doctorates he supervised include the former director of antiquities for the Saarland, Alfons Kolling, the mining archaeologist Gerd Weisgerber and François Bertemes, professor of prehistoric archaeology at Martin Luther University of Halle-Wittenberg. Hachmann was a regular member of the German Archaeological Institute and was also an honorary member of the Romanian Academy from 1993. On June 30, 2007, the University of the Saarland held a colloquium on the occasion of his 90th birthday.
Publications (selected)
Die Goten und Skandinavien. de Gruyter, Berlin 1970.
Die Germanen. Nagel, Munich 1971.
as publisher: Ausgewählte Bibliographie zur Vorgeschichte von Mitteleuropa. Steiner-Verlag-Wiesbaden-GmbH, Stuttgart 1984,.
as publisher: Frühe Phöniker im Libanon. 20 Jahre deutsche Ausgrabungen in Kāmid el-Lōz. von Zabern, Mainz 1983,.
as publisher: Studien zum Kulturbegriff in der Vor- und Frühgeschichtsforschung. Habelt, Bonn 1987,.
Gundestrup-Studien. Untersuchungen zu den spätkeltischen Grundlagen der frühgermanischen Kunst. In: Bericht der Römisch-Germanischen Kommission. Vol. 71, 1991,, pp. 568–903.